aa A type of lava flow characterized by its lumpiness and viscosity. When it cools, the rock it forms may be sharp.
absolute dating A technique that geologists use to assign specific dates to rock formations and geologic events.
ash A substance that shoots out of a volcano that's two millimeters or smaller in diameter.
cleavage The way crystalline minerals split or break along an even plane (planar surfaces).
cross cutting relationships A fundamental principle of geology, which states that rocks that cut through other rocks are younger than the rocks being cut.
faunal & floral
succession A fundamental principle of geology, which states that unless disturbed, the oldest fossils in a rock bed should be at the bottom.
fracture In rocks, a break along an uneven (non-planar) surface.
geologists Cool guys and gals who study rocks and the Earth.
hardness A rock or mineral's ability to resist abrasion.
igneous rock A rock that forms when molten rock cools down and crystallizes.
lateral continuity A fundamental principle of geology, which states that sediments are deposited initially in a layer that extends horizontally in all directions.
lava Hot, melted rocks that reach the Earth's surface.
lithification The process that turns sediment into sedimentary rock.
luster The way that minerals reflect light. There are two types of luster: metallic and nonmetallic.
magma Hot, melted rock that's below the Earth's surface.
Mohs hardness scale A scale that measures and compares the hardness of minerals.
original horizontality A fundamental principle of geology, which states that sediments settle and accumulate horizontally on the sea floor.
pahoehoe Congealed surface lava is dragged along over hot-moving lava. The congealed part rolls over the hot part, forming folds that look like ropes.
pumice A form of volcanic glass that's filled with holes. These holes form when gases escape from lava.
relative dating A technique geologists use to assign a sequential order to the age of rocks and geologic events. The oldest comes first, and subsequent events follow on a relative dating line.
rocks Rocks formed from things like little pieces of gravel, sand, silt, and clay, as well as the remains of animals and plants.
A fundamental principle of geology, which states that if a rock bed hasn't been disturbed since it was formed, it is younger than the layer of rock below it.
tuff A type of igneous rock formed by the consolidation of ash.
A fundamental principle of geology, which states that we can understand geologic events of the past by looking at geologic events of the present. According to uniformitarianism, a volcano eruption that happens today is pretty much like a volcano eruption that happened million years ago.