Plates
A section of the lithosphere that slowly moves, carrying pieces of continental and oceanic crust.
Pangaea
The name of the single land mass that broke apart 225 years ago and gave rise to today’s continents.
Plate Tectonics
The theory that states that Earth’s plates are in constant, slow motion.
Plate Boundaries
A crack in the lithosphere where two of Earth’s plates meet.
Earthquake
The shaking that results from the movement of rock beneath Earth’s surface.
Stress
A force that acts on a rock to change its shape for volume.
Shearing
Stress that pushes a mass of rock in opposite direction.
Tension
Stress that stretches rock so that it becomes thinner in the middle.
Compression
Stress that squeezes rock until it folds or breaks.
Fault
A break or crack in Earth’s lithosphere along which rocks move.
Strike-slip fault
A type of fault where the rocks on either side move past each other sideways with little up-or-down motion.
Normal fault
A type of fault where the hanging wall slides downward; caused by tension in the crust.
Hanging wall
The block of rock that forms the upper half of a fault.
Footwall
The block of rock that forms the lower half of a fault.
Reverse fault
A type of fault where the hanging wall slides upward; caused by compression in the crust.
Fold
A bend in rock that forms where part of the Earth’s crust is compressed.
Plateau
A landform that has a more or less level surface and is elevated high above sea level.
Anticline
A fold that bends upward into an arch.
Syncline
A fold in rock that bends downward in the middle to form a bowl.
Magma chamber
The pocket beneath a volcano where magma collects.
Pipe
A long tube through which magma moves from the magma chamber to Earth’s surface.
Vent
An opening through which molten rock and gas leave a volcano.
Lava flow
The area covered by lava as it pours out of a volcano’s vent.
Crater
A bowl-shaped area that forms around a volcano’s central opening.
Silica
A material that is formed from the elements oxygen and silicon; silica is found in magma.
Pahoehoe
A hot, fast-moving type of lava that hardens to form smooth, ropelike coils.
Aa
A slow-moving type of lava that hardens to form rough chunks; cooler than pahoehoe.
Pyroclastic flow
The expulsion of ash, cinders, bombs, and gases during an explosive volcanic eruption.
Active
A volcano that is erupting or shows signs of eruption in the near future.
Dormant
A volcano that does not show signs of erupting in the near future.
Extinct
A volcano that is unlikely to erupt again.
Geyser
A type of hot springs that builds up pressure underground an erupts at regular intervals as a fountain of water and steam.
Geothermal energy
Heat energy in Earth’s interior from water or steam that has been heated by magma.