Earth's History

Uniformitarianism

·      The principal that most of the geologic events of the past are similar to processes that we can observe in the present.

Law of Superposition

·      The principle that the lowest layers in a sequence of rock strata must have been deposited before the layers above, unless the rock strata have been turned upside down. 

·      The oldest rocks are generally found at the bottom of an outcrop.

Outcrop

·      Bedrock that is exposed at the surface because it is not covered by soil.

Orogeny

·      The process of mountain building.

Extrusion

·      Molten, liquid rock (lava) flowing out onto the surface of the Earth.

·      A fine-grained igneous rock formed by the rapid crystallization of lava at or near the surface of the Earth.

Intrusion

·      Molten, liquid rock (magma) being pushed into cracks within the Earth.

·      A body of coarse-grained igneous rock formed by slow cooling in the Earth.

Lava

·      Molten rock at the surface of the Earth.

Folded Strata

·      Layers of rock that have been bent by forces within the Earth.

Fault

·      A break in the rock of the Earth’s crust along which there has been displacement (movement).

Uplift

·      The rising of the Earth’s crust from forces within the Earth, generally related to motions of the tectonic plates.

Joint

·      A crack in a rock produced by shrinking or uneven pressure. 

·      Unlike a fault, no displacement along a joint surface occurs.

Fossil

·      Any preserved remains or traces of life.

Species

·      A group of living organisms with similar characteristics, a common name, and the capability to interbreed.

Correlation

·      A matchup of rock layers in different locations by age or by rock types.

Index Fossil

·      A fossil that can be found over a large geographic area but existed for a brief period of geologic time. 

·      Useful in determining the geologic are of the rock in which it is found.

Organic Evolution

·      The principle that living things have changed in form through the history of the Earth from a few simple organisms to a great diversity of organisms.

Dendritec

·      A pattern resembling tree branches.

Unconformity

·      A gap in the geologic record caused by the erosion of sediments or rock before they are protected by layers above.

Absolute Age

·      The age of a rock unit, a fossil, or an event expressed in units of time, such as years.

Relative Age

·      A comparative age

·      Age expressed as before or after other events without specifying the age in units of measure.

Radioactivity

·      The emission of energy rays or nuclear particles from the breakdown of an unstable isotope.

Radiative Balance

·      Equilibrium between absorbed radiant energy and radiant energy given off.

Isotope

·      A form of an element with more or fewer neutrons than other forms of the same element.

Decay Product

·      The element produced by the decay of a radioactive isotope.

Half-Life

·      The time required for half of a radioactive element’s atoms in a sample to change to the decay product.

Decay-Product Ratio

·      The ratio between the mass of a radioactive element and its decay product.

Carbon 14

·      A radioactive form of the element carbon that has been used to find the absolute age of recent fossils and geologic events.

Reversed Magnetic Polarity

·      Refers to an igneous rock that was formed at a time in the geologic past when the north and south magnetic poles of the Earth had the opposite polarity that they have at the present.

Geologic Time Scale

·      Division of the history of the Earth originally based upon observations of fossil evidence. 

·      Through the use of radioactive isotope measurements, it has changed from a relative scale to an absolute scale.

Era

·      A large division of geologic time.

Period

·      In geologic time, a subdivision of an era.

Event

·      A change or series of changes in the Earth’s environment.

Time

·      A measurable period in which an event or process occurs.

Pangaea

·      The ancient supercontinent that broke apart millions of years ago to form the present continents.

Ice Age

·      A long period of Earth’s history when ice sheets cover large areas of the continents.