Chapter 4: A Trip Through Geologic Time

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