Earth's Moving Puzzle Pieces
Have you ever looked at a world map and noticed how the coastlines of South America and Africa look like they could fit together? You're not the first! This observation is a key piece of evidence for the theory of plate tectonics.
This theory states that Earth's outer layer, the lithosphere, is not one solid piece. Instead, it is broken into several large and small tectonic plates that are slowly and constantly moving, floating on the semi-liquid asthenosphere below. Where these plates meet, we see most of the world's geologic action.
Plate Boundaries
A plate boundary is the region where two tectonic plates meet. There are three main types, classified by how the plates move relative to each other.
1. Divergent Boundaries
At a divergent boundary, two plates are moving apart from each other.
Process: As the plates pull apart, magma from the mantle rises up to fill the gap. This magma cools and forms new crust.
What it Forms:
On the ocean floor, this process is called seafloor spreading and it creates mid-ocean ridges (like the Mid-Atlantic Ridge).
On land, it can create a rift valley (like the East African Rift Valley).
Geologic Events: These boundaries are associated with many small earthquakes and volcanic activity.
2. Convergent Boundaries
At a convergent boundary, two plates are moving towards each other, or colliding. What happens next depends on the types of plates involved.
Oceanic-Continental Convergence: A dense oceanic plate collides with a less dense continental plate. The denser oceanic plate sinks beneath the continental plate in a process called subduction.
What it Forms: A deep ocean trench and a line of volcanic mountains on the continent (like the Andes Mountains).
Geologic Events: Major earthquakes and explosive volcanoes.
Oceanic-Oceanic Convergence: Two oceanic plates collide. The older, colder, and denser plate will subduct beneath the younger one.
What it Forms: A deep ocean trench and a chain of volcanic islands called a volcanic island arc (like Japan or the Aleutian Islands).
Geologic Events: Major earthquakes and volcanoes.
Continental-Continental Convergence: Two continental plates collide. Since both are low-density, neither can subduct. Instead, the crust crumples and is pushed upwards.
What it Forms: Huge, non-volcanic mountain ranges (like the Himalayas, formed by the collision of India and Asia).
Geologic Events: Major earthquakes.
3. Transform Boundaries
At a transform boundary, two plates are sliding past each other horizontally.
Process: The plates grind against each other. They can get stuck, building up stress. When the stress is released suddenly, the ground shakes.
What it Forms: A fault, which is a break in the Earth's crust.
Geologic Events: Lots of earthquakes, but typically no volcanoes. The San Andreas Fault in California is a famous example.