Transform Boundaries

Lesson Plan - Get It!

Audio:

The third and final tectonic plate movement is transform boundaries.

In a transform boundary, the earth's plates do not move toward or away from one another. Rather, the plates slide past each other in opposite directions.

These boundaries typically do not create large geological structures, such as mountains and rifts, although they can create small stream beds or valleys.

The greatest threat posed by transform boundaries are earthquakes.

Rub your hands together quickly again.

• Do your hands move exactly in a straight line?

The palms of our hands are not perfectly flat, so there should be a slight shift in how your hands move.

Tectonic plates are similar. They are not perfectly smooth along the edges. Therefore, as the plates slide past one another, they can shift, or sometimes pieces can break off. When these plates shift or break, that can cause an earthquake.

Transform boundaries are places where two tectonic plates slide past each other. They do not crash together, and they do not pull apart. Instead, they scrape along side by side.

This movement is not always smooth. The edges of tectonic plates are rough, like two broken pieces of cracker rubbing together. Sometimes the plates slide slowly. Other times, the rough edges get stuck.

When the plates get stuck, pressure builds. The plates keep trying to move, but the stuck edges hold them in place. Eventually, the pressure becomes too strong, and the rocks suddenly slip or break. That sudden movement sends energy through the ground as seismic waves.

Those waves are what people feel during an earthquake.

Transform boundaries usually do not build tall mountains or create deep ocean trenches. However, they can still change the land. Over time, they may shift stream channels, crack rock, form narrow valleys, or create uneven land along a fault.

Think of it this way: transform boundaries may not be the flashiest plate boundary, but they are definitely not boring. They are the quiet pressure-builders of plate tectonics — until they shake things up.

The most well-known transform boundary is the San Andreas Fault, which is 800 miles long and runs through California. While the boundary has only produced a few large-scale earthquakes, it produces thousands of small ones yearly.

The San Andreas Fault is one of the most famous examples of a transform boundary. It runs through California and marks the boundary between two tectonic plates: the Pacific Plate and the North American Plate.

These plates are moving past each other very slowly. Their average movement is only a couple of inches each year, about the speed your fingernails grow. That may not sound dramatic, but Earth likes to play the long game.

The movement is not steady all the time. For years, parts of the fault may be locked in place. While the plates are stuck, pressure builds underground. When the rocks finally break or slip, the stored energy is released as an earthquake.

The San Andreas Fault can also be seen in some places at the earth’s surface. Along parts of the fault, scientists can observe landforms such as scarps, pressure ridges, offset streams, and crushed rock. These features show that the land on one side of the fault has slowly shifted past the land on the other side.

A common myth says the San Andreas Fault could split open and cause California to fall into the ocean. That cannot happen. The plates are sliding past each other, not pulling California away into the sea.

So, the San Andreas Fault is not a giant trapdoor. It is a real-world example of how transform boundaries move, lock, build pressure, and sometimes release that pressure through earthquakes.

Look at the map above.

• Where do you see examples of transform boundaries?
• Is there an increase in earthquakes in these locations?
• Can you find the San Andreas Fault on the map?
• Do you live near a transform boundary?

When you are ready, move on to the Got It? section to assess what you have learned.