Basalt
A type of igneous rock. Basalt is the most common rock type in the earth's crust and makes up most of the ocean floor.


Compacting and Cementing
A process that turns sediment into sedimentary rock. Over time, sediment accumulates in oceans, lakes, and valleys, eventually building up in layers and weighing down the material underneath. This weight presses the sediment particles together, compacting them. Water passing through the spaces in between the particles helps to cement them together even more.


Conglomerate
A type of sedimentary rock made up of pebbles, stones, and smaller particles pressed together by the action of waves or water. Conglomerate is often found in large expanses or beds. Finding a bed of conglomerate is a great clue that a river or beach once existed in that location.


Cooling
The process that hardens magma and turns it into igneous rock. The cooling of magma can occur either underneath or on the earth's surface.


Erosion
The process by which soil, sediment, and small pieces of rock are carried away from their original locations and transferred elsewhere by the actions of wind, water, ice, or living organisms.


Fossils
Imprints of leaves, shells, insects, or other items left in rock.


Gneiss
A type of metamorphic rock that usually has ribbonlike layers. Gneiss (pronounced "nice") can often be seen on mountainsides, where rocks formed below the surface have been pushed up by movements in the earth's crust.


Heat and Pressure
The process that occurs under the earth's surface and turns any kind of rock into metamorphic rock.


Igneous Rock
One of the three main rock types, formed from the cooling and hardening of magma. Sometimes the magma cools deep within the earth, and other times it erupts onto the earth's surface from volcanoes (in this case, it is called lava).


Limestone
A type of sedimentary rock often found near oceans and lakes.


Magma
Hot, molten rock found deep inside the earth.


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.


Marble
A type of metamorphic rock formed when limestone is pushed down into the earth and subjected to intense heat and pressure for a long period of time.


Melting
The process that turns any rock into magma. Melting a rock requires extremely high temperatures, which only occur far beneath the earth's surface.


Metamorphic Rock
One of the three main rock types, formed deep within the earth, where rock material is subjected to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals on their surface, formed by minerals growing slowly over time.


Metamorphosis
From the Greek "to transform," metamorphosis is the change that occurs when rocks under the earth's surface are subjected to intense heat and pressure, turning them into metamorphic rocks.


Obsidian
A type of extrusive igneous rock, created from lava that cools so quickly that no crystals can form on its surface. It can be found near volcanic lava flows.


The Rock Cycle
The recurring series of events that rocks undergo, over time, that transforms them from one type to another.


Sediment
Loose pieces of minerals and rocks.


Sedimentary Rock
One of the three main rock types, formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils.


Volcano
An opening in the earth's surface that allows hot magma, ash, and gases to escape from deep below the surface. When magma reaches the surface through a volcano, it is called lava. When lava cools, it forms extrusive igneous rock.


Weathering
The process by which rocks are worn down by wind and water, creating sediment.


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