Kid-friendly explanation

Imagine Earth's outer layer as a giant cracked eggshell. A fault is one of those cracks. The rock on each side of the crack can slide, rise, or sink. When the rocks suddenly move along a fault, the ground can shake and cause an earthquake!

Worked examples

Example 1 (simpler): Seeing a fault at a cliff

Picture a rocky cliff where you can see layers of rock like stripes: brown, gray, and red. On the left side of a crack, the red stripe is higher. On the right side of the crack, the same red stripe is lower.

This shows a fault. The crack is the fault, and the layers are no longer lined up. The rock on one side of the fault moved down compared to the other side. That movement along the crack is what makes it a fault, not just an ordinary crack.

Example 2 (more advanced): Faults at plate boundaries and earthquakes

Earth's outer shell is broken into large pieces called tectonic plates. Where two plates meet, they often grind past each other along big faults. One famous example is the San Andreas Fault in California.

As the plates try to move, friction holds the rocks along the fault in place. Stress builds up in the rocks, like stretching a rubber band. When the rocks finally slip suddenly along the fault, the stored energy is released as seismic waves, which we feel as an earthquake. The place underground where the rock first slips is called the focus, and the crack where the slip happens is the fault.

Going deeper

Faults are a key part of plate tectonics, the idea that Earth's plates are slowly moving over time. Most earthquakes happen on or near faults where plates meet or where the crust is being stretched or squeezed.

Different kinds of forces create different types of faults. When rocks are pulled apart, one block may slide down, forming a normal fault. When rocks are pushed together, one block may be pushed up over another, forming a reverse or thrust fault. When rocks slide past each other sideways, that is a strike-slip fault.

Over long periods, movement along faults can build mountains, create valleys, and change landforms. Faults can also affect where rivers flow and where caves or other features form, because broken rock can be easier for water to move through.

Common mistakes

  • Mistake: Thinking any crack in a rock is a fault.
    Correction: A fault is a crack where the rocks on each side have actually moved past each other, not just a small surface crack with no movement.
  • Mistake: Believing faults only exist where there are big, famous earthquakes today.
    Correction: Faults can be found in many places, including areas that are quiet now; some faults are active, and others have not moved for a very long time.
  • Mistake: Assuming faults always move up and down like a trapdoor.
    Correction: Some faults move up and down, some move sideways, and some move in a mix of directions, depending on how the crust is being pushed or pulled.

Everyday connection

If your child has ever seen news about an earthquake, there was almost always a fault involved. When you look at a map that shows a long, straight valley or a sharp line of mountains, those shapes may have formed along old faults where Earth's crust moved and changed the land over millions of years.

Related terms

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