In past episodes, we’ve talked about convergent and divergent plate boundaries.
Seafloor spreading creates new oceanic lithosphere at mid-ocean ridges
that occur at true divergent plate boundaries,
and this creation is balanced by destruction of old oceanic lithosphere at convergent
plate boundaries. These two types of plate boundaries are marked with ridges and trenches
and explain the birth and death of the plates.
However, if we project these plate boundaries on the map,
we can see that there are gaps in the outline of the plates.
There must be another kind of plate boundary. The third type of plate boundary is called a
‘transform plate boundary’ and is marked by transform faults.
Transform plate boundaries do not create or destroy lithosphere. Instead,
transform faults allow lithospheric plates to slide past each other.
Transform faults can connect with either divergent or convergent plate boundaries.
There are many transform plate boundaries all around the world, both on land and in the oceans,
just as there are for other types of plate boundaries. Transform plate boundaries can
therefore be subdivided into two groups: oceanic and continental transform plate boundaries.
Let’s talk about continental transform boundaries first. The San Andreas fault zone in California
is the only transform plate boundary in the United States and is one of the most famous in the world.
This transform fault can be traced for 750 miles or 1,200 km from near the border with Mexico
to north of San Francisco, where it continues on the seafloor. The San Andreas Fault connects
two divergent plate boundaries: the East Pacific Rise in the south with the Juan de Fuca Ridge in
the north. The San Andreas Fault separates two of Earth’s biggest plates: the Pacific plate in the
west and the North American plate in the east. The fault is often easy to see because it makes long,
straight depressions and uplifts. Depressions along the fault are often filled with water to
form long, narrow lakes or bays. You might have heard an idea that California west of the fault
might fall off into the ocean. Because of how transform faults work, this would never happen.
However at the slow rate of about 5 cm each year, the western side is moving north as the
two plates slide past each other. Over millions of years, rocks that formed in Southern California
have moved to Northern California. Because of their side by side style of motion,
transform plate boundaries are different than the other plate boundaries in that they don’t
produce volcanoes. However, like the other types of plate boundaries, they do produce earthquakes.
Earthquakes happen when there is motion along a fault. Small amounts of motion cause small
earthquakes and large amounts of motion cause big earthquakes. About 10,000 earthquakes happen along
the San Andreas fault every year but most are so small they are not even felt by people nearby.
Large earthquakes happen less often but when they do, they can cause a lot of problems.
Because cities like San Francisco and Los Angeles lie near the San Andreas fault, big earthquakes
along it can cause a lot of damage. Such a big earthquake happened in 1906 near San Francisco,
killing 3,000 people and leaving half of the city’s 400,000 people homeless. A much more deadly
earthquake struck the Caribbean Island of Haiti in 2010. Haiti lies near a transform plate boundary
between the North American plate to the north and the Caribbean plate to the south.
Buildings were not designed to survive such an earthquake
and many collapsed. Over 100,000 people died as a result of this terrible earthquake.
People that live near transform faults need to be prepared for big earthquakes!
Oceanic transform faults are also important. The most spectacular examples of oceanic transform
faults are found in the Mid-Atlantic Ridge between Africa and South America.
These connect the spreading ridge and divergent plate boundary in the northern Atlantic Ocean with
the spreading ridge and divergent plate boundary in the South Atlantic. The northern and southern
parts of the Atlantic Ocean opened separately and these oceanic transform faults connect them.
The offset of the divergent plate boundaries – farther west in the north, farther east in
the south – was accommodated by these oceanic transform faults. This difference in position
of the spreading ridges in the Atlantic Ocean is also what causes the shoreline in the southern
part of West Africa to trend East-West, parallel the oceanic transform faults of the region.
There are many other examples of transform plate boundaries around the world. Take a look at a map
and see if you can find the one in New Zealand and between Israel and Jordan. They are not hard
to find! Just follow the plate boundaries! Now, we have covered all three types of plate
boundaries: convergent, divergent, and transform. In the next episode,
we will talk about how scientists came up with the theory of Plate Tectonics. See you next time!