BasicPrinciples2009

BasicPrinciples2009 - Basic Geologic Principles and...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Basic Geologic Principles and Geologic Time (4 lectures) Actualism The Development of the Earth System Uniformitarianism • The present is the key to the past. • Is the present the key to the past? 1750’s to 1850’s • Developed ways to understand Earth processes – Werner (1749-1817) – Hutton (1726-1797) – Cuvier (1769-1832) – Lyell (1797-1875) Abraham Werner (1749-1817) • Mining engineer and popular professor in Germany – First to classify minerals Neptunism • All rocks precipitated from a primeval, worldwide ocean • hence the name… • Precipitated in a particular sequence • the stratigraphic column/time scale Neptunism’s Column • Primitive rocks • Oldest granite and metamorphic Contributions • Understood erosion and deposition • Limestone was precipitated from water • Gradual processes • Directional, but not necessarily biblical • Was a “whole earth” theory • Transition rocks • hard sedimentary rocks like quartzite • Secondary Rocks • softer sandstones, limestones, coal • Tertiary rocks • unconsolidated sediments and volcanic rocks Neptunism sinks • It didn’t fit the patterns everywhere • Where did the water go?? • Static Earth Catastrophism • Rock units caused by a series of catastrophic events. – Long periods of stasis with short periods of upheaval. Catastrophism Catastrophism crunched • Many more than six catastrophes to account for all the rocks • Didn’t explain igneous rocks seen forming in present day – mainly by French geologists! James Hutton 1726-1797 Emergence of Gradualism • Gentleman Farmer – soil, where it came from, where it was going. James Hutton 1726-1797 Emergence of Gradualism • The Rock Cycle – Sedimentary rocks were made of bits of weathered older rocks. – The unlithified counterparts to the strata were still forming and would eventually form rocks. – Cyclic • He noticed the weathering and erosion of rocks. James Hutton 1726-1797 Emergence of Gradualism • Revolution: a cyclic view – “no vestige of a beginning, no prospect of an end” • The Rock Cycle • Suggested the Earth was old Hutton’s (additional) contributions • Saw that processes that were operating today operated in the past (erosion, deposition) • The Earth was much older than 6000 years – erosion rates (~.03 mm/year) • Earth processes were cyclic: – Erosion-deposition-upheaval, repeat Charles Lyell (1797-1875) Quintessential Gradualist • Articulated Hutton’s ideas – The Principles of Geology (~1830) • -the book that defined modern geology – Based largely on Hutton – Lots of observation and examples – Presented in a “lawyer-like” fashion Lyell’s Uniformitarianism • All mechanisms that acted in the present, acted in the past • No mechanism existed in the past doesn’t exist today • All mechanisms operated at the same rate in the present and in the past • Never ending cycle, completely cyclical, gradual view of the earth – Staunch “gradualist”, very dogmatic – Darwin’s teacher Notes on Lyell’s Uniformitarianism • Dominated many areas of science (particularly biology) for 100 years! • He argued (to a fault) for infinite cyclicity, – no directional component in Earth history – a kind of “steady state” • Aside: this caused him to doubt Darwin’s theory of evolution, since it implied a directional component. Lord Kelvin (1866) Challenges Gradualism • The earth was losing heat from its interior • measurable • Kelvin calculated age of the earth by using: • melting temp. of rocks, size of the earth • Rate of heat loss • = 20 m.y. to 400 m.y. • Not enough time for gradualism William Thomson Radioactivity burns Kelvin t = (T0/G)2 !" T0, based on melting experiments on rocks (~3900 °C) " # 1.2 ! 10"6 m2 s"1 lab measurement, thermal diffusivity (how fast a substance conducts heat) G = Geothermal Gradient • Antoine Henri Becquerel discovers radioactivity (1896) Henri (aka Hank) Becquerel Marie Curie (1867-1934) • Radioactive decay produces heat – falsified Kelvin’s hypothesis? • Radioactive elements are found within the earth. • Kelvin’s assumptions were incorrect J. Perry (1895) • Little known fact: J. Perry beat radioactivity to the punch • Proposed convection Marya Sklodowska aka Marie Curie Lake Missoula/Scablands catastrophes do occur Lake Missoula/Scablands catastrophes do occur Lake Missoula/Scablands catastrophes do occur • Peak discharge for the earliest and largest floods is estimated at 20 times the flow of the Amazon River, • or more than the total discharge of all the rivers in the world. • The floods were 300 meters deep and moving at 100 km/hr. – That's water a thousand feet deep with the force of a fire hose. Modern Uniformitarianism: “Actualism” (~1920’s-today) • Uniformity in process, but not rate – inviolable laws of nature that have operated since the beginning of time • Geological processes are governed by these laws Modern Uniformitarianism: “Actualism” (~1920’s-today) – Combination of Hutton/Lyell, Darwin, and Cuvier’s viewpoints Rates of Geologic Events • Most happen so slow that they are nearly imperceptible and take millions of years to happen • Some events are instantaneous Rates of Geologic Events • Most happen so slow that they are nearly imperceptible and take millions of years to happen • Some events are instantaneous Your fingernails Average erosion of the continents .03 mm/y Cutting of the Grand Canyon 0.7mm/y Postglacial rise of sealevel 5mm/y Fast-spreading Mid-ocean ridge 200mm/y Advance TigrisEuphrates Delta 25,000mm/y Fundamental Geologic Principles Da Vinci on fossils Da Vinci on fossils • They resembled living shells. • Found too far from the ocean to be carried by a flood • didn’t floods carry things out to sea?? Nicolaus Steno • • • • Fossils Superposition Original Horizontality Lateral Continuity • Found in groups and orientations like living shells, not jumbled up from flood deposits • He was correct, but largely ignored... Nils Stensen 1638-1686 Nicolaus Steno • • • • Fossils Superposition Original Horizontality Lateral Continuity Nicolaus Steno • Concluded fossils were remains of past organisms – De solido intra solidum naturaliter contento dissertationis prodromus • Preliminary discourse to a dissertation on a solid body naturally contained within a solid. Nils Stensen 1638-1686 Superposition • The oldest layer is at the bottom, with successively younger layers above. Original Horizontality • Sedimentary rocks form horizontal layers • Thus, inclined sed. rocks suffered subsequent disturbance (tectonics!) – because particles settle from a fluid under the influence of gravity – Note sequence of events: – sed rocks came first, followed by tectonic disturbance Lateral Continuity • Sedimentary rocks are laterally continuous – to a certain extent (not forever!) Lateral Continuity • Sedimentary rocks are laterally continuous – to a certain extent (not forever!) Crosscutting Relationships • Something that cuts across or affects another must be younger than the material that’s being affected. • Faults • Folds • Igneous intrusions Crosscutting Relationships Faults The rocks had to be there before the fault Crosscutting Relationships Faults Strike-Slip Hanging wall Crosscutting Relationships Faults Normal Strike-Slip Hanging wall Normal Normal Foot wall fault Reverse (if low angle: thrust) Reverse (if low angle: thrust) Foot wall Crosscutting Relationships Faults Strike-Slip Hanging wall Crosscutting Relationships Faults Strike-Slip Hanging wall Normal Foot wall fault Normal Foot wall fault Reverse Reverse (if low angle: thrust) Reverse (if low angle: thrust) Crosscutting Relationships Folds The rocks had to be there before being folded Crosscutting Relationships Folds Crosscutting Relationships Folds ng you est syncline anticline old est The rocks had to be there before being folded Crosscutting Relationships Intrusions sedimentary igneous Crosscutting Relationships Intrusions Unconformities Missing Time in Rock Record • Rocks are formed episodically, not continuously sedimentary • Gaps are called unconformities o ne us ig – Nonconformity – Disconformity – Angular unconformity The molten nature of the igneous rock allows it to “burn” its way through the red sandstone (sandstone was there 1st) Nonconformity • The erosional contact between igneous or metamorphic units below and sediments above. • Implications: – Uplift of the igneous/metamorphic rocks followed by deposition of the sedimentary rocks – tectonic timescales Nonconformity sedimentary schist Disconformity • Missing time between two sedimentary units • Implications: Erosion or non-deposition took place across the unconformity Disconformity sedimentary sedimentary Angular Unconformity • Displays angular discordance across the unconformity. • Implication: – Deposition of unit 1 – Uplift/Tilting/Erosion unit 1 – Deposition of unit 2 Angular Unconformity unit 2 sedimentary The Great Unconformity sedimen unit 1 tary Angular Unconformity Siccar Point (Hutton’s locality) The Grand Canyon has it all! How do we tell if there is an unconformity? • Angular: look for the angle between the sedimentary rocks! Disconformity Angular Unconformity • Nonconformity: between seds and ig/met! • Disconformity: – Need a marker that will tell us the “age” of the sed. rocks. Nonconformity Robert Hooke (Late 1600’s) • Speculated that fossils might be useful for correlating rocks from place to place • Roman coins were used to date successive human historical events • fossils could be used in the same way George Cuvier (1769-1832) • Recognized that fossils seemed to occur in a particular sequence • studied mammal fossils in the Paris Basin. • Demonstrated that extinction occurred • He was largely ignored... William Smith (1769-1839) • “Father of modern geology" – Well, at least one of them, anyway… Fossil Succession • No two species are exactly alike. • Species become extinct and never “reappear” • think of the coins… – the rock types change from place to place, but the succession of fossils was the same. • Largely ignored – lower-class upbringing – recognized for his contributions later in life with the GSL Wollaston Medal in 1831. • Thus, fossils can be diagnostic of a certain age. – Index fossils • Widespread • Short-lived Geologic Time Era Cenozoic Period Neogene Paleogene Cretaceous 142 Ma today How geologists display geologic information • Geologic maps 65 Ma age of mammals age of dinosaurs Mesozoic – two dimensional plan view Jurassic Triassic Permian 290 Ma 206 Ma 251 Ma • Stratigraphic columns and cross sections – two dimensional vertical view age of trilobites Paleozoic Carboniferous Devonian Silurian Ordovician Cambrian 495 Ma 544 Ma 354 Ma 417 Ma 443 Ma Jean-Étienne Guettard 1715 –1786 • First “Geologic” map (1746) Antoine-Laurent Lavoisier 1743-1794 • Worked with Guettard on the Geologic atlas of France--first stratigraphic column William Smith 1769-1839 • First Geologic map of England – "father of modern geology" – Well, at least one of them, anyway… 1819 1820 Despite the fact that Lavoisier and Cuvier beat him to the punch… The Formation • A Formation is a mappable unit of rock, usually distinctive from other units and regional in extent • Formations can be composed of members • Formations can be grouped together to form Groups Map view each color represents a formation The geologic time scale and the immensity of geologic time Sedgwick vs. Murchison The first geologic systems • Sedgwick and Murchison (Brits) • Realized the need to “organize” the vast amount of fossil and rock data • Proposed the idea of naming “systems” of rock • based on fossil content and other lithologic criteria Cambrian and Silurian • Sedgwick named the Cambrian system • few index fossils • more “lithologic in nature Sedgwick vs. Murchison $%&" '(%! • Murchison named the Silurian System • based on fossil content )*+ µ,! • Debate!! • Cambrian the lower part of the Silurian? • Silurian the upper part of the Cambrian? Debate resolved! • Charles Lapworth (in 1879) – showed that there were THREE distinctive groups of fossils: • Cambrian • ORDOVICIAN (intermediate fossils) • Silurian Debate resolved! Ordovician Cambrian Organization of the Time Scale • “time vs. rock” units – Time is continuous • “abstract” – Rocks are not continuous • riddled with unconformities • "tangible" – Thus, a distinction is made between time units and time-rock units Cenozoic The geologic time scale is a hierarchy Era Period Neogene Paleogene Cretaceous today 65 Ma 142 Ma Mesozoic Eon Era Period Epoch Age Jurassic Triassic Permian 290 Ma 206 Ma 251 Ma Paleozoic Carboniferous Devonian Silurian Ordovician Cambrian 495 Ma 544 Ma 354 Ma 417 Ma 443 Ma The geologic time scale is a RELATIVE time scale • The time scale was constructed using basic geologic principles (not "dates") • • • • • Superposition Lateral Continuity Original Horizontality Fossils Succession (index fossils) Cross-Cutting Relationships The geologic time scale is a RELATIVE time scale • The base of a geologic time unit is usually defined by the first occurrence of a fossil or groups of fossils • Example: Base of Triassic is defined by first occurrence of H. parvus Era Cenozoic Period Neogene Paleogene Cretaceous today 65 Ma 142 Ma Mesozoic Jurassic Triassic Permian 290 Ma 206 Ma 251 Ma Paleozoic Carboniferous Devonian Silurian Ordovician Cambrian 495 Ma 544 Ma 354 Ma 417 Ma 443 Ma Relative vs. “Absolute” Time • Relative time – sequence of events, without dates – chronostratigraphic Fossils vs. Radioactivity!! • Not all rocks can be radiometrically dated – few sedimentary rocks can be dated… • “Absolute” time – sequence of events, with dates – radiometric age dating • covered in lab – chronometric Eon Phanerozoic today Era Cenozoic Period Neogene Paleogene Cretaceous today Meso Neo 65 Ma 142 Ma 1.0 Ga Proterozoic Mesozoic Jurassic Triassic Permian 290 Ma 206 Ma 251 Ma 2.0 Ga Paleo Paleozoic Carboniferous Devonian Silurian Ordovician Cambrian 495 Ma 544 Ma 354 Ma 417 Ma 443 Ma 3.0 Ga 4.0 Ga Archean A calibrated time scale (in millions of years) Eon Phanerozoic today The Whole Time Scale PRECAMBRIAN Geologic Time + Grand Canyon Meso Neo 1.0 Ga Proterozoic • The Precambrian is 87% of Earth’s History!! • “Precambrian” is subdivided using chronometric means – Archean and Proterozoic Eons Hermit Shale Supai Group Redwall Limestone Muav Limestone Bright Angel Shale Tapeats Sandstone 2.0 Ga Paleo 3.0 Ga Archean • Phanerozoic Eon is subdivided using chronostratigraphic means – Fossil succession 4.0 Ga Ch u Gr ar ou p Geologic Time + Grand Canyon Era Cenozoic Period Neogene Paleogene Cretaceous 142 Ma today Ways to visualize Geologic time • Geologic time is a long, long, long time!! 65 Ma Hermit Shale Supai Group Redwall Limestone Mesozoic • Much longer than we can perceive without a lot of imagination. – Drink analogy – Arm analogy – TP analogy Jurassic Triassic Permian 290 Ma 206 Ma 251 Ma Paleozoic Muav Limestone Bright Angel Shale Tapeats Sandstone Carboniferous Devonian Silurian Ordovician Cambrian 495 Ma 544 Ma 354 Ma 417 Ma 443 Ma Ch u Gr ar ou p Ways to visualize Geologic time The last 4.6 Million Years ...
View Full Document

Ask a homework question - tutors are online