GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
POWERPOINT SLIDE SHOW NOTES
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Topic 4: Plate Tectonics—Plate Theory
Plate tectonics is an important topic and tells us why the oceans look the way they do. It
is broken into two topics.
A few of the important thinkers and events leading to the development of the plate
tectonic theory.
The jigsaw puzzle question illustrated.
Earth’s interior is layered. We saw a little of that already.
Two ways to classify the layers. By composition and by properties. By composition there
are only three layers, not four as was shown in Topic 2. Inner and outer cores have the
same composition. By physical property, there are more layers. The core has a solid
inner core and a liquid outer core. Though the composition is the same, the inner core is
under so much pressure that it cannot expand and melt. The mantle is broken into three
layers, the mesosphere (rigid), the asthenosphere (plastic), and a thin sliver at the top
which is rigid. The thin, rigid, sliver of upper mantle, together with the crust (oceanic,
continental, or both), make up the lithosphere. The lithosphere is rigid and brittle. It is
the layer broken into the “plates” of plate tectonics. Breaking or cracking the lithosphere
releases energy as earthquakes.
Inner core
Outer core
Mantle
Crust
Lithosphere
Asthenosphere
Details of lithosphere and asthenosphere
How do we know what Earth’s internal structure is?
Earthquakes occurring all over the world and recorded by seismic stations all over the
world give us information just like a sonogram in a doctor’s office. Nuclear bomb tests
have also provided the energy source just like earthquakes.
Density is one of the most important characteristics of matter in oceanography. We’ll
come back to it several times. It controls layering in the Earth’s interior, in the oceans,
and in the atmosphere. And drives much atmospheric and oceanic circulation.
Major Earth zones by density
Continental drift, precursor to plate tectonics. Not really well accepted for decades
because a driving mechanism could not be found. What force could move continents?
The jigsaw puzzle again. South America and Africa fit well, but not perfectly.
Not quite as good a fit when North America and Europe are added.
Wegener realized that evidence of glaciers on the southern continents meant glaciers
would be contiguous if continents had once been together.
Glacial striations—evidence of glacial activity
Diamictite deposits indicate glacial activity
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Diamictite
Fauna refers to all the plants and animals in an area. The Glossopteris fauna is an
assemblage of specific plant and animal fossils. Fossil distribution indicates continents
were assembled at one time.
Pangaea is the name Wegener gave to the supercontinent that existed 250 million years
ago (myBP means million years before present). Also spelled Pangaea. The word means
“all earth.” Likewise, if the land is all assembled into a single continent, that must leave
Earth with a single ocean—Panthalassa, or “all sea.” As Pangaea broke apart and
continents moved, Panthalassa has been divided into the oceans we have today.
Continental arrangement today versus 200 million years ago.
Sea floor spreading. The force that drives continental movement and ocean changes.
Much of the evidence was obtained thanks to oceanic observations during World War II,
and by major research spending after the war.
Sea floor spreading. Rising up of molten rock to form new sea floor at mid-ocean ridges,
and sinking and recycling of old sea floor at subduction zones (trenches).
Convection is driven by differences in density.
Sea floor spreading explains many observed phenomena.
Note that seismic events (earthquakes) are mainly restricted to narrow belts.
Same with volcanoes
Age of sea floor increases with distance away from mid ocean ridges (spreading centers).
Sea floor spreading moves ocean floor (oceanic crust) away from where it formed. There
is also a magnetic signal recorded in the rocks. All rocks of the same age have the same
magnetic polarity.
Iceland is on the mid-ocean ridge
Paleomagnetism. Earth’s magnetic field reverses relatively often compared to the rate
of creation of new ocean crust. The reversals are recorded in the rocks.
As molten rock cools, Earth’s magnetic field direction is recorded in magnetic minerals in
the rock.
We also see this record on land in stacked lava flows.
As sea floor spreading progresses, magnetic reversals are recorded as “magnetic stripes”
in the oceanic crust.
The very uniform and predictable oceanic crust age and magnetic polarity relationship
made it obvious that the sea floor is spreading away from mid-ocean ridges, the
spreading centers.
Paleomagnetic patterns. The age/polarity relationship and magnetic stripes.
Magnetic striping in the Pacific Ocean off northwestern USA and southwestern Canada.
The Juan de Fuca and Gorda Ridges are spreading centers. Note that the stripes east and
west of the ridges are mirror images.