Fossil
Preserved remains or traces of living things
Sedimentary Rock
Type of rock made from hardened sediment
Mold
Hollow area in sediment in the shape of an organism or part of an organism
Cast
Solid copy of the shape of an organism
Petrified Fossil
Fossil in which minerals replace all or part of an organism
Carbon Film
Extremely thin coating of carbon on rock
Trace Fossil
Provide evidence of the activities of ancient organisms
Paleontologist
Scientist who studies fossils
Scientific Theory
A well-tested concept that explains a wide range of observations
Evolution
The gradual change in living things over long periods of time
Extinct
Organisms that no longer exist and will never again live on earth
Relative Age
Age of a rock compared to the ages of other rocks
Absolute Age
The number of years since the rock formed
Law of Superposition
The oldest layer of rock is at the bottom
Extrusion
Lava that hardens on the surface. Always younger than the rocks below it
Intrusion
Magma that pushes through bodies of rock and hardens underground. Always younger than the rocks around it
Unconformity
Surface where new rock layers meet a much older rock surface beneath them
Index Fossil
Help geologists match rock layers. Must be widely distributed and have only existed briefly
Geologic Time Scale
A record of the life forms and geologic events in Earth's history
Periods
Eras are divided into these time units
Eras
Three long units of time
Invertebrate
Animal without a backbone
Vertebrate
Animal with a backbone
Amphibian
Animal that lives part of its life on land and part of its life in water
Reptile
Animal with scaly skin that lays eggs with tough, leathery shells
Mass Extinction
Many types of living things become extinct at the same time
Mammal
Warm-blooded vertebrate that feeds its young milk