| A | B |
|---|
| absolute age | the age of a rock given as the number of years since the rock formed |
| amphibian | a vertebrate that lives part of its life on land and part of its life in the water |
| atom | the smallest unit of of an element that retains all of the properties of that element |
| carbon film | a type of fossil consisting of an extremely thin coating of carbon on rock |
| cast | a fossil that is a copy of an ourganism's shape; formed when minerals seep into a mold |
| element | a substance composed of a single kind of atom |
| epoch | a subdivision of the periods of the geologic time scale |
| era | one of the three long units of geologic time- longer than a period, but shorter than an eon |
| evolution | the process by which all the different kinds of living things have changed over time |
| extinct | a type of organism that no longer is alive anywhere on Earth |
| extrusion | an igneous rock layer that is formed when lava flows onto Earth's surface and hardens |
| fault | a break or crack in Earth's lithosphere (surface) along which rocks move |
| fossil | the preserved remains or traces of living things |
| geologic time scale | a record of the geologic events and life forms in Earth's history |
| half-life | the time it takes for half of the atoms of a radioactive element to decay |
| index fossil | fossil of widely distributed organisms that lived during only one short period |
| intrusion | an igneous rock layer that forms when magma hardens beneath Earth's surface |
| invertebrate | an animal without a backbone |
| law of superposition | each layer is older than the layer above it (in horizontal rock) |
| mammal | a warm-blooded vertebrate that feeds its young milk |
| mass extinction | when many types of living things become extinct at the same time |
| mold | a fossil formed when an organism buried in sediment dissolves, leaving a hollow area |
| paleontologist | a scientist who studies fossils to learn about organisms that lived long ago |
| period | one of the units of geologic time, shorter than an era but onger than an epoch |
| petrified fossil | a fossil in which minerals replace all or part of an organism |
| radioactive decay | the breakdown of a radioactive element, releasing particles and energy |
| relative age | the age of a rock compared to the ages of rock layers |
| reptile | a vertebrate with scaly skin that lays eggs with tough, leathery shells |
| scientific theory | a well-tested concept that explains a wide range of observations |
| sedimentary rock | forms when particles of other rock (or the remains of plants and animals) are pressed and cemented together |
| trace fossil | a type of fossil that provides evidence of the activities of ancient organisms |
| unconformity | a place where old, eroded rock surface in in contact with a newer rock layer |
| vertebrate | an animal with a backbone |
