The earth is layered. A thin rocky crust sits on the mantle, a thick layer of hot rock that flows very slowly, which surrounds a metallic core with a liquid outer part and a solid inner part. Everything we live on is the crust, and it is broken into pieces.
Plates and their boundaries
The crust is divided into large slabs called tectonic plates that move a few centimetres a year, carried by slow motion in the mantle. Most of the world's dramatic geology happens where two plates meet.
| Boundary | What the plates do | What forms |
|---|---|---|
| Convergent | push together | mountain ranges, deep ocean trenches, volcanoes |
| Divergent | pull apart | rift valleys on land, new sea floor at mid ocean ridges |
| Transform | slide past each other | earthquake faults |
Earthquakes and volcanoes
An earthquake happens when rock that has been slowly bending under stress suddenly slips, releasing the stored energy as waves that shake the ground. A volcano forms where molten rock from below reaches the surface. Both cluster along plate boundaries, which is why a world map of earthquakes traces the plate edges almost exactly.
Building up and wearing down
| Process | What it does | Examples |
|---|---|---|
| Constructive | builds land up | volcanic eruptions creating new rock, plate collisions raising mountains, rivers depositing deltas |
| Destructive | wears land down | weathering breaking rock apart, erosion carrying the pieces away |
Weathering breaks rock into smaller pieces where it sits. Water freezing in a crack expands and widens it. Plant roots push rock apart. Acids in rain slowly dissolve limestone. Erosion is the transport step: moving water, wind, ice and gravity carry the broken material somewhere else. Deposition is where it finally settles, building beaches, sandbars and deltas.
Weathering and erosion are constantly confused. Weathering breaks it. Erosion moves it. A cliff face crumbling is weathering, and the river carrying the fragments to the sea is erosion.
The pace of change
Some changes are sudden: an earthquake lasts seconds, and a landslide takes minutes. Others are imperceptible: the Grand Canyon was cut by a river working for millions of years. Both kinds of change are ordinary, and geology is largely the study of what slow processes accomplish given enough time.
Common mistakes
Swapping weathering and erosion
Break, then move. Weathering first, erosion second.
Assuming plates move fast
A few centimetres a year, about the rate fingernails grow. The results are dramatic because the process has run for hundreds of millions of years.
Check yourself
What is the difference between weathering and erosion?
Weathering breaks rock into pieces where it is. Erosion carries those pieces away.
Why do earthquakes cluster in narrow bands across the globe?
Because they occur where tectonic plates meet, and those boundaries form long lines across the earth.
Water gets into a crack in a rock and freezes. What happens?
The ice expands and widens the crack, which is a form of weathering. Repeated over many winters it splits the rock apart.
Name one constructive and one destructive process.
Constructive: a volcanic eruption adding new rock. Destructive: a river eroding a valley.
See also
Sources
- Plate tectonics, landform processes, weathering and erosion as taught in United States upper elementary earth science. Examples on this page are our own.