The Modern Theory of Plate Tectonics
The Modern Theory of Plate Tectonics
The Modern Theory of
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.
The Ring of Fire
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
There are two types of crust
“riding” on the plates –
continental crust and oceanic
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.
Makes up most of the continents
(actually a little bigger than the
Makes up most of the seafloor
The oldest crust
Made of mostly basalt (an igneous rock)
Made of all three rock types (igneous,
Granite is the most common rock in the
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
There are THREE types of plate boundaries:
Convergent plate boundaries
Divergent plate boundaries
Transform plate boundaries
Divergent Plate Boundaries
Wherever plates are
Mostly at mid-ocean ridges.
Formation of new crust at
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
Where plates are colliding
into each other
What happens depends on
the two types of crust
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
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
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
Obviously there is a lot of
heat under the crust!
Molten rock is called
Hot liquids (even gooey
magma) can form
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
Plate tectonics certainly explains why
earthquakes and volcanoes happen where they
And as the plates
continue to slowly
move, what will the
Earth look like in the
Stick around long enough and we will all
(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