4.6 Earth's Geology and Natural Hazards

Earth's Layers Drive Plate Tectonics

Earth has a layered structure: the crust, mantle, and core. Heat from the core drives convection currents in the mantle, moving rigid tectonic plates on the underlying asthenosphere. Plate boundaries are classified as convergent (plates collide), divergent (plates separate), and transform (plates slide past each other), each producing characteristic geologic activity.

Volcanoes and Earthquakes

Volcanoes form at divergent boundaries, convergent (subduction) zones, and hot spots, releasing magma, ash, and gases that can affect climate and soil fertility. Earthquakes occur mainly along transform and convergent boundaries as stress builds and releases along faults. Both hazards can trigger secondary events like tsunamis, landslides, and fires.

Other Natural Hazards

Tsunamis are large ocean waves often triggered by undersea earthquakes or volcanic eruptions. Landslides and mass wasting occur when gravity moves loosened rock/soil, worsened by heavy rain, deforestation, or earthquakes. Floods result from excess water overwhelming drainage systems. Human development in hazard-prone zones increases vulnerability and economic/human losses.

Key terms

Convection current

Circular movement of material in the mantle caused by heat from the core, driving plate tectonics.

Convergent boundary

Plate boundary where plates collide, often causing subduction, mountain building, or volcanic activity.

Divergent boundary

Plate boundary where plates move apart, creating new crust (e.g., mid-ocean ridges).

Fault

A fracture in Earth's crust where blocks of rock move relative to each other, often causing earthquakes.

Tsunami

A large ocean wave typically triggered by an undersea earthquake, volcanic eruption, or landslide.

Try an AP-style question

Which plate boundary forms volcanoes like the Andes?

1. Hot spot (mantle plume)
2. Divergent (spreading)
3. Convergent (subduction)
4. Transform (sliding)