Unformatted text preview: The Sea Floor
The Sea Floor
The Sea Floor is geologically distinct from the Continents Ocean occupies 71% of surface area Ocean occupies 71% of surface area of planet Earth 61% of surface in Northern Hemisphere
80% in Southern Hemisphere
Pacific Ocean is largest & deepest
Atlantic & Indian Oceans about same size
Arctic is smallest & shallowest
The Southern Ocean surrounds Antarctica
Basically, the Southern Hemisphere is almost all water Mariana Trench in western Pacific
Mariana Trench in western Pacific Deepest point in ocean at 36,000 ft
If Mt Everest (29,000 ft) were placed there,
It would be covered by 7,000 ft of water.
What caused deep trenches?
26 major trenches in oceans When Earth was formed, rocks had varying When Earth was formed, rocks had varying densities, some lighter, some heavier Heavier rocks tended to flow deeper in planet Lighter rocks on top Thus the heavier rocks wound up at the bottom of the ocean and the lighter rocks became the land Oceanic (Basalt) vs Continental Oceanic (Basalt) vs Continental (Granite) crust Density 3 g/cm3 vs 2.7 g/cm3
About 5 km thick vs 2050 km thick
Younger vs older
Dark vs light
Rich in iron & magnesium vs rich in sodium, potassium, calcium, aluminum
Basalt vs Granite The earth has layers that were formed The earth has layers that were formed because of differences in density of materials
Solid inner core Liquid outer core Inner Mantle semisolid Outer Mantle semisolid Continental & Oceanic Crusts are solid The Asthenosphere is part of the The Asthenosphere is part of the outer Mantle
The Asthenosphere is a relatively fluid layer, sort of like warm taffy Above this is the
Pull the crust and
Form the midocean
Ridges Mid Ocean Ridge
Mid Ocean Ridge
Earthquakes are common at mid ocean ridges Rising lava from the Mantle causes the astheno
sphere to pull in two
Directions, thus forming
The ridge The movement of the asthenosphere The movement of the asthenosphere pulls the crust along with it This causes the movement of continents
This is the basis of the Theory of Plate Tectonics There were some suggestions that There were some suggestions that Africa and South America were once linked Fossils of Mesosaurus found in both. Also folded sedimentary rocks in both Alfred Wegener was a German scientist who Alfred Wegener was a German scientist who proposed in 1912 that the earth’s crust was composed of plates that were moving He couldn’t explain how they moved, so his hypothesis was not widely accepted
In 1970s scientists confirmed his idea
Theory of “plate tectonics” In Mid Ocean Ridges the youngest rocks In Mid Ocean Ridges the youngest rocks (newly formed from lava) are at the ridge and older are farther away.
Magma comes to surface in rift valley The mid Atlantic Ridge runs The mid Atlantic Ridge runs through the middle of Iceland The two plates are moving in opposite directions In The Pacific, the spreading zone is In The Pacific, the spreading zone is the East Pacific Rise Note the Plates Because the sea floor is younger near the Because the sea floor is younger near the mid ocean ridge, the sediments are thinner there The farther from the ridge, the thicker the sediments because the sea floor farther away is older.
Blue = thin
Red = thickest Trenches form where an oceanic plate is subducted under a Trenches form where an oceanic plate is subducted under a continental plate.
Volcanoes and earthquakes common. Why does it go under? Because it is denser than the continental crust San Andreas Fault
San Andreas Fault The San Andreas Fault is the sliding boundary between the Pacific Plate and the North American Plate. It slices California in two from Cape Mendocino to the Mexican border. San Diego, Los Angeles and Big Sur are on the Pacific Plate. San Francisco, Sacramento and the Sierra Nevada are on the North American Plate. And despite San Francisco’s legendary 1906 earthquake, the San Andreas Fault does not go through the city. West coast earthquakes occur along West coast earthquakes occur along fault lines Earthquakes globally, color shows Earthquakes globally, color shows how deep in earth they occurred Note differences in shaking Note differences in shaking amplification Most of the submarine trenches are Most of the submarine trenches are in the Pacific Note the location of the trenches Note the location of the trenches (previous slide) relative to plates In the Bay Area we had major In the Bay Area we had major earthquakes in 1838, 1868, 1906 and 1989. The Pacific Plate is slowly moving N and the North American plate less so, thus putting strains on faults Note how much faster N the Pacific Note how much faster N the Pacific Plate is moving
The plates are locked
now but someday they
Will have to move and
This will be an
The movement is detected
Using satellites and GPS.
Movement is about as fast
As your fingernails grow When the quake occurs there will be When the quake occurs there will be much more shaking and destruction in Bay Areas with mud and fill These areas shake like Jello
The severity of an earthquake is measured on the Richter Scale The SF Marina area was made by filling in an area The SF Marina area was made by filling in an area of the Bay with gravel and mud. In the 1989 earthquake this area of the city suffered the most destruction Richter Scale
Richter Scale The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs (adjustments are included to compensate for the variation in the distance between the various seismographs and the epicenter of the earthquake). Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; in terms of energy, each whole number increase corresponds to an increase of about 31.6 times the amount of energy released. Know this!
Thus an earthquake of Richter scale 7 has 10x more amplitude and 31.6 x more energy released than one of scale 6 Some Islands form over “hot spots”
Some Islands form over “hot spots”
As the crust slowly moves, the hot spot stays in same place and new islands are formed The hot spot is a place where hot magma forces its way through the lithosphere to form volcanoes Most hot spots are in ocean Hawaiian islands formed
This way Lava on the Big Island of Hawaii Lava on the Big Island of Hawaii This island is now over a hot spot Continental Drift has shaped the Continental Drift has shaped the Earth About 200 million years ago the continents were joined as one and called Pangea
Gradually Pangea broke up into continents of Laurasia and Gondwana The Atlantic and Pacific oceans gradually formed
Laurasia and Gondwana broke up to form present day continents There are two major types of There are two major types of sediments in the sea Lithogenous sediments are derived from the breakdown or weathering of rocks. These come from dust from deserts or from sediments in rivers.
Biogenous sediments are made up of the skeletons and shells of marine planktonic organisms
We will talk about these organisms later in the course.
Biogenous are either of calcium carbonate (major component of shells of snails etc) or silica (similar to glass) depending on the organisms they originated from. Africa, Mongolia and Patagonia are major Africa, Mongolia and Patagonia are major sources of lithogenic sediments because of the dust that blows off their deserts River plumes also bring lithogenous River plumes also bring lithogenous sediments Abyssal sediments may be of biogenic (carbonaceous or Abyssal sediments may be of biogenic (carbonaceous or silaceous) or lithogenic (clays or neritic) origin. Neritic = waters over continental shelves Fossils in ocean sediments plus presence of oxygen isotopes Fossils in ocean sediments plus presence of oxygen isotopes indicate that the earth has gone through many glacial (cold) and interglacial (warm) cycles During a glacial cycle more ice forms on land, and sea level drops
The last ice age was about 18,000 years ago and sea level was about 425 ft lower.
We are presently in an interglacial period.
Global warming causes sea level to increase (from thermal expansion and from glacial melting).
Sea level has been increasing about 2 to 3 mm/yr for the past several decades Greenland is now melting faster Greenland is now melting faster than any scientist expected If all of Greenland melts, the ocean will rise 7 meters.
If all of Antarctica melts it will rise another 75 meters.
Pink shows extent of melting in summer. Greenland summer melt stream
Greenland summer melt stream
More every summer Greenland Greenland 2005 Melt extent increasing Data from Vostok Antarctic ice core
Data from Vostok Antarctic ice core Shows relationship between temperature and atmospheric CO2. Temperature is determined from the isotopic composition of the water molecules in the ice. In the last glacial period so much In the last glacial period so much water was locked up in the ice that the continental shelves were exposed Scientists can infer glacial/interglacial periods Scientists can infer glacial/interglacial periods from microfossils and isotopes in ice cores Note changes in climate over the past 450,000 years Shelves extend out to about 200 m depth and at the Shelves
shelf break, the continental slope starts and leads to the deep ocean. Shelves make up 8% of area of the oceans. The shelf on the US west coast is narrow, The shelf on the US west coast is narrow, often less than 1 km (0.6 mile) The US East Coast has a much wider shelf.
The shelf ends at the shelf break (about 150 to 200 m depth)
Shelves are highly productive and support commercial fisheries.
The slope descends to the deep sea floor.
The slope has a grade of 56%, about the same grade that the highway (Route I80) is on the road up to Lake Tahoe Note extent of shelf off New England
Note extent of shelf off New England During Ice ages the continental During Ice ages the continental shelves were exposed Rivers drained out over the shelves and eroded them and formed deep Canyons
Monterey Canyon is a good example of a submarine canyon In Monterey Bay, the deep canyon was In Monterey Bay, the deep canyon was formed by erosion from an old river when sea level was lower Turbidity currents erode the slope Turbidity currents erode the slope and also submarine canyons Continents may have an active or a Continents may have an passive margin West coast of South America is active, while east coast is passive
West coast is colliding with the Nazca Plate
This is creating a Trench, and geological activity is very high.
The east coast is not a boundary between plates and is geologically inactive. This coast has a gentle slope and wide shelf. Active (upper) and Passive (lower) Active (upper) and Passive (lower) margins The Abyssal Plain in the depths of the ocean is The Abyssal Plain in the depths of the ocean is very flat except for the presence of seamounts which are submarine volcanoes The submarine USS San Francisco The submarine USS San Francisco hit an uncharted seamount The Pacific has lots of flat topped The Pacific has lots of flat topped seamounts called guyots (geeohs)
They originate from coral atolls which have sunk beneath the sea surface These are sites of high
Biological activity The Mid Ocean Ridges have The Mid Ocean Ridges have numerous hydrothermal vents Seawater percolates through rock and warms from contact with magma.
The seawater picks up reduced chemicals which can sustain life
We will cover these vent communities later Study Guide
Study Guide Where deepest place in the ocean is, how deep and in what geological feature.
Differences between oceanic and terrestrial crusts.
What is asthenosphere and characteristics. What are processes that form the mid ocean ridges?
Who/What is Alfred Wegener, East Pacific Rise & age of sediments related to sea floor spreading.
Why is it that when the Pacific Plate hits the North American plate it goes under it?
Location of San Andreas Fault and directions and speed at which the plates on either side are moving.
In Bay Area, what type of land will sustain the most damage?
Understand Richter Scale
What are “hot spots”, particularly in relation to island formation Study Guide
Pangea (when?) Laurasia, Gondwana Differences between Lithogenous, Biogenous,
Calcium carbonate & silaceous sediments and how they originate
When was last glacial period and what happened to sea level. What is happening to Greenland now? How fast is sea level rising?
What formed the canyon in Monterey Bay?
What are Turbidity currents, Difference between Active & Passive Margins?
What is a Guyot? ...
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- Fall '08
- sea floor, mid ocean ridges