| A | B |
|---|
| FOSSIL | The preserved remains or traces of living things. |
| PALEONTOLOGIST | A scientist who studies fossils to learn about organisms that lived long ago. |
| SEDIMENTARY ROCK | A type of rock that forms when particles from other rocks or the remains of plants and animals are pressed and cemented together. |
| PETRIFIED FOSSIL | A fossil in which minerals replace all or part of an organism. |
| MOLD | A fossil formed when an organism buried in sediment dissolves, leaving a hollow area. |
| CAST | A fossil that is a copy of an organism’s shape formed when minerals seep into a mold. |
| CARBON FILM | A type of fossil consisting of an extremely thin coating of carbon on rock. |
| TRACE FOSSIL | A type of fossil that provides evidence of the activity of ancient organisms. |
| SCIENTIFIC THEORY | A well-tested concept that explains a wide range of observations. |
| EVOLUTION | The process by which all the different kinds of living things have changed over time. |
| EXTINCT | Describes a type of organism that no longer exists anywhere on Earth. |
| RELATIVE AGE | The age of a rock compared to the ages of rock layers. |
| ABSOLUTE AGE | The age of a rock given as the number of years since the rock formed. |
| LAW OF SUPERPOSITION | The geologic principle that states that: In horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it. |
| UNCONFORMITY | A place where an old, eroded rock surface is in contact with a newer rock layer. |
| FAULT | A break or crack in Earth’s lithosphere along which the rock moves. |
| INTRUSION | An igneous rock layer formed when magma hardens beneath Earth’s surface. |
| EXTRUSION | An igneous rock layer formed when lava flows onto Earth’s surface and hardens. |
| INDEX FOSSIL | Fossils of widely distributed organisms that lived during only one short period. |
| ATOM | The smallest unit of an element that retains the properties of that element. |
| ELEMENT | A substance composed of a single kind of atom. |
| RADIOACTIVE DECAY | The breakdown of a radioactive element, releasing particles and energy. |
| HALF-LIFE | The time it takes for half of the atoms of a radioactive element to decay. |
| GEOLOGIC TIME SCALE | A record of the geologic events and life forms in Earth’s history. |
| ERA | One of the three long units of geologic time between the Precambrian and the present. |
| INVERTEBRATE | An animal without a backbone. |
| PERIOD | One of the units of geologic time into which geologists divide eras. |
| EPOCH | Subdivisions of the periods of the geologic time scale. |
| VERTEBRATE | An animal with a backbone. |
| AMPHIBIAN | A vertebrate that lives part of its life on land and part of its life in water. |
| REPTILE | A vertebrate with scaly skin that lays eggs with tough, leathery shells. |
| MASS EXTINCTION | When many types of living things become extinct at the same time. |
| MAMMAL | A warm-blooded vertebrate that feeds its young milk. |
