The Modern Theory of Plate Tectonics

Remember that it took Alfred Wegener’s Theory of Continental Drift (1915) and

Harry Hess’s Theory of Seafloor Spreading (1960) to finally add up to the Theory of

Plate Tectonics.

And as Plate Tectonics swept through the scientific world, scientists from all branches of science found that it explained so many things so well.

Mountains

Volcanoes

Earthquakes

The Ring of Fire

Fossils

Other deposits

Mid-ocean ridges

Deep Sea Trenches

The main points:

That the Earths’outermost layer (the lithosphere) is broken into pieces, called plates. (seven major ones, several small ones)

For some reason(s), these plates are moving.

There are two types of crust “riding” on the plates – continental crust and oceanic crust.

New crust can form (at spreading plate boundaries) and older crust can be destroyed at subduction zones, where plates are coming together.

The two types of crust.

Continental CrustMakes up most of the continents (actually a little bigger than the continents.

The oldest crust

Made of all three rock types (igneous, sedimentary, metamorphic)

Granite is the most common rock in the continental crust

Much thicker

Less dense

Oceanic CrustMakes up most of the seafloor

Much younger

Made of mostly basalt (an igneous rock)

Much thinner

More dense, so it “rides” lower.

The lithosphere moves over a gooey layer of the upper mantle known as the

asthenosphere. Notice the thickness of the two types of crusts.

But the most interesting activity is at the boundaries of the plates,

where they meet each other!

There are THREE types of plate boundaries:

Convergent plate boundaries

Divergent plate boundaries

Transform plate boundaries

Divergent Plate Boundaries

Wherever plates are separating.

Mostly at mid-ocean ridges.

Formation of new crust at these boundaries.

Divergent plate boundaries explain how “new” oceans formed as

Pangaea split up.

One place where we can study a divergent plate boundary on land is

the African Rift Valley.

Someday, it might separate enough, so that a new ocean will form, and Africa will be broken into two

pieces!This is how the Atlantic Ocean once formed too.

Another place where we can study divergence on land is Iceland, where the

mid-Atlantic Ridge is above sea level.

Iceland is a fascinating place, with a great deal of geothermal, volcanic,

and seismic activity.

Convergent Plate Boundaries

Perhaps the most interesting boundary!

Where plates are colliding into each other (converging).

What happens depends on the two types of crust colliding.

This is where you might find subduction zones, volcanoes, or

deep sea trenches.

Oceanic-oceanic convergent plate boundaries explains the many chains of volcanic islands in the world’s oceans.

Oceanic-continental convergent boundaries explains chains of volcanoes along the edges of continents, such as along the west coast of North America

And continental-continental convergent plate boundaries explains the formation of

the mighty Himalaya Mountains.

Transform Plate Boundaries

Where two plates are sliding next to each other.

Usually come with a good deal of seismic activity!

The best example of a transform boundary is the

San Andreas Fault Zone in California.

But what is still difficult to explain

is what force drives the plates to move?

One theory is that it is the heat within the mantle.

Obviously there is a lot of heat under the crust!

Molten rock is called magma.

Hot liquids (even gooey magma) can form convection currents.

Convection currents might be what causes the plates to move.

Convection currents will also form in a pot of boiling water, or even in the

air.

Could convection be what drives the plates to move?

Plate tectonics certainly explains why earthquakes and volcanoes

happen where they do.

And as the plates continue to slowly

move, what will the Earth look like

in the future?

Stick around long enough and we will all find out!

(Click on image for video of plate tectonics – 1:20.)

This is a short video (10:24) – click on image - showing how Plate

Tectonics might have begun and how it continues to influence our

planet and our lives.