Plate Tectonics (College Board AP® Environmental Science): Study Guide

Tectonic plates

• The surface of the Earth is divided into tectonic plates that have moved throughout geological time

  • The Earth's crust is broken into 15 large tectonic plates and a number of smaller ones

  • The place where plates meet is called a plate boundary or plate margin

• Tectonic plates move apart, slide against each other, or collide

Convergent plate boundaries

Geological changes and events at convergent boundaries

• Definition: Convergent plate boundaries occur where two tectonic plates collide

  • This leads to significant geological activity

• These boundaries can result in:

Subduction

• The denser, heavier oceanic plate subducts under the lighter, less dense continental plate

• Example: The Nazca Plate is being subducted beneath the South American Plate at their convergent boundary

Mountain formation

• At a collision boundary, two plates of similar density move towards each other

  • When these two continental plates collide, neither is dense enough to subduct

  • This means the crust is forced upward, creating mountains

• Example: The Himalayas formed due to the collision of the Indian and Eurasian plates

Island arcs

• When an oceanic plate subducts beneath another oceanic plate, a curved chain of volcanic islands forms

• Example: The Mariana Islands were created by subduction in the Pacific Ocean

Earthquakes

• The intense pressure at convergent boundaries leads to frequent, powerful earthquakes

• Example: The 2011 Tōhoku earthquake in Japan occurred due to subduction at a convergent boundary

Volcanoes

• Subduction of an oceanic plate beneath a continental plate causes magma to rise, forming volcanic mountain ranges

• Example: The Andes Mountains contain numerous active volcanoes due to the subduction of the Nazca Plate beneath the South American Plate

Divergent plate boundaries

Geological changes and events at divergent boundaries

• Definition: Divergent plate boundaries occur where two tectonic plates move apart

  • This creates new crust and geological activity

• These boundaries can result in:

Seafloor spreading

• As plates pull apart, magma rises and solidifies, forming new oceanic crust

• Example: The Mid-Atlantic Ridge is a major seafloor spreading center

Rift valleys

• When continental plates diverge, the crust thins and forms a depression known as a rift valley

• Example: The East African Rift is a continental rift valley that may eventually become a new ocean basin

Volcanoes

• Rising magma at divergent boundaries can create moderate volcanic activity along mid-ocean ridges and rift zones

• Example: Iceland, located on the Mid-Atlantic Ridge, has active volcanoes due to divergent boundary activity

Earthquakes

• As plates move apart, stress can cause earthquakes along the boundary

• Example: The frequent earthquakes along the Mid-Atlantic Ridge result from the continuous movement of tectonic plates

Transform plate boundaries

Geological changes and events at transform boundaries

• Definition: Transform plate boundaries occur where two tectonic plates slide past each other horizontally

  • They can move past each other in opposite directions or in the same direction at different speeds

  • This causes intense friction and seismic activity

• These boundaries mainly result in:

Earthquakes

• The friction between sliding plates builds up stress, which is released as earthquakes

• Example: The San Andreas Fault in California is a well-known transform boundary that experiences frequent earthquakes

Global distribution of plate boundaries

Importance of plate boundary maps

• Plate boundary maps show the locations where Earth's tectonic plates interact

• These maps help predict geological activity, including:

  • Volcanoes

  • Island arcs

  • Earthquakes

  • Hot spots occur where magma rises through the crust, independent of plate boundaries (e.g., Hawaii)

  • Faults are fractures in Earth’s crust where two blocks of rock move relative to each other due to tectonic forces

Earthquakes

• The majority of earthquakes (about 95%) occur close to or at a plate boundary

Volcanoes

• Like earthquakes, most active volcanoes occur at or near plate boundaries

• Many (about 75%) occur around the 'Ring of Fire' surrounding the Pacific Ocean

Examples of plate boundary distribution

• Pacific Ring of Fire:

  • A tectonically active region surrounding the Pacific Ocean, known for earthquakes and volcanoes

• Mid-Atlantic Ridge:

  • A divergent boundary where new oceanic crust forms, creating volcanic activity and seafloor spreading

• San Andreas Fault:

  • A transform boundary in California known for frequent earthquakes

Earthquakes caused by locked faults

What are locked faults?

• A locked fault is a section of a fault that is stuck due to friction

  • This prevents movement between tectonic plates

• Over time, stress builds up as the plates continue trying to move

• When the stress overcomes friction, the fault suddenly slips

  • This releases stored energy as an earthquake

How locked faults cause earthquakes

• The longer a fault remains locked, the more stress accumulates

  • This leads to a more powerful earthquake when it finally moves

• Sudden fault movement releases seismic waves, which cause ground shaking and potential destruction

Impacts of locked fault earthquakes

• Can generate major earthquakes with significant damage to buildings and infrastructure

• May trigger aftershocks as stress redistributes along the fault

• In some cases, locked faults near oceanic regions can cause tsunamis

  • Over 70% of tsunamis occur around the Pacific Ocean