Glossary

Asthenosphere
A hot, malleable semiliquid zone in the upper mantle, directly underneath the lithosphere, on which the plates of the lithosphere move (or float).


Boundary
The border between two tectonic plates.


Collision Zone
The place where a collision between two continental plates crunches and folds the rocks at the boundary, lifting them up and leading to mountain formation.


Continental Crust
A layer of the earth's crust that lies under the seven continents. It is about 20 to 40 miles (35 to 70 km) thick and very old.


Convergent Boundary
The boundary that occurs where two plates are pushing toward each other.


Crust
Hard and rigid, the earth's outermost and thinnest layer. It is only a few miles (5 km) thick under the oceans and averages 20 miles (30 km) thick under the continents.


Divergent Boundary
The boundary that occurs where two plates are moving apart from each other.


Earthquake
A trembling and shaking of the earth's surface resulting from the sudden release of energy in the crust, either along fault lines or from volcanic activity.


Fault
A crack or fracture in Earth's crust where two tectonic plates grind past each other in a horizontal direction.


Inner Core
The innermost layer of the earth, an extremely hot, solid sphere of mostly iron and nickel. The inner core is 3,200 to 3,960 miles (5,150 to 6,378 km) below the surface and about 750 miles (1,200 km) thick.


Lithosphere
Made up of the crust and a tiny bit of the upper mantle, this zone is divided into several constantly (very slowly) moving plates of solid rock that hold the continents and oceans.


Magma
Molten rock that flows beneath the earth's surface and is made up of gases, liquids, and crystals. When magma reaches the surface, it is called lava.


Mantle
This dense layer of the earth is made of hot, semisolid rock and is located directly below the crust. It is about 1,800 miles (2,900 km) thick.


Mid-ocean Ridge
A raised area or mountain range under the oceans formed when magma fills the space between two tectonic plates that are spreading apart.


Oceanic Crust
The type of crust lying under the oceans of the world. It is only 4 to 6 miles (7 to 10 km) thick and usually younger than continental crust.


Outer Core
This layer of the earth lies between the mantle and the solid inner core. It is the only liquid layer, a sea of mostly iron and nickel about 1,400 miles (2,300 km) thick.


Pangaea
The name given to the supercontinent that existed more than 225 million years ago, in which the present-day continents were joined together in one large landmass.


Rift
A dropped zone where two tectonic plates are pulling apart.


Seafloor Spreading
The process that forms new ocean floor and oceanic crust. Magma oozes up from the mantle through a crack in the ocean floor, filling in the space between the plates and spreading out from the plate boundary.


Subduction zone
The place where one plate is getting bent and pulled under the edge of another plate.


Tectonic Plates
Several large slabs of the lithosphere that hold the continents and oceans and are slowly but constantly moving around the earth.


Transform Boundary
The boundary that occurs where two plates slide past each other.


Trench
A deep valley that forms at the edge of a continent when an oceanic plate sinks underneath a continental plate.


Tsunami
A huge ocean wave caused by a sudden, powerful shift on the ocean floor, like an undersea earthquake, landslide, or volcanic eruption.