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Earth’s Crust
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Movable, unique in the solar system
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Tectonics - field that deals with the structures and deformation of the Earth’s crust.
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Hydrologic cycle: Continually eroding continents
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Plate Tectonics: Build up Continents
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Average elevation of a continents: .84 km
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Average depth of oceans: 3.74 km
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Crust is divided into large pieces called plates. Plates “float” on top of deeper, more dense
mantle material. Mantle is believed to move by convection currents (warm rises, cold sinks)
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Alfred Wegner - Began idea of continental drift. He noticed that South America and Africa fit
together like “pieces of a puzzle.” Coined the word Pangea or super-continent. It is believed that it
came together in the Permian period and started to split apart 200 million years ago.
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Two types of crust: Continental and Oceanic
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Continental Crust
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Very old; 3.8 billion years old
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Very earliest rocks were destroyed by erosion
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Complex reactions; layers have been bent or folded
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Consists mainly of Silicate (SiO2) based minerals; mainly granite, sandstone, and shale
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Continental crust is less dense and tends to “float” on top denser mantle. Mantle material is
what makes up Oceanic crust.
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Averages about 35-50 km thick.
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Oceanic Crust
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Young; none older than 250 million years old
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Very little deformation as compared to continental crust.
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Constantly being produced at spreading centers (ridge)
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Made up of dense iron/magnesium minerals ; mainly basalt, gabbro, olivene
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5-15 km thick
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Continental Drift
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Now accepted; but at one time it was not because no power known could force continents to
plow through solid rock.
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1st proposed by Pelegrini in 1858. He used fossil evidence that he concluded was remnants of
the Great Flood.
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Wegner used more evidence:
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Fossils from various plants and dinosaurs
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Rocks; Appalachian Mountains seem to appear in Ireland.
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Glacier deposits and grooves: Glaciers appear to come out of sea and onto land (w/o
continental drift theory) Which is impossible!
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Paleomagnetism
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Earth is a dipole magnet
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time
Magnetic North is 11o off true north; it has been shown that magnetic north has moved through
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This magnetic field is believed to be produced by movement in liquid core.
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TRM: Thermal Remnant Magnetism
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In Igneous rocks, iron atoms line up North to South while in the molten state; then frozen in
position when the rock cools. Igneous rocks have strong TRM, and these rocks can be dated
radiometrically.
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Careful examination of “old” rocks shows that the poles appear to have migrated. Looking at
rocks from different continents, the pole’s migration path is different.
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This apparent enigma can be explained by continent drift.
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Magnetic Pole Reversals
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Using TRM detection techniques it was discovered that the magnetic poles had reversed.
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Rocks showing magnetic pole reversals were 1st discovered on land, and then later on the ocean
floor. The pole reversal patterns formed symmetrical “zebra” stripes on both sides of the spreading
zones. This was considered to be valid evidence that the ocean floor was spreading.
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EARTH = Giant Recycling Machine!
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Divergent Plate Boundaries:
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Plates created!
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Convergent Plate Boundaries:
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Plates destroyed!
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*Most earthquakes occur at plate boundaries.
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Areas of Subduction
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Denser ocean crust is forced under lighter continental crust.
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Some of this material is remelted and erupts as volcanoes. Ex: Cascade Mountains
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Deep sea trenches occur at areas of subduction
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Hot Spots
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Large “upwellings” of hot mantle material under plates can cause volcanic eruptions on the
moving plates
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As plates move over hot spot, more volcanoes are formed
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Ex: Hawaii
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Ocean Floors
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Almost exclusively made by volcanism at spreading zones.
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Large areas covered by sediments brought from rivers.
* For many years, inaccessible, unknown!
There were many myths, mainly from stories from fishermen and sailors
- Now ocean floor has been completely mapped by bathymetry which is mapping of the ocean floor
topography by using sound reflections; to produce seismic reflection profiles.
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The Atlantic Floor consists of 3 Divisions:
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Continental Margin (shelf)
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Deep Ocean Basin
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Mid Ocean Ridge
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Continental Margin
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Continental Shelf- submerged portion of continental platform
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Continental Slope- steep edge of continent; many submarine canyons cut by turbidity currents
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Continental Rise- raised edge of ocean floor
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Deep Ocean Basins
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Abyssal Plains- Flat surface built by deposits from turbidity currents
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Abyssal hills- < 900 m (2700 ft) in height
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Unburied portions of the ocean floor, also small seamonts buried by sediment
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Covers 80% of the Pacific Ocean; most widespread land form on earth
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Seamounts > 900 m; cone shaped,
volcanic in origin
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Guyots- flat topped seamont
“tablemounts”
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Ocean Ridges
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Spreading Zone- “Rift”; these are active volcanoes; many earthquakes. New ocean crust is
continually produced here.
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Crest- Rift valley
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Flank-sides
-Forms a continual system of ridges 46, 000 miles long that winds around the earth “like seams in a
baseball” This range is 600-10,000 feet high.