Plate Tectonics

All images fromhttp://pubs.usgs.gov/publications/text/understanding.html#anchor5567033

unless otherwise noted.

Close examination of a globe often results in the observation that most of the continents seem to fit together like a puzzle: the west African coastline seems to snuggle nicely into the east coast of South America and the Caribbean sea; and a similar fit appears across the Pacific. The fit is even more striking when the submerged continental shelves are compared rather than the coastlines.

In 1912 Alfred Wegener (1880-1930) noticed the same thing and proposed that the continents were once compressed into a single protocontinent which he called Pangaea (meaning "all lands"), and over time they have drifted apart into their current distribution. He believed that Pangaea was intact until about 300 million years ago, when it began to break up and drift apart.

Wegener had four main pieces of evidence.

First he noted the jigsaw fit of South America and Africa, especially, but also elsewhere.

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

He found fossils that were the same on both continents. After a certain period, whoever, the fossils begin to evolve differently on the different continents.

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

He found that on both sides of the Atlantic, mountains were the same both in terms of age and structure.

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

He found that ice sheets covered parts of Africa, India, Australia and South America 250 million years ago. How could this happen in places that are so warm today?

Making Connections: Canada’s Geography. Clark & Wallace. Prentice Hall Ginn, 1999.

Wegener's hypothesis of continental drift lacked a geological mechanism to explain how the continents could drift across the earth's surface.

It wasn’t until the the 1960s that the theory of plate tectonics was advanced to explain how the continents could separate..

http://www.pbs.org/wnet/savageearth/animations/hellscrust/index.html

The thin crust (20 km under the ocean) and the very top of the mantle are called the lithosphere. Beneath this is the asthenosphere.

•The Earth's crust is broken into a series of plates or pieces.•The ocean floors are continually, moving, spreading from the center, sinking at the edges, and being regenerated. •Convection currents beneath the plates move the crustal plates in different directions. •The source of heat driving the convection currents is radioactivity deep in the Earth's mantle.

The main features of plate tectonics are:

So, a few key concepts so far:

•The earth radiates energy from the radioactive decay of elements in the core.

•The energy released keeps the earthg’s surface in constant motion and change, albeit very slowly most of the time.

•Some geological change is abrupt: volcanoes and earthquakes for example.

“Advances in sonic depth recording during World War II (SONAR) and the subsequent development of the nuclear resonance type magnometer led to detailed mapping of the ocean floor. Among the seafloor features that supported the sea-floor spreading hypothesis were: mid-oceanic ridges, deep sea trenches, island arcs, geomagnetic patterns, and fault patterns.”

http://www.ucmp.berkeley.edu/geology/tecmech.html

How do we know this?

The Surface of the Earthhttp://www.ngdc.noaa.gov/mgg/image/mggd.gif

“The mid-oceanic ridges rise 3000 meters from the ocean floor and are more than 2000 kilometers wide surpassing the Himalayas in size. The mapping of the seafloor also revealed that these huge underwater mountain ranges have a deep trench which bisects the length of the ridges and in places is more than 2000 meters deep.”

http://www.ucmp.berkeley.edu/geology/tecmech.html

http://www.ucmp.berkeley.edu/geology/tectonics.html

“Research into the heat flow from the ocean floor during the early 1960s revealed that the greatest heat flow was centered at the crests of these mid-oceanic ridges. Seismic studies show that the mid-oceanic ridges experience an elevated number of earthquakes. All these observations indicate intense geological activity at the mid-oceanic ridges.”

http://www.ucmp.berkeley.edu/geology/tecmech.html

Oceanic trenches, which are as deep as 35,000 feet below the ocean surface, are long and narrow, and run parallel to and near the oceans margins. They are associated with and parallel to large continental mountain ranges. There is also an parallel association of trenches and island arcs. Like the mid-oceanic ridges, the trenches are geologically active, but unlike the ridges they have low levels of heat flow.

http://www.ucmp.berkeley.edu/geology/tecmech.html

Scientists also learned that the youngest regions of the ocean floor were along the mid-oceanic ridges, and that the age of the ocean floor increased as the distance from the ridges increased.It has also been found that the oldest seafloor often ends in the deep-sea trenches.

http://www.ucmp.berkeley.edu/geology/tecmech.html

So what’s happening?

•the outer surface of the Earth is a thin crust of fragile rock, fractured like the cracked shell of an egg•the pieces of the shell are Earth's tectonic plates -- there are 12 major ones -- and they float along on vast convection currents in the asthenosphere•the asthenosphere churns like a fluid

There are actually two types of crust:

•oceanic crust extends all over the earth and is broken into the 12 large and many smaller plates; and,

•continental crust which rides around on top of the oceanic crustal plates

•the currents in the asthenosphere are generated by heat rising to the earth’s surface from the hot radioactive core•at their boundaries, the plates spread apart, converge, and slide past one another•this makes these areas the most geologically active: earthquakes and volcanoes

•in a single year, earthquakes alone release 1026 ergs of energy, or the energy of 100,000 Hiroshima-sized nuclear bombs•that is just one percent of the total amount of energy that reaches the surface from Earth's innardshttp://www.pbs.org/wnet/savageearth/hellscrust/

index.html

Convection currents rise up from the radioactive core, carrying heat to the thin crust of the earth.

Where the plates move apart, new magma wells up to the surface, forming new oceanic crust.

The main types of plate boundaries.

The Mid-Atlantic Ridge: a divergent zone.

Iceland: On the Mid-Atlantic Ridge

Mid Ocean Convergence Zone

Convergence Zone: Oceanic Crust and Continental Crust

Convergence Zone: Continental Crust to Continental Crust

Indian Plate collides with Eurasian Plate

The result: the Himalayas and Mt. Everest

The Pacific Ring of Fire

Transform plate margins: where two plates slip past one another.

The San Andreas Fault, California

http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/figures.htm

Tectonic setting of western British Columbia and Washington state. The oceanic Juan de Fuca plate is moving beneath the continental North America plate at a rate of about 4 cm/year. Great earthquakes occur along part of the boundary between the two plates.

http://sts.gsc.nrcan.gc.ca/page1/geoh/quake/fig2.htm

Diagram illustrating deformation associated with a subduction thrust fault. Top: elastic deformation builds up between earthquakes if the thrust fault is locked; the edge of the overriding plate is dragged down and a bulge forms farther landward. Bottom: during a large earthquake, the leading edge of the plate is uplifted and the bulge collapses.

http://www.pbs.org/wnet/savageearth/hellscrust/index.html

This map, which shows 20th-century earthquakes in red, illustrates how they cluster on the edges of the major tectonic plates (outlined in yellow).

http://www.pbs.org/wnet/savageearth/animations/hellscrust/index.html