GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 5: Plate Tectonics—Plate Dynamics
POWERPOINT SLIDE SHOW NOTES
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Topic 5: Plate Tectonics—Plate Dynamics
Theory of plate tectonics
The lithosphere is broken up into several major and minor plates. Remember the
lithosphere is rigid and can crack. The lithosphere is often described as being like the
shell of a hard-boiled egg.
Major and minor tectonic plates
Convection cells drive plate tectonics. It’s the same process that occurs in a pot of
boiling water, only at a much slower scale.
An overview of plate tectonics. Note: convection cells, molten rock moving to the
surface and creating new ocean crust, old ocean crust subducting at oceanic trenches,
volcanic island arcs where subduction occurs.
Seafloor spreading moves ocean crust away from zone of creation at mid-ocean ridges.
Seamounts are undersea volcanos. A seamount grown large enough breaks the ocean
surface as a volcanic island. Islands eroded flat at sea level and moved to deep water by
sea-floor spreading are called guyots (gee-yohs) or table mounts. Note age of ocean
floor.
Plate boundaries are where the action is. There are three types of boundaries.
Features of a divergent plate boundary that formed within a continent (some for in the
ocean) when magma rises to the surface: upwarping of continental crust, stretching and
thinning the continental crust with down-dropped blocks forming a rift valley (East Africa
is an example), continued divergence allows sea water to flood the thinned continental
crust to form a linear sea (Red Sea), and finally a mature ocean with a well-developed
min-ocean ridge.
Divergent boundaries
Artist’s conception of the North Atlantic Ocean sea floor. This is called a physiographic
map. Note the well-developed rift valley along the center of the mid-ocean ridge. The
rift valley is offset by numerous transform faults. This portion of the mid-ocean ridge is
called the Mid-Atlantic Ridge.
Small transform fault boundaries offset the mid-ocean ridge rift valley. Transform faults
occur when rocks slide horizontally against each other instead of up or down.
The mid-ocean ridge circles most of the globe. It has different names in different parts of
the ocean.
Detail of a mid-ocean ridge.
This is called “pillow basalt” or “pillow lava.” It forms when lava erupts under water and
the molten rock in contact with water freezes immediately to form a rind, kind of like the
rind of a watermelon. These are called pillows. When first formed, there is still molten
rock inside a pillow which can exert pressure on the rind and push open cracks to create
another pillow. This is a common characteristic of submarine volcanism.
Ancient pillow basalt in a rock outcrop along a road in Puerto Rico. The pillow basalt in
the previous slide is brand new. This pillow basalt may be as old as 100 million years.
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Another photo of the pillow basalt from Puerto Rico. Note the “rind” surrounding some
of the pillows has weathered away.
Young ocean crust is warm and has a low density so it is up at a higher level than the
surrounding sea floor (look back at slide 9, figure d). Conversely, as the ocean crust
moves (spreads) away from the ridge, it gets farther from the mantle heat source, cools,
becomes denser, and sits at a greater depth than younger crust.
The process of atoll formation was theorized by Charles Darwin. With the post-World
War II study of the oceans, we learned he was correct. Coral reefs form around volcanic
islands if they are in warm seas which corals love, and form a fringing coral reef. If the
volcanic island moves away from the lava source, it will be eroded and it will sink, but the
corals can grow fast and keep pace with the sinking to create a barrier reef. Finally, the
remnants of the original volcanic island sink below sea level leaving only a circular
growing coral reef with shallow water in the center known as an atoll.
Hydrothermal vents
Hydrothermal vents
A vent chimney consists of layers of minerals precipitated from sea water.
The “smoke” is hot sea water colored by dissolved minerals.
Project FAMOUS was the first to study mid-ocean ridge rift valleys.
Continental rifting to form a new ocean.
The East Africa Rift Valley is an example of continental rifting, and the Red Sea illustrates
the continued rifting forming a narrow linear sea.
Convergent plate boundaries. Three types.
A lithospheric plate contains either continental or oceanic crust, and sometimes both.
But usually plates converge in one of three relationships: ocean crust collides with
continental crust, ocean with ocean, or continental with continental. Each type of
collision results in different geography and geology.
Ocean-ocean convergence.
Ocean-ocean convergence. Note the trench, the island arc, melting of subducting plate.
Note also the relative movements of the plates and the depth scale.
The Aleutian Islands off of Alaska are a classic example of a modern ocean-ocean
collision. The subduction of the Pacific plate results in the Aleutian Trench.
Ocean-continent convergence.
Convergence off the west coast of South America is a classic example of a modern oceancontinent collision. Note the subducting lithosphere, the trench, the Andes mountains,
and the melting of the subducting plate. The volcanoes associated with convergent
plates are comprised of lava that is a mixture of granite and basalt called andesite.
Andesite lava has physical characteristics that make andesitic volcanoes erupt violently.
Most of the world’s great volcanic disasters have been eruptions of andesitic volcanoes.
Ocean-continent convergence
The classic type of ocean-continent convergence is referred to as “Andean-Type”
subduction, named for the Andes Mountains.
The Juan de Fuca Ridge is part of the mid-ocean ridge and a spreading center. The Juan
de Fuca and Gorda plates are small, but as they subduct beneath North America, melting
of the rocks form the Cascade Mountain range of the northwestern U. S. and
southwestern Canada.
The Cascade volcanoes
Mount St. Helens is part of the Cascade range and is an andesitic volcano.
Ocean trenches are formed by subduction of the lithosphere at convergent plate
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boundaries. Notice that most of the world’s subduction is in the Pacific Ocean. The
Pacific is the remnant of Panthalassa, and it has been growing smaller as the Atlantic has
grown by sea-floor spreading.
The deepest spot on Earth is in the Mariana Trench in the western Pacific.
The Challenger Deep is the deepest spot on Earth. A “deep” is a geographical term for
the deepest spot in an ocean basin. The Challenger Deep is deeper than Mount Everest
is high.
Comparisons of Earth’s deepest spot, average depth of the ocean, average height of
land, and highest elevation.
Continent-continent convergence
Forming the Himalayas. The Appalachian Mountains looked like this 300 million years
ago.
India colliding with Asia formed the Himalayas. Collision is ongoing.
Himalayan foothills
Continental collisions
Transform fault boundaries. We’ve already seen small transform faults along the midocean ridges (see slides 11 and 12). The San Andreas Fault is perhaps the most famous.
The San Andreas Fault is only one of several transform faults stretching along western
North America. The several faults are offsetting the East Pacific Rise, part of the midocean ridge system.
Aerial view of the San Andreas Fault.
Overview of plate tectonic processes.
The Pacific Ocean has so much tectonic activity (earthquakes and volcanoes) because of
the dominance of plate convergence, it is referred to as the “Ring of Fire.”
Measuring plate motion by satellites. Plates move at about the same speed as our
fingernails grow.
Continental margins from a plate tectonic perspective.
Passive and active continental margins.
The geography of a passive continental margin, of an ocean-ocean active margin
(Marianas type), and an ocean-continent active margin (Andean type).
Hot spots or intraplate (not at margins) volcanism. Hot spots are in the mantle and
supply molten rock up into the lithosphere. Hot spots are fixed in location.
There are many hot spots around the world. Perhaps two of the most famous give rise
to the Galapagos Islands and to Yellowstone.
The Midway Islands and the Hawaiian Islands formed as the Pacific Plate moves over a
hot spot.
Formation of a volcanic island chain by hot spot, or intraplate, volcanism.
Notice the progression of ages of the Hawaiian Island chain. A new island is already
forming as the big island of Hawaii moves off the hot spot. The Lōʻihi Seamount may
form a new island in another 100,000 years or so.
This single hot spot has been active for over 70 million years and has given rise to the
Emperor Seamounts as well as the Hawaiian Islands.
Hawaiian hot spot
Another view of major hot spots
Summary slide of plate tectonic activity. This is taken This Dynamic Earth: The Story of
Plate Tectonics produced by the United States Geological Survey (USGS) and available on
line at http://pubs.usgs.gov/gip/dynamic/.
Pangaea was not the first time continents had been assembled.
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Plate collisions (and thus new mountain belts) are around the margins. The interior of
the continents get farther and farther from the margins, unless they rift apart.
Note how the older, stable portions (cratons) of reassembled South America and Africa
line up.
As continents collide, break apart, and collide again, they grow by a process of
continental accretion. Terranes can be added as India is now being added to Asia.
Western North America is comprised of several accreted terranes.
The big picture of the history of continental accretion in North America. An “orogeny” is
a mountain belt. Each of these orogens was at the margin of the continent when they
were active.
Plate tectonics helps us explain the global distribution of earthquakes and volcanoes.
Unfortunately, many people live in active tectonic zones.
Earthquakes don’t occur only in the western US. West coast earthquakes are
understandable. That’s an active margin. The ring of fire and all that. But some of the
strongest earthquakes ever to hit the U. S. were in Missouri and South Carolina, far from
a plate boundary. Why? These are actually along old faults from the breakup of Pangaea
and possibly even before. As the forces of plate tectonics continue to put stress upon
the continent, old faults are zones of weakness which can crack the lithosphere
creating earthquakes.