Geologic Terms

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Orogeny

Period of mountain building in which tectonic forces cause rocks to be faulted, folded and/or metamorphosed

Shield

Ancient cores of continents made of Precambrian igneous & metamorphic rocks

Mold

Empty cavity formed when the hard parts of a buried organism are dissolved/reacted away

Trace Fossil

Indirect evidence for ancient life such as footprints, trails or burrows

Dendrochronology

Study of tree rings to calculate dates of past events and to determine climatic changes in the past

Varves

Alternating bands of light- and dark-colored sediments deposited respectively during summer and winter in large lakes

Paleozoic

Era of "Ancient Life"

Mesozoic

Era of "Middle Life"

Cenozoic

Era of "Recent Life"

Phanerozoic

Eon of "Visible Life"; Eon of multicellular organisms with hard parts

Proterozoic

Eon of one-celled organisms

Archean

Eon of "Ancient" Time

Hadean

Eon of "Hellish" Time

Geologic Time Scale

Chart used to describe earth's history by dividing it into various eons, eras, periods & epochs, originally based only on the fossil record

Cast

A 3-D copy of the hard parts of an organism formed when the hard parts of a buried organism are dissolved/reacted away and the resulting cavity is filled with sediments

Stromatolites

The most common Precambrian fossils (of algae) found in numerous places around the world and dating back hundreds of millions to billions of years

Mass extinction

When many types of living things (species) disappear from earth at the same time

Evolution

Change in living things through time

Paleontologist

Scientist that studies fossils and ancient organisms

Petrified

Turned to stone; fossilized in rock

Principle of Superposition

In undisturbed sedimentary rocks, the oldest rocks are on the bottom and the youngest are on the top

Principle of Cross-Cutting Relationships

Younger features cut across or through older features

Principle of Contact Metamorphism

Rocks metamorphosed by the magma are older than the intrusion or lava flow, unmetamorphosed rocks are younger than the lava flow

Principle of Fossil Succession

Fossils and the rocks they are found in can be arranged in a definite order according to their age

Principle of Original Horizontality

Sediments are deposited in originally flat layers

Daughter Isotope

Isotope formed from the decay of an unstable isotope

Uniformitarianism

Fundamental principle of geology that states the processes which occur on Earth today are the same processes that occurred in the past; the present is the key to the past

Correlation

Matching a rock layer or rock unit in one place with the same rock layer in another place using rock type, thickness, fossils, etc.

Unconformity

Any break in the rock record when erosion took place instead of deposition

Relative Date

The age of a rock formation or event compared to the surrounding rock formations or events

Fossils

Remains or traces of ancient life

Index Fossils

Fossils that are widespread over a large area of earth and only existed for a short amount of time and are used to give approximate numerical dates to rock formations and geologic events

Radiometric Date

The absolute age of a rock or mineral calculated using the ratios of parent and daughter isotopes

Absolute Date

Date that gives the number of years that have passed since a rock formed or an event occurred

Radioactive Decay

The spontaneous breaking apart of nuclei of atoms through time to form new & different atoms

Parent Isotope

An unstable isotope that has not yet decayed

Half life

The time it takes half the atoms of a parent isotope to decay into atoms of the daughter isotope

Isotope

Atoms that have the same number of protons, but different numbers of neutrons

Eon

The largest time expanse in geologic time consisting of 100s of millions of years

Epoch

The smallest time expanse in geologic time consisting of millions of years

Era

The second largest time expanse in geologic time consisting of many tens of millions of years

Period

The second smallest time expanse in geologic time consisting of tens of millions of years