4.1 - Tectonic Plates

Plate Tectonics

Earth’s moving tectonic plates shape landforms and cause geological events. The lithosphere—the brittle outer layer of Earth—is divided into plates that move above hotter, softer material in the upper mantle. Heat from Earth’s interior drives convection: hot material rises, cools and spreads, then sinks, repeating the cycle.

Three Types of Plate Boundaries

Divergent Boundaries

At divergent boundaries, plates move apart as rising magma pushes them away from one another. Cooling magma creates new lithosphere. This process can produce mid-ocean ridges, seafloor spreading, and volcanic activity. On land, divergence can form a rift valley. If magma reaches the surface and builds up, it may form a volcanic island.

Convergent Boundaries

At convergent boundaries, plates move toward one another. When an oceanic plate meets another plate, the denser plate may sink beneath it in a process called subduction.

• Oceanic–oceanic convergence: Subduction can produce volcanoes, island arcs, and deep-ocean trenches.
• Oceanic–continental convergence: The oceanic plate subducts beneath the continental plate. Rising magma can form volcanoes and coastal mountain ranges, such as the Andes.
• Continental–continental convergence: The crust is compressed and pushed upward, forming large mountain ranges such as the Himalayas.

Subduction zones can also produce earthquakes and tsunamis.

Transform Boundaries

At transform boundaries, plates slide alongside one another in opposite directions. Their rough edges can catch and lock while the plates continue trying to move. Stress builds until a fault slips suddenly, releasing energy as seismic waves. The resulting shaking is an earthquake.

Using Plate Maps to Anticipate Geological Activity

A map of plate boundaries can help identify where certain geological events are likely:

• The Ring of Fire around the Pacific is associated with convergent boundaries and subduction, which produce many volcanoes.
• Transform faults are likely locations for earthquakes. California and Haiti are examples discussed in the video.
• Divergent boundaries are associated with mid-ocean ridges, seafloor spreading, and sometimes volcanoes or new islands. Iceland is located at a plate boundary.
• Volcanic island chains can also form over hot spots away from plate boundaries. Hawaii is an example.

Study Cues and Practice

Hand motions serve as memory aids: move hands apart for divergence, bring them together for convergence, and slide them past each other for transform boundaries.

The practice prompt asks viewers to use diagrams to explain how subduction can lead to volcanic activity.

Speakers and Sources

• Presenter: Jordan Dischinger-Smedes
• Visual sources: Diagrams of Earth’s layers, plate-boundary interactions, and convection; a global tectonic-plate map highlighting the Ring of Fire, California, Haiti, Iceland, and Hawaii.