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1013 E. GEO R. SWANSON spring 2009 A SHORT GEOLOGIC HISTORY OF THE EARTH The Geologic Time Scale The concept of relative dating and the discovery that fossils could be used to determine the age of rocks quickly led to the construction of the geologic time scale. This time scale, based on fossils, was divided into eras according to the life forms (fossils) found in rocks, and these time scale names make more sense if you understand that the zoic refers to life and that paleo = old, meso = middle and ceno = recent. The major divisions of the geologic time scale are the Paleozoic, Mesozoic and Cenozoic eras. These eras are divided further into time periods with names typically derived from the regions were rocks of the various time periods were first described. The Paleozoic Era, for example, is divided into the Cambrian, Ordovician, Silurian and Devonian periods, all defined in England. Devonian rocks were first described in Devonshire. Younger Paleozoic periods include the Pennsylvanian and Mississippian rocks from the U.S.A., and Permian rocks from the Perm province of Russia. There are no hard and fast rules however. Most of the world refers to our Pennsylvanian and Mississippian rocks as Carboniferous for the abundant coal beds in these rocks. Likewise, Cretaceous (from the Latin creta meaning chalk), the last period in the Mesozoic, is so named because limestone (including chalk) is abundant in rocks of this age. All of these periods are based on major changes in the fossil record, although not as great as the radical fossil changes (mass extinctions) found between rocks of the Paleozoic, Mesozoic and Cenozoic Eras. And what about rocks that are older than Paleozoic? What are they called, how are they divided, and are these rocks older than life? Well, these rocks are all older than the first Paleozoic period, the Cambrian. For this reason, Pre-Paleozoic rocks are said to belong to Precambrian time. Some evidence for life can be found in Precambrian rocks, but fossils in Precambrian rocks are extremely rare and primitive. Also, most Precambrian rocks are igneous, which would not have had fossils, or they are sedimentary rocks that have been metamorphosed, a process that would have destroyed any existing fossils. Fossils, therefore, are not particularly useful in dating Precambrian rocks, but dating based on the decay of radioactive elements is particularly helpful here. Just as we know less about the distant reaches of the Universe than about our own solar system, we know very much less about Precambrian earth history than we do about the history of the Cenozoic Era, a time closer to our own. This represents a fairly significant lack of knowledge because the Precambrian Era represents 88% of Earth's history, compared to one 1.4% of the geologic time belonging to the Cenozoic. Things ancient, therefore, tend to get lumped together, and we play fast and lose in the Precambrian talking about 4.3 or 4.4 billion as if 100 million years was next to nothing when, in fact, it represents much more time than has passed since the days of the dinosaurs. The Geologic Time Scale {66.4 Million - Present Cenozoic (Recent Life) Phanerozoic {245 Million - 66.4 Million Mesozoic (Middle Life) Visible life {570 Million - 250 Million Paleozoic (Old Life) -----------------------------------------PRECAMBRIAN TIME ---------------------2.5 Billion - 570 Million Proterozoic (First Life) 1 GEO 1013 3.8 Billion - 2.5 Billion Formation of Earth - 3.8 Billion Precambrian Geologic History of the Earth E. R. SWANSON spring 2009 Archean (Ancient) Hadean The world's oldest rocks have been assigned to the time period known as the Archaean Eon. The time before that is known as the Hadean Eon, and its name is derived from Hades Greek for "unseen" or "hell". It was once thought that this period of time was preserved only in the rock record found on the lunar surface. Recent discoveries, however, have shown otherwise. The Hadean Eon began after the Earth formed from the solar system nebula, and it officially starts at the time when the original crust of the earth formed. We know from the first section of the book that vast regions of the moon are covered by remnants of the ancient lunar crust, but are there any crumbs of the original earth crust left? We have already covered the formation of the solar system under the astronomy portion (recall the nebular hypothesis). Let's see what is known about the Precambrian events that followed on earth. Differentiation of the Earth Geochemical studies on rocks of the Earth's crust, the age of the oldest moon rocks and the age of most meteorites all suggest an age for the Earth of about 4.6 billion years. Earth formed through the addition of material from space in a process known as accretion. We know, however, from geophysical studies that earth materials must have quickly separated into a core, mantle and crust; great shells of the Earth arranged outward in order of decreasing density. Evidently, impact heating from the accretion process combined with heat produced by radioactive decay allowed the temperature of the early Earth's interior to rise to the melting point of iron whereupon the heavy metal sank to form Earth's dense iron and nickel-rich core. The core forms roughly half the diameter of the whole Earth. Nearly all of the remaining material coalesced to form a mantle around the core in what is called, well, the mantle. The lighter elements, including magnesium, silicon and oxygen, are abundantly found in Earth's mantle. It should be noted that the oxygen mentioned here is bonded with other elements to form minerals and is not free and uncombined like breathable atmospheric oxygen. Earth has a third, extremely thin, layer above the mantle. It is the least dense layer, and it is composed largely of light elements like aluminum, silicon and oxygen. This layer is known as the Earth's crust, and it is the source of the resources on which life and civilization depend. The denser mantle below is separated from the lighter crust above by what is called the Moho (short for Mohorovicic Discontinuity and named after Andrija Mohorovicic, a Croatian seismologist who discovered it in 1909). Earth's crust may have originally congealed from an early magma ocean upon which lighter minerals floated. The crust is thinner, relative to the Earth, than the crust on bread and it has been constantly changing since its formation. You may have rejected bread crust as a kid, but it is all we have to live on, and since Precambrian days, it has come in two types, deep pan and thin & crispy (to change our metaphor slightly to pizza). Continental crust, average thickness of about 20 miles, would be the deep pan and ocean crust, with an average thickness of about 3 miles, is obviously thin but you will have to wait until the next paragraph to see why it is crispy? It turns out that continental crust is not only thicker, but it is lighter and more buoyant (specific gravity ~2.8). The crust under the oceans is denser (specific gravity ~3.0). Now a density difference between 2.8 grams/cm3 and 3.0 grams/cm3 might not seem like much, but it makes 2 GEO 1013 E. R. SWANSON spring 2009 the difference between continents and oceans, and it determines if the crust has a long life at the surface of the earth or will soon plunge back into the mantle as a descending plate. Precambrian Plate Tectonics Ocean crust is relatively dense compared to continental crust, and it weighs on the hot underlying mantle causing Earth's surface to sag. Gravity pulls water toward these regions of sagging crust so oceans are located mostly over areas of ocean crust. Ocean crust can even locally detach and sink back into Earth's mantle, but no matter, new ocean crust is continually being generated elsewhere as melting mantle sends up magma to replenish the supply of ocean crust. So, crustal rock hidden by Earth's ocean is constantly forming a new, hot (crispy), thin crustal layer that moves laterally only to be destroyed and taken back into the Earth. Continental crust, however, is composed of material just buoyant enough to survive. Unable to sink into the Earth, continental crust is carried along as part of Earth's great plates. It rides with the underlying motion of the mantle, having igneous rocks added to it, perhaps colliding and merging with other continental crustal fragments, enjoying a long history at the surface of the Earth, and providing a relatively dry home for non-marine organisms such as ourselves. Geologists agree that plate movement existed in late Precambrian time, but the evidence for earlier plate motion is less conclusive. Some would have plate movements as early as 4.4 billion years ago; the earliest known time that Earth had a solid crust. Age of the Earth As mentioned, Precambrian time (the biggest chunk of geologic time) can not be divided on the basis of fossils. Radiometric dating is very helpful here as ages can be determined for ancient igneous and metamorphic rocks, even to the age of the Earth. The oldest Meteorites give an age of 4.56 billion years, an age that is thought to approximate the age of the Earth and of the other planets. A study of the isotopes of lead in very old lead-bearing minerals, suggests that the lead was isolated from the rest of the solar system about 4.55 billion years ago, an age that is remarkably consistent with the age given by meteorites. These dates are considered to approximate the age of the Earth (we tend to round up to 4.6). Also, the oldest rocks brought back from the original lunar crust are between 4.4 and 4.5 billion years old. There are, however, no rocks of that age known from Earth. So what is the world's oldest earth rock? World's Oldest Rock A Rock from Hell There are ancient Archean rocks in Greenland, Australia, Southern Africa and Minnesota, but none yet discovered is as ancient as that found in Canada's Northwest Territories near Great Slave Lake along the Acasta River. These rocks date, in fact, to the Hadean Eon. Their discovery also shows how the scientific method sometimes proceeds, which is to say, "not as advertised". It is true that very old rocks were already known from the region, and that geologists had flown there to systematically look for old rocks, but the Acasta rocks were a chance discovery. A forced landing due to engine trouble was viewed by the grounded geologists as a golden opportunity to pick up a few additional rocks. What they chanced to bring back, however, turned out to be the oldest known rock in the world. Another case of better to be lucky than smart, although it is better still to be lucky and smart. The rock didn't look much different than many others. It was a metamorphic rocks called the Acasta Gneiss (pronounced nice and along with its metamorphic friend, schist, provides geology 3 GEO 1013 E. R. SWANSON spring 2009 students with endless hours of entertainment e.g. "have a gneiss day", "it's not gneiss when you step on schist", etc.). The gneiss contained zircons, the mineral of choice for dating really old rocks. While gneiss is actually produced by the metamorphism of an earlier rock, in this case an igneous rock, the tough, refractory zircon grains don't seem to mind the later heat and pressure. They still produce the age of the original rock, which turned out to be 4.03 billion years! The folks at Washington's Smithsonian Institute were so impressed that a 1,600 pound stone was selected to mark the northern cardinal point of a symbolic compass on the grounds of the nation's new National Museum of the American Indian. With the help of the Canadian Government and the local native people, a proper stone was selected. It was said to be an appropriate choice because, as you recall from our earlier philosophical discussion, the local people's mythology tells them that they came from stone and that they have existed since the beginning of time. What better symbol than a stone from as close as possible to the beginning. But is it the oldest part of Earth? The Oldest Mineral The formula for Zircon is ZrSi04. That means that zircon contains the elements zirconium, silicon and oxygen. Fortunately for its use in dating, however, its crystal structure also takes in small amounts of the radioactive elements uranium and thorium. Also fortunately for the rest of this story, the mineral does not accept lead when it is formed. Zircon is, therefore, ideal for dating by the uranium-lead method. Uranium and thorium have very long, but known, half-lives. With time, these radioactive elements disappear and lead accumulates in the zircon. All you need are a couple of zircons and access to a sensitive high mass resolution ion microprobe (SHRIMP for short) to accurately measure the concentration of uranium, thorium and lead. With these measurements made, it is possible to calculate just how long it took to produce the accumulated lead and, therefore, the age of the zircon. The age of the zircon in an igneous or metamorphosed igneous rock like the Acasta Gneiss give the age that the rock formed, but sedimentary rocks are different. Grains in sedimentary rocks were once part of some older rock from which they were removed by erosion. Sedimentary rocks can contain grains derived from older rocks of various ages. Western Australia has some very old sedimentary rocks, about 3 billion years, but zircons found in these rocks are as old as 4.404 billion years old (plus or minus 8 million years the last significant number in the age given). From 3 to 4.4 billion years might not seem all that much, but that would be like saying the oldest rocks in the Central Mineral Region of Texas are about the same age as the age of the sediment forming in the local streams (which could have been last Tuesday). So the Canadians have the oldest rock, but the Aussies have the oldest mineral grain in the world. But where is that 4.4 billion year old rock the produced that zircon from "down under"? Well, it may be down under, hidden beneath younger rocks, or it may no longer exist, a victim of erosion or remelting. When all you have to go on is a single grain, the science can get intense. Scientists are saying that this single zircon grain tells us that Earth's crust had solidified by 4.4 billion years ago at the latest, and that there probably was even some continental crust (favorite haunt of zircons) and perhaps even liquid water. If there was continental crust, then perhaps there was a plate tectonic process to form it. If it is true about the water, then life (microscopic of course) is considered possible. All this from a single grain of zircon! It may not matter about the life, however, because what happened next would likely have been hell for any living thing. The Late Heavy Bombardment 4 GEO 1013 E. R. SWANSON spring 2009 The Lunar Highlands, as mentioned in the first section of this course, is completely covered with craters of all sizes. The Lunar Maria, by comparison, show relatively few impacts for their 3.2 to 3.8 billion years of existence. The Moon clearly shows that the process of accretion continued for some time after the first lunar crust formed. An analysis of the craters of the Moon, particularly those continental size craters that hold the Mare basins and of the breccia formed by those giant impacts, shows that there was a particularly violent upsurge in the rate of impacts between 3.8 and 4 billion years ago. This period is known as the Late Heavy Bombardment. The process certainly made its mark on the primitive lunar crust and it should have affected the Earth as well. The reason for this period of renewed bombardment after a relatively calm period of several hundred million years is a matter of some speculation. An accusing finger has been pointed at giant Jupiter, the tag-team of Jupiter and Saturn or maybe even all of the outer gas giants. It may be that the outter planets settled into somewhat nearer solar orbits 3.9 billion years ago, positions from which they deflected asteroid belt objects to cause havoc elsewhere in the solar system. It could also be that some cosmic collision sent new asteroids fragments flying in our direction. Estimates are that collisions during this period were frequent enough to produce serious environmental damage about every 100 years on average, and that several impacts occurred that created craters as large as the continental U.S.A. With all that chaos, life had about as much chance as a Hadeon snowball. Early Atmosphere and Ocean When did Earth get its atmosphere, what was the composition of that atmosphere, and how would our Canadian friends from the beginning of time have faired under such conditions? As previously mentioned, the early Sun striped the inner planets of their primordial solar system atmospheres. Hydrogen and helium, the two most abundant elements in the solar system, were too light to be held by Earth's weak gravity. They were also swept away by the direct impact of strong solar wind (ions from the Sun), at least until Earth developed a core and the associated magnetic field capable of deflecting the solar winds. Thus, our atmosphere is secondary and the result to some unknown extent of continued impacts, especially by comets, but mostly it is due to degassing of the Earth through igneous activity. The question then becomes what are the most abundant constituents of volcanic gas and where are they now? Volcanic gases contain a wide variety of constituents but hydrogen, nitrogen, carbon dioxide, sulfur dioxide and water vapor are common. Oxygen is present, but it is combined with other elements as in water (hydrogen dioxide), sulfur dioxide and carbon dioxide. This is good if you are a plant but not so good if you are an animal. So it makes sense that plants formed first in Earth's carbon dioxide-rich environment. Besides, animals would want something to eat as well as to breathe and plants certainly make fine dining. The hydrogen injected into the atmosphere would escape into space unless it combined with oxygen to make steam or water vapor or perhaps something else. The atmosphere of Saturn's largest moon, Titan, is rich in nitrogen but it also contains methane (hydrogen combined with nitrogen). Titan's thick, hazy, methane-rich atmosphere may be like that of our early Earth. It has also been suggested that Earth's very early atmosphere might have been like the hot, carbon dioxide-rich atmosphere of our sister planet, Venus, where droplets of sulfuric acid producing a Hadean scene seeming to lack only little red guys with pitchforks. But Earth is further from the Sun 5 GEO 1013 E. R. SWANSON spring 2009 than Venus. Most water, 97% at last count, condensed to form the world's oceans. Ocean water absorbs enormous amounts of carbon dioxide and sulfur dioxide and it can exit the ocean to be deposited in sedimentary minerals such as calcite (CaC03) and gypsum (CaS04 .2H20). So volcanic hydrogen, carbon dioxide, sulfur dioxide and water can all mostly leave earth's atmosphere for other watery or stony locations. Nitrogen's fate, with no particular place to go it seems, remains "up in the air", and nitrogen, therefore, is our atmosphere's most abundant gas. An oxygen-deficient, reducing environment surely persisted throughout Archean time. Remember that Australian zircon grain that gave so much information? We turn now to South Africa for a few grains of truth about the Precambrian atmosphere. In the next section of the course we will study gold, including that of Earth's largest gold deposit in the Republic of South Africa. South African gold is found in sandstone, the grains of which originated in mountains to the north. Most of these sandstone grains are common quartz, rounded by abrasion during their trip down river, just as such grains are rounded in streams today. But these ancient rocks also contain a few odd rounded grains of pyrite (FeS2 - also known as fool's gold) and uraninite (U02 - an ore of uranium). These minerals do not exist in streams today or in young sandstone because these minerals oxidize rapidly in the presence of today's oxygen-rich atmosphere. The pyrite, for example, would oxidize into a pile of rust. These minerals could only have been exposed, eroded, transported and deposited in Precambrian days in an oxygen-free environment. So, these single grains of evidence are more that enough to indicate that the Archean atmosphere was essentially free of free oxygen. At 3.8 billion years old, metamorphosed sedimentary rocks from Isua in southwestern Greenland are among some of the oldest rocks known on the planet, and they are the oldest sedimentary rocks. These rocks show the sedimentary textures and structures indicating that they were deposited in water. Although almost nothing is known about the size and distribution of Earth's earliest oceans at that time, the Isua rocks show that oceans have existed on the planet for at least 3.8 billion years. We should probably stop to consider why Earth has oceans and Venus and Mars do not. Much of the answer, it appears, lies in why Baby Bear preferred one particular bowl of porridge. If you remember, one bowel was too hot, one bowel was too cold, but one bowl seemed just right to the pint-sized, picky bruin life form. Venus is too hot for liquid water, while Mars is too small and cold. Early Earth, it seems, was just the right temperature for liquid water, although it would take a very long time to make a bear. Was there life in that 3.8 billion-year-old ocean? The Isua rocks do contain carbon, but it is found metamorphosed as the mineral graphite (like that originally used in writing pencils). It is not known with certainty if the graphite came from living things or if it was inorganic, but the graphite's isotopic composition suggests that it was once in a living form. What are the first definite signs of life? First Signs of Life The Oldest Fossils The early Earth would have been hostile to life as we know it, but perhaps not to life as they knew it. Sure there would be little or no free oxygen and the UV radiation would be withering, but that is not a great problem if you are an anaerobic bacteria living in a deep subsea hydrothermal vent or inside some hot, water saturated rock. The odd asteroid impact would have been inconvenient, and perhaps life had several false starts until things calmed down a bit, but some suggest that the basic building blocks of life arrived via complex molecules found in comets. 6 GEO 1013 E. R. SWANSON spring 2009 While the debate over Greenland's 3.8 billion year old carbon goes on, real fossils of organisms practicing some sort of metabolism and capable of reproduction are found in rocks beginning as early as 3.5 billion years ago. These fossils include mound-like structures called stromatolites. Stromatolites form when sediment is trapped on sticky mats of blue-green algae (cyanobacteria). They were relatively abundant in Precambrian days and they may still be found today in rare places were hungry snails can not exist as in salty Shark Bay, Australia. These modern stromatolites even still show their remarkable tolerance for ultraviolet radiation. The importance of these hardy little guys or colonies of these little guys is that the metabolism they practiced was photosynthesis, a way to live in a carbon dioxide-rich world that produced oxygen as a by-product. It must have been very slow going, but stromatilites contributed to the oxygen buildup in the ocean and in the atmosphere, and they paved the way for the more complicated organisms that utilized oxygen in their metabolism. The Oxygen Revolution The Great Precambrian Iron Deposits By 2.3 billion years ago, oxygen producing stromatolites had become relatively common. In spite of that, geologists estimate that oxygen levels were only about 1% of that today at the start of the Proterozoic (placed at 2.5 billion years ago and meaning first life, although bacteria existed prior to this time) and probably no more than 10% of today's level at the beginning of Paleozoic time when life in the oceans became diverse and abundant. Today, pure iron almost never occurs in nature. It is a highly reactive element that readily combines with oxygen to form iron oxides (like rust for example). In a world devoid of free oxygen, however, iron is quite soluble and great amounts of iron could accumulate in seawater. As oxygen levels began to rise, it combined with iron in the world's oceans and great quantities of iron oxides were precipitated to form widespread Precambrian iron deposits. The iron is found, for reasons that still keep geologists awake at night, alternating with layers of chert (flint to an anthropologist). The layered nature of these rocks has caused them to be called banded iron formation which has led to the acronym BIF. 92% of all BIF formed from 2.5 to 2.0 billion years ago. BIF is found in Precambrian rocks world-wide and BIFs constitute the largest source of iron for mining. BIF deposition marked a time in earth's history known as the oxygen revolution, when the world was changing from its early anaerobic (oxygen-free) environment to the aerobic world we live in today. Anaerobic organisms are still around, but they have mostly gone underground for safety as new organisms evolved that could take advantage of the freely available oxygen. Back in Precambrian days, When you couldn't see the mist for the haze, The sediments had the first geo-fad, Depositing iron was the craze. In Early Precambrian times, If it's true that our ancestors were slimes, With the sediment that would settle, so loaded with metal, I'm surprised we're not quarters and dimes. The Real Sexual Revolution? 7 GEO 1013 E. R. SWANSON spring 2009 The earliest fossils were prokaryotes, meaning that they were composed of cells that lacked a nucleus. They were also anaerobic (they required no oxygen). Fortunately for sex and eventually for oxygen users like us, early stromatolites had the ability to produce oxygen as a by-product of photosynthesis. This led, in later Precambrian time, to more complex, mulicellular organisms with eukaryotic cells. These organisms were able to use the newly available oxygen for their metabolic processes. They not only survived in an environment with freely available oxygen, but they flourished and in marked contrast to prokaryotes, they reproduce sexually. This last trick, however, took some time. For more than 80 percent of its history, the Earth not only lacked even the simplest multicellular organisms, it was also celibate. Phanerozoic History of the Earth Phanerozoic means visible life. So rocks belonging to the Phanerozoic Eon contain visible remains, or more likely traces of life. Visible, of course, depends on who or what is looking. Some people can barely see a Mack Truck, but a scanning electron microscope can see a microbe. Today's technology has pushed back the limit of what is visible into the microbial world. In addition, there are the rare Precambrian imprints and trails of soft-bodied organisms to consider (trace fossils). Although some Precambrian stromatolite colonies were about the size of a Mack truck, traditionally Phanerozoic time has been applied to the age when visible fossils were relatively abundant in the rocks. That would begin with the Cambrian Period. The Phanerozoic Eon is divided into three very unequal Eras based on the fossils being identified as old (Paleozoic), middle (Mesozoic) or recent (Cenozoic) life forms. The even finer subdivisions into Periods, Epochs, etc. are made based upon characteristic fossil assemblages or sometimes even on small variations in fossil features that Paleontologist find particularly exciting. Paleozoic (Old life but not all that old in the scheme of things) About 540 million years ago (the end of Precambrian and the beginning of Phanerozoic time), oxygen levels may have risen to roughly 7% of today's value. Although oxygen values like that are less than those found in what is called "The Zone of Death" high on Mount Everest, there were animals adapted to those conditions. Early Paleozoic rocks, in fact, show what geologists like to call "an explosion of life". Suddenly, it seems as though the rocks are alive with fossils, but suddenly to a geologist means that it may have taken 10 million years. Certainly life forms progressed as rapidly the oxygen arrived, but what appears to be a biological explosion might have had more to do with the arrival of carnivores and their gastronomic preference for other animals or to some other unknown factor. It was a world, like always, were it helps to be thick skinned to survive, and solid protective hard parts became a strategy for success. An exoskeleton or shell allowed a critter to put up a good defense, or it may have allowed life to extend to shallow water exposed to high ultraviolet radiation levels, or even to the intertidal zone where a shell might just keep you from drying to death. Trilobites, a marine animal that looked somewhat like a flat doodlebug, appeared about 540 million years ago, dressed in an external skeleton, and stole the show. Trilobites became the dominant life form of the Cambrian Period, although they were not as popular throughout the rest of the Paleozoic Era, and they were not invited to the Mesozoic. Most early Paleozoic animals were invertebrates like the sponges and brachiopods (a clam-like animal). All of them lived in the ocean. The Paleozoic Era lasted roughly 300 million years. During that time, life evolved from 8 GEO 1013 E. R. SWANSON spring 2009 primitive creatures to the forerunners of the dinosaurs. Paleozoic time began with virtually all life in the ocean. The land was bare of everything but algae and microscopic organisms, with only rock and sediment at the surface, about as barren as the surface of Mars, a geologist's dream. By middle Paleozoic time, some marine organisms were so abundant that their remains formed great reefs. The oceans also held the first sharks and fish with jaws, some of them enormous. One bizarre example was well over 30 feet long and heavily armored with large, bony plates. These fish had backbones (vertebrates like us) and early versions of nervous systems that we would one day be put to use to figure out what they were and when they lived. Also by middle Paleozoic time, plants had become established on land, and they were beginning to attract the attention of some hungry animals. Scientists calculate that increasing levels of atmospheric oxygen began the production of a stratospheric ozone layer that corresponded nicely with a general biological movement to dry land in the time before sun block. Winged insects also became part of the Paleozoic picture as did the first amphibians somewhat later in Paleozoic time. The first amphibians walked or crawled onto a land without competition, and they diversified rapidly. Amphibians eventually gave rise to reptiles and ultimately to birds, important organisms in the Mesozoic and Cenozoic to come. The development of a thick skin was again the key, but this time it was a thick skin in the form of a shell around an egg. The traditional Paleozoic egg (like a fish egg) left on land tended to dry out, and expectant mothers couldn't always depend on there being a nearby supply of water when it came time for the blessed event. A tough outer cover would keep junior from drying and dieing, and why not pack him a lunch in the form of an internal supply of nutrients while you're at it? And with little lizzy's needs meet, Mom was free to continue the freewheel lifestyle that the late Paleozoic land had to offer. Because by late Paleozoic time, there were numerous tall forests of plants standing in those inviting swamps that reptiles find irresistible. Throughout much of Paleozoic time, North America was a low-lying continent over which shallow seas came and went. These seas were responsible for the deposition of the mile thick sequence of sedimentary rock seen at the Grand Canyon, for example. Late in Paleozoic time, some continental fragments were at the South Pole, like Antarctica is today, accumulating thousands of feet of ice. Meanwhile, temperate and tropical zones had coastal swamps populated with giant ferns. The coming and going of glaciers may have causes rhythmic sea-level changes that periodically drowned the forest and ultimately turning them to coal. Rocks of this age remain great sources of carbon, and the age is known to most of the world as the Carboniferous Period, although U.S. geologists, as mentioned, prefer to divide this into Mississippian and Pennsylvanian Periods. No wonder the state of Pennsylvanian has so much coal. In England, Carboniferous-age coal was a major reason that William Smith built canals for the Industrial Revolution, and it resulted in his noticing the orderly faunal succession of primitive to more complex organisms in the rocks through which he blasted. Toward the end of Paleozoic time the continents came together to produce the Appalachian Mountains and metamorphosing some of that Pennsylvanian coal to high grade anthracite (hard coal). Of course it was too good to last. Paleozoic time ended with the extinction of most marine animals and land plants, probably the greatest extinction episode of all time, but more on that later. Mesozoic The Age of Reptiles The Mesozoic Era began about 250 million years ago with most of the current continents united in the giant landmass known as Pangaea, but when the Mesozoic Era came to a crashing end, 9 GEO 1013 E. R. SWANSON spring 2009 66 million years ago, the physical world did not look all that different than today. The Atlantic Ocean was somewhat narrower than today; Indian, following a recent broken up with Pangaea, was an equatorial island headed northward for a date with Asia; and Australia was still attached to Antarctica but was about to go walkabout. The great end-of-Paleozoic extinction left the early Mesozoic Earth impoverished in terms of fauna and flora. Eventually many clams and coiled ammonites developed and spread throughout the seas while reptiles such as the dinosaurs and crocodiles proliferated. Speaking of breakups and dinosaurs, around 1820, Gideon Mantell (1790-1852), English physician and friend of Charles Lyell, was hunting fossils when he came across something that he had never seen before. There, along the side of the road, was a group of large fossil teeth that looked to Mantell to be similar to iguana teeth, only much larger. It appeared that some huge Mesozoic animal had definitely bit the dust. Mantell published his results in 1925, and he gave the name Iguanodon to the creature that left his dentures behind in Mesozoic time. It was the first description of what would later be called a dinosaur. As for the breakup, Mantell just couldn't stop collecting fossils, even as his medical practice faced extinction. His home became crammed with specimens, and other fossil fanatics kept dropping by. Finally, his wife had had enough. She packed up the kids and moved out. The name dinosaur was created from the Greek words for terrible (dino) and lizard (saur). Some dinosaurs were terrible (or magnificent) but they weren't all so terrible. Early Mesozoic (Triassic) dinosaurs were no bigger than chickens and, like chickens, they walked on two legs. By mid-Mesozoic time some larger dinosaurs, included Tyrannosaurus, had evolved, and the late Mesozoic (Cretaceous Period) also had its share of heavy weights. The largest dinosaurs may have weighed as much as 100 tons, about as much as the great blue whale. It might seem strange, but paleontologists seem attracted to dinosaur hips, and so they divide dinosaurs into groups based mainly on their hips. Lizard-hipped dinosaurs include both the giant carnivores like T-rex as well as some plant-eating types. Bird-hipped dinosaurs were all herbivores and they include the familiar Stegosaurs with its large triangular plates and Triceratops with its three horns. Mary Anning (1799-1847) was a fossil collector par excellence who lived along the fossil-rich Mesozoic cliffs of the southern coast of England. She is believed to be the source of the tonguetwister, "She sells sea shells by the sea shore", but she collected more than mere shells. At age twelve, Mary Anning discovered the first complete skeleton of an ichthyosaur and later the first long-neck fossil plesiosaur. She collected all her life, living from whatever tourists visiting the coast would shell out for her fossils. Completely self-trained, scholars of the day beat a path to her door, and she made many contributions to paleontology. At the time it was generally thought that animals didn't evolve and they didn't go extinct. Each was a special creation so that there was no need for evolution. God didn't make mistakes, so there was not way anything could go extinct. If some impertinent rock hound would be bold enough to point out that no one had ever seen a live trilobite, someone else could just say "Oh they're out there someplace", but if nightmare monsters like Mary's marine reptiles or Mantell's Iguanodon still existed, they would be hard overlook. The tide of evidence turned in favor of extinction. If giant swimming reptiles were not enough, giant flying reptiles, the pterosaurs, must have been a common sight in the Mesozoic skies. These include a forty-foot wingspan and a fifty-foot crocodile, both Texas-size specimens found in Mesozoic rocks of far west Texas. There are a number of other Mesozoic events, but we plan to save them for the next section on natural resources because they are associated with the resources that profoundly affected aspects of 10 GEO 1013 E. R. SWANSON spring 2009 modern human history. The Western U.S. experienced volcanism, including the intrusion of igneous rocks found in the Sierra Nevada Mountains on the California-Nevada border. Gold veins formed along these intrusions that would later give our nation a rush. In South African, Mesozoic diamond bearing pipes were being emplaced that contained diamonds that would attract some unwanted attention from the outside world. Also, the high biological productivity of the Cretaceous oceans resulted in an enormous amount of organic material that eventually resulted in some of Earth's major ocean fields. These include giant oil fields of the Gulf of Mexico, Russia and especially those of the Persian Gulf region. The Mesozoic (middle life) ended with a mid-life crisis. This was another mass extinction that put an end to the dinosaurs among others, but allowing Cenozoic mammals to rise and diversify. Cenozoic Recent Life (Composed of Tertiary and Quaternary) The Cenozoic Era, the last 66 million years, is very short compared to the time periods that preceded it. We are living in Cenozoic time, and just as we know more about our parents than about our great, great grandparents, we know more about Cenozoic time than we do about eras long past. The Cenozoic Era has seen, among other things, the spectacular development of mammals, birds and flowering plants, the great Ice ages and the coming of mankind. Cats, horses, bears, elephants and humans all arrived in Cenozoic time, but not necessarily in that order. Many people think that the Spanish introduced the horse to North America, but that is not strictly true. Horses originated in North American and much of the modern horse's development occurred here and then spread during low Ice Age water levels to Asia and to much of the Old World. Horses, however, disappeared from North America about eight to ten thousand years ago. By that time, humans had already arrived, but without fresh horses. Years later, horses would escape from early Spanish explorers to begin repopulating North America. . Grasses evolved during early Cenozoic time allowing the evolution of life adapted to a home on the range. Horses and others have already been mentioned but there were giant ground sloths over six feet long and giant mammoths. As for the ocean mammals, perhaps the largest animal ever to live and certainly the most intelligent animal the planet had see to that point arrived. Whales evolved in the oceans where they developed brains larger than our own, but they did not seem to sea the need for tools and certainly did not use fire. Mammals of extraordinary size attracted some serious carnivores such as saber-tooth cats and giant dire wolves. The end of Cenozoic time is marked by a series of great glacial advances during an age of ice age known as the Pleistocene. Many giant mammals faced Ice Age extinction, but mankind did relatively well thanks to not being a picky eater (omnivorous apatite), a remarkable tool-making ability and the use of fire. Hominoids and Hominids Johann Jacob Scheuchzer (1672-1733), mathematic teacher, town physician and Zurich fossil zealot, described hundreds of fossils from Germany, all of which he considered as remains of the great Universal Deluge described in the Bible. In 1708, he even announced that he had found the vertebral remains of a miserable sinner who had perished in the flood. That fossil was later shown to be the 11 GEO 1013 E. R. SWANSON spring 2009 partial fossil remains of a large salamander. Apparently the salamander tale was caused by the quarryman fossil suppliers having neglected to give the salamander's tail to Herr Scheuchzer. And speaking of detailed studies More recent and more detailed studies have shown that Hominoids consists of the following three taxonomic families 1) the great apes (chimpanzees our closest living relatives, gorillas, etc.), 2) lesser apes like gibbons and 3) the hominid family, that would be us and various extinct ancestor species such as Neanderthals, etc. If chimpanzees are in another family and they are our closest living relatives, then you might guess that we are alone with no other species in the family. There currently are no other hominid species or even any other hominid genus, but that hasn't always been so. The total number of hominid species that have existed is not yet known, the hominid fossil record is still incomplete, but what exists is very well documented. Hominids have a fossil record extending back almost seven million years. Hominids exhibit quite a range in physical features but compared to the apes, Hominids walk upright, show a trend toward a large brain, possess good manual dexterity as demonstrated through the use and construction of tools, a small face, reduced canine teeth and will eat most anything (are omnivorous). Because we are the only surviving hominid species, we got to choose our name. The result was sexist and not particularly modest, but we have chosen the title of Homo sapiens, meaning wise men, for ourselves. So what happen to the rest of the family? The hominid fossil record all leads back to Africa, home of the apes, just as Charles Darwin predicted. So when did upright walking get started? The oldest hominid consists of a seven million year old skull and dental remains from northern Chad, but you can't tell from teeth if upright walking was its preferred method of locomotion. More abundant fossil fragments have been found in Ethiopia that indicate a chimp-sized hominid that lived when forests covered the region between 5.8 and 5.2 million years ago. The creature had the longer toe bone indicative of upright walking. More evidence was found in the summer of 1976 in the form of an amazing volcanic ash layer exposed by erosion in northern Tanzania. Among the thousands of giraffes, elephants, rhinoceroses, etc. footprints found in the ash, were the unmistakably clear hominid footprints of two adults and possible those of a child. One interpretation is that these tracks were made by an actual family group. In any event, these hominids left a trail in a wet layer of volcanic ash which later hardened and was preserved by a second dusting of volcanic ash. The volcanic layer below the bed was dated at 3.8 million years and a layer from higher in the sequence above what has been dubbed the Footprint Tuff (Tuff is a hard volcanic rock) is 3.6 million years old. The form of the foot is exactly like our own. These creatures walked upright. The bones found are classified as Australopithecus afarensis, but no tools have yet been found. Perhaps the most famous hominid fossil is Lucy found in 1974 just 50 miles away from the Ethiopian fossil above. Lucy is remarkably well preserved for a lady of her age (3.2 million years), and her skeleton is nearly complete. All these creatures were hominid members of the family, but they did not belong to our genus or species. The earliest member of our own genus (Homo) is Homo habilis. This creature is considered the first human species, but Homo habilis is not us. Homo habilis is also known from the East Africa Rift valley. It had a larger brain, apparently used crude hand axes, and lived from 2.5 to 1.6 million years ago in an environment populated by large predators. It must have had a tough life. Out of Africa 12 GEO 1013 E. R. SWANSON spring 2009 By 1.8 million years ago, Homo erectus came on the seen. About comparable in size to modern humans, the archaeological record also indicates that this hominid made and used tools, used fire, lived in caves and that it walked right out of Africa. Homo erectus spread into Europe, India, China ("Peking Man") and Indonesia ("Java Man"). For all his apparent talents, Homo erectus had an average brain size that was still not quite up to that of modern man. The transition from Homo erectus to Homo sapiens did not occur until at least 300,000 to 400,000 years ago, but we were not alone. Homo erectus may have continued to survive until 100,000 years ago or later, and there were other species as well. . In 1856, bones were found in the Neander Valley near Dusseldorf, Germany. The find was immediately acclaimed as the missing link between ape and man, but we know it today as Neanderthal Man. From about 200,000 to 30,000 years ago, Neanderthals lived in Europe where they must have coexisted with Homo erectus and a later bunch, Homo sapiens called Cro-Magnons, the immediate ancestor of modern Europeans. The Neanderthals were somewhat shorter but more massive and heavily muscled than your typical modern man. More suited, perhaps, to the cold climate of the last Ice Age, they were more a robust version of us, in the time before steroids. They lived in caves or hut rock shelters. They wore clothing, made stone tools and weapons, buried their dead with selected items and so apparently believed in an afterlife, and occasionally indulged in cannibalism. CroMagnons moved into the region inhabited by Neanderthals about 30,000 years ago and completely replaced them. They lived from about 35,000 to 10,000 years ago and they were anatomically modern. They were highly skilled nomadic hunters and they used a variety of specialized tools, perhaps even the bow and arrow. They occupied caves where they left cave paintings still seen today in France and Spain. It would seem strange to us today to share the world with another intelligent species like the Neanderthals. Would we fear them or fight them? Would we wonder if they were human? I am betting that we would wonder if they were human because we have done that before. Shortly after the Conquistador's first encounter with inhabitants of the "New World", the Spanish held a council to debate whether the people that the Spanish called Indians deserved to have human rights. I wonder if somewhere at that time on a remote mountain top retreat in the Americas a less well-heeled group was having the same argument about the Spanish. (For the record, the Spanish council decided that Native Americans were human and that they had souls that ought to be saved.) A Second Look at Geologic Time There are a number of levels at which we are conscious of time. From our own experience and from that of others, we have some feeling for the amount of time it takes for one century to pass. A centenarian has lived a long time. Historians must deal with even larger chunks of time. Columbus sailed the ocean blue in fourteen hundred and ninety-two (in fifteen hundred and twenty-six Columbus sailed the river Styx). Historic time is somewhat ill defined. It goes back thousands of years in places like China, but far less in places like Antarctica or the Moon, or College Station. It encompasses the time in which humans began to keep records of their existence. For most of us, and for those Enlightenment Age intellectuals, historic time is about all we can handle. But, there are much larger pieces of time, even in the human record. Human existence prior to historic time is the realm of anthropologists and archeologists. These social scientists (and even the antisocial ones) work 13 GEO 1013 E. R. SWANSON spring 2009 somewhat like historians to piece together the "history" of mankind's existence on the planet. They may dig through dirt rather than pages, and they commonly deal in thousands of years, and far more time then that in place like the East Africa Rift Valley. Geologic time, however, requires a major mental leap. How long is 4.6 billion years? Imagine taking a penny (0.05625 inches-thick). Now stick that penny end on at the water's edge in a sandy North Carolina beach, maybe where Wilber (or was it Orville) took that first flight a century ago. Then image that you come back each year to place another penny right next to the first. After 100 years (a long life) you only have a row of copper less than 6 inches in length, but if someone had been doing this since the birth of Christ (historic time), you would have twenty dollars worth of pennies stretching almost nine feet of beach. But consider that 4.6 billion pennies aligned in this manner would stretch from the Atlantic to the Pacific Ocean. 4.6 billion years is time of geologic proportions, but only about 1% of the dollars in Bill Gate's fortune. Earlier we considered the size of the Universe compared to the 3rd Planet from the Sun, and now we can see that Geologic time is similarly vast compared to that of a human life or of the time on Earth spent by all of humanity. Still, everything is relative. The Universe is certainly large compared to us and 100 years is short compared to 4.6 billion, but the average human probably seems like a universe to a germ, and they say that it takes only twenty minutes for a baby germ to become a grandfather. We have tried to get some handle on the length of geologic time. Now let's attempt to put some of the major events in the geologic history of the earth into perspective. Let's look at 4.6 billion years as if it were just one year. Imagine that the Earth was formed at the stroke of midnight on New Year's Eve. If so, there is absolutely no record of anything until February 18, the age of that oldest Canadian rock. The oldest known life, in the form of microfossils and stromatolites (bacteria and algae) arrived on March 28th, but Precambrian time was not over until mid-November. Nearly the whole year has gone without a living thing above a microbe on the land surface of the Earth and before the first shelled animals appeared in the seas. Land plants arrived on December first and the dinosaurs existed from December 18 until going extinct the day after Christmas. The first hominids didn't stand up straight until late on the last day of the year, and Columbus sailed the ocean blue just 3 seconds prior to midnight on December 31. Mass Extinction The existence of many extinction episodes have been proposed and debated, but most geologists agree that there are five times in the Phanerozoic history that can rightly be labeled as mass extinction episodes. The lengthy Paleozoic time saw three such events. About 440 million years ago marine organisms like the trilobites, graptolites, and conodonts perished. Some 365 million years ago saw a decimation in brachiopods, calcareous foraminifera and organisms that formed coral reefs. But the biggest extinction of all time occurred 250 million years ago, at the end of Paleozoic time, when an estimated 96% of all marine species went extinct. The cause for this is not known. An asteroid impact has been proposed, but it was also a time that saw extremely vast amounts of basaltic lava poured out in the region of Siberia. Mesozoic time saw two major extinctions. At 210 million years ago nearly one quarter of both marine and non-marine animal families went extinct, and the Mesozoic Era ended 65 million years ago when about two-thirds of Earth's organisms were lost from a wide variety of habitats. 14 GEO 1013 E. R. SWANSON spring 2009 We will discuss the Mesozoic mass extinction in more detail in the last section of the course as we cover geologic disasters. Are We Living in Interesting Times? Apparently the Pleistocene extinction event in which saber toothed cats, etc. were involved is not viewed as a mass extinction event. It primarily affected mammals, although birds and some reptiles were also affected. It was also not of global extent, but what about the present day? There is an old curse from Indian that says, "May you live in interesting times". A mass extinction episode would certainly make for interesting times? "Natural" extinctions go on all the time, but presumably at relatively low rates. It is natural for things to go extinct. Most species that have ever existed have gone extinct to be replaced by new species. The average length a species exists is estimated to be about 2 million years. There is an estimate that the natural extinction rate for birds is one species per 400 years, but there is no need to raise a big flap as long as new species keep taking off. The problem today is that scientists estimate that somewhere between 200 and 2,000 birds have gone extinct over the last 800 years, not the 2 that you would expect from the fossil record. I don't want to spread panic, but rates like that have caused many scientists to conclude that we are entering what has been called the Sixth Extinction. As for the future, the climate will rapidly warm and the seas will rise. Some life will perish, some will persist and some will adapt. It is harder to predict into which one of the groups we humans will fall. THE ANSWERS by Robert Clairmont "When did the world begin and how?" I asked a lamb, a goat, a cow: "What's it all about and why?" I asked a hog as he went by: "Where will the whole thing end, and when?" I asked a duck, a goose, a hen: And I copied all the answers too, A quack, a honk, and oink, a moo. HISTORY OF THE EARTH What is the geologic time scale, what are its major eras and on what are they based? What is the oldest geologic era called? What is the age of Earth and on what scientific evidence is it based? What is differentiation and what three Earth layers did it produce? How do these layers differ in terms of composition? What two kinds of crust does Earth have, which is thicker and which is denser? What is the boundary between the curst and mantle called? Which lasts longer, continental or oceanic crust? How old are the oldest known Earth rocks and where are they found? 15 GEO 1013 E. R. SWANSON spring 2009 What is the age of the oldest known (non-meteorite) mineral grain on Earth? What was the late heavy bombardment? What kind of atmosphere did the early Earth likely have? How was it formed? When did the Earth have an ocean? What are the first signs of life on Earth? What caused oxygen levels to increase? When did that begin? What evidence is there that early oxygen levels were much lower than today? What are stromatolites? What was the oxygen revolution and when did it occur? How is the oxygen revolution related to Earth's major iron deposits? What are eukaryotic cells and how did organisms with them differ from earlier organisms? What does Phanerozoic mean? To what geologic age do the trilobites belong? In what geologic age did land plants first appear? In what geologic age were the world's greatest coal beds deposited? What occurred at the end of Paleozoic time? What sorts of large creatures were common in Mesozoic time? What sorts of natural resources were formed during Mesozoic time? What occurred to end the Mesozoic era? Humans, birds and flowers belong to what geologic age? What is the Pleistocene? Are we the only human species on the planet? Has that always been true or is more complicated than that? Have there been episodes of mass extinction? When? Are we in one now? All rights reserved. No part of this work may be reproduced, in any form or by any means beyond that permitted by Sections 107 and 108 of the U.S. copyright law without the written permission of the author. 16 ... View Full Document

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