Transform plate boundaries represent some of the most dangerous geological features in North America but they can be so deceptively modest in their physical appearance that we build major US cities on top of them. In this video we will introduce you to transform plate boundaries and discuss the geological features and processes that characterize them. We’ll start by reviewing some major examples of these features.
Plates slide past each other along transform boundaries, here illustrated by the green lines. While they are rarer than their divergent and convergent cousins, they can be recognized in several locations, typically forming links between longer segments of convergent or divergent boundaries. Let’s take a look closer look at some of these examples.
The most famous example of a transform plate boundary runs through California and separates the North American plate from the Pacific Plate. We will return to take a closer look at this boundary in the second half of the video. Some transform boundaries occur almost exclusively in the oceans and are largely hidden from view.
For example, the small Scotia plate is separated from the Antarctic and South American plates by transform boundaries along its north and south margins. The Caribbean plate is about the same size as the Scotia plate and its northern edge is a transform boundary separating it from the North American plate. Movement along a fault parallel to this boundary resulted in a magnitude-7 earthquake that struck the nation of Haiti in 2010.
An estimated 3 million people were affected by the earthquake which resulted in widespread destruction and many deaths in the capital of Port au Prince. Some transform boundaries are much smaller than these examples. Examination of the ridge system in any ocean basin reveals that it is often offset by short transform boundary segments.
Plates move in opposite directions along these segments, making them a source of shallow earthquakes in the ocean basins. These offsets are essential for allowing the rigid tectonic plates to move over a spherical Earth. By way of analogy, imagine trying to wrap a ball with a rectangular sheet of paper.
We have to make many small cuts in the paper to fit it to the ball’s rounded surface. Earth’s plates are rigid slabs of rock that have to make similar adjustments to slide at different rates and in different directions over the planet’s surface. These small transform segments represent some of those adjustments.
In previous videos we have described the features of convergent and divergent plate boundaries. The mountains, volcanoes, and rift valleys of these boundary types produce dramatic landforms. But what about transform plate boundaries?
What features can we expect to see where these boundaries cut across a continent? Let’s take a look at the transform plate boundary as represented by the San Andreas fault system in California. The San Andreas fault system links to oceanic ridge systems from the coastal waters of Oregon in the North to the Gulf of California in the South.
The North American continent is actually on two plates. Most of it is, as expected, on the North American Plate, but there is a sliver of the continent that represents the eastern margin of the Pacific Plate. If we look more closely we can see that two major cities lie adjacent to the transform boundary on opposing plates.
Los Angeles can be found west of the boundary on the Pacific plate, while San Francisco is located just east of the fault on the North American plate. The relative plate motions will slowly move the site of Los Angeles northward along the boundary so that it will eventually be parallel with San Francisco. Given enough time, it will continue its journey northward and may eventually collide with the convergent boundary south of Alaska.
In the meantime, earthquakes of various sizes occur along the San Andreas fault system, posing a potential danger for any cities nearby. The transform boundary is so unassuming that communities are built right on top of it. Both Los Angeles and San Francisco sprawl outward over the location of boundary.
Segments of the fault have ruptured to produce three major earthquakes in the last 400 years. The most recent event occurred in 1906 and resulted in a fire that destroyed large sections of San Francisco. The city burned for three days, destroying nearly 500 city blocks, leaving 250,000 homeless and killing more than 500 people.
The same event today would be disastrous, with the potential to kill thousands of people and costing many billions of dollars in damages. Let’s take a closer look at what the boundary in the desert of southern California. The white rectangle represents the field of view for the next image.
Zooming in from above on a segment of the fault in the desert we can make out a linear trace that runs northwest-southeast through the image. Examining the feature on the ground may reveal a linear fault scarp produced by erosion along the line of the boundary or may just show a relatively nondescript surface with little to indicate that a major plate boundary is present. Unlike the depictions in some movies, movement along a transform plate boundary doesn’t generate a gap or chasm at Earth’s surface.
While earthquakes may occur along these boundaries, there are no volcanoes, rift valleys or mountain ranges characteristic of other boundary types. Ok, let’s see if you can identify the expression of the Alpine Fault which represents a transform plate boundary in New Zealand. Can you identify where the fault is located?
How about now? We can see it here on the western side of the Southern Alps on the South Island of New Zealand. We had two learning objectives for this lesson.
How well do you think you can respond to the learning objectives?
