Around 1912, a German scientist named Alfred Wegener
theorized that all of the Earth's continents were once joined
together in a single, large landmass. He further proposed that
the continents have separated and collided as they have moved
around over the last few million years. He called this theory
continental drift. He provided several pieces of
evidence to support his theory:
Continent Shapes- The continents appear to be shaped in
such a way that they would fit together nicely, like a jigsaw
2) Rock Formations- There are rock formations on
different continents that match up beautifully when the
continents are put back together.
3) Fossils- There are fossils found on different
continents that would also match up nicely if the continents
were all once together.
People of the time mostly thought Wegener was crazy!
In the 1950's, scientists discovered some
surprising evidence in support of Wegener's theory.
While mapping the ocean floor, scientists discovered two
important, and unexpected things:
First, the age of the rocks that make up the ocean
floor gets older as you move away from the ridges at the
center. This meant that the youngest rocks were found
near the ridges, and the oldest rocks near the
Below is a graph of the rock
ages for the map on top.
Second, there are stripes of alternating magnetic
polarity on each side of the ridge. When the molten
rock hardens, the magnetic minerals in the rock align
themselves with the Earth's magnetic field. Scientists
discovered that the Earth's magnetic field has reversed
itself many times, at intervals of roughly every 100,000
years. The pattern they observed makes sense if the
ocean floor is being formed at the ridge and gradually
pushed outward in both directions.
These discoveries gave rise to the now respectable
science of Plate Tectonics. This is the theory
that the Earth's seemingly solid crust is actually made
up of several pieces, or plates, that move around
The places where the different plates meet, called
plate boundaries, are where the tectonic action really is.
There are three basic types: convergent, divergent, and
Convergent Boundaries: This a when two plates
are moving toward each other, as shown above.
If the two plates are of relatively low, and similar
densities, the plates will form a Collision Boundary.
In this scenario, the crust is forced upward by the
collision, resulting in mountain building. The
diagram above shows how this type of collision
between India and China forced the formation of the
If one of the plates is more dense than the other, as
happens when oceanic and continental crust meet, then the
more dense plate will be forced under the less
dense plate. This forms a trench, or deep valley,
where the plates meet. This is called
subduction, and is shown in the diagram above. This often results in a chain of volcanoes
running parallel to the trench.
Divergent Boundaries: As you might expect, this
is essentially the opposite of a convergent boundary.
This occurs when two plates are moving away from
one another, as shown above. This is seen at mid-ocean ridges and
Transform Boundaries: This type of boundary
forms when two plates are sliding past one
another. The diagram above illustrates this
motion. The most popular example of this is the San
Andreas Fault in California.
All of the different boundaries and their locations
are found on
5 of the
Earth Science Reference Tables, shown
below. Notice the key that shows the different
boundaries and their symbols.
movement of the plates is driven by convection
currents in the mantle. These currents cause the
solid plates to float along on top of the semi-molten
Sometimes, there is an opening in the middle of a
plate that allows the molten material to flow through
it. This is called a hot spot, and usually
results in a chain of volcanic islands that form as the
plate moves over the hot spot. The Hawaiian Islands
are a great example of this.