Uniformitarianism
Geologic processes that happened in the past can be explained by current geologic processes.
Fossils
The trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock. May be skeletons or body parts, shells, burrows, or ancient coral reefs.
Amber
Is formed when when hardened tree sap is buried and preserved in sediment.
Asphalt
wells up at Earth's surface in thick, sticky pools and trap and preserve many fossils.
Petrification
Happens when an organism's tissues are replaced by minerals.
Trace Fossil
A fossilized structure that formed in sedimentary rock by animal activity on or in soft sediment. Include tracks, burrows and animal dung.
Ripple Marks
A feature on sedimentary rock which records the motion of wind or water waves over sediment.
Mud Cracks
A feature on sedimentary rock which forms when fine grained sediments at the bottom of a shallow body of water are exposed to the air and dry out. They show that an ancient lake, stream, or ocean shoreline was once part of an area.
Pangea
The single land mass which was formed by all the current continents being together 200 million years ago.
Climate
Describes the weather conditions in an area over a long period of time. It is mostly determined by temperature and precipitation.
Tree Rings
The new layer of wood which is added to the trunk every year. They tell the age of the tree.
Sea Floor Sediment
Removed from the ocean floor and studied by scientists because they contain microscopic organisms that have died and settled there.
Ice Core
A long cylinder of ice gathered by scientists when they drill down into ice caps. They give a history of the Earth's climate over time through their bands.
Ice Caps
Found in places such as Iceland and the islands of the Arctic. They form as older snow is squeezed into ice by new snow falling on top of it.
Relative Dating
The determination of whether an object or an event is older or younger than other objects.
Superposition
A law which states the principle that younger rocks lie above older rocks if the layers have not been disturbed.
Tilting
Occurs when Earth's forces move rock layers up or down unevenly. As a result, the layers of rock become slanted.
Folding
The bending of rocks that can occur when rock layers are squeezed together. Sometimes this causes the rock layers to be turned over so that the older layers end up on top of younger layers.
Fault
A break or crack in Earth's crust where rocks can move.
Intrussion
Igneous rock that forms when magma is injected into rock and then cools and becomes hard.
Unconformity
A break in the geologic record that is made when rock layers are eroded or when sediment is not deposited for a long period of time.
Crosscutting Relationships
A law that states that a fault or a body of rock, such as an intrusion, must be younger than any feature or layer of rock that the fault or rock body cuts through.
Geologic Column
An ordered arrangement of rock layers that is based on the relative ages of the rocks, with the oldest rocks at the bottom of the column. Represents an ideal image of a rock layer sequence that doesn't actually exist in any once place on Earth.
Absolute Dating
The process of finding the actual age of an event or object in years. One process uses radioactive isotopes.
Radioactive
What unstable isotopes are called.
Isotopes
Atoms of the same element which have a different number of neutrons.
Radioactive Decay
The breakdown of a radioactive isotope into a stable isotope of the same element or of another element. For many isotopes, this occurs when a neutron is converted into a proton.
Parent Isotope
This is what a radioactive isotope is called.
Daughter Isotope
This is what the stable isotope formed by radioactive decay, or breakdown, is called.
Half-Life
This is the time that is needed for half of a sample of a radioactive substance to undergo radioactive decay to form daughter isotopes. This is always given in units of time.
Radiometric Dating
This is the process of finding the absolute age of a sample by determining the relative percentages of a radioactive parent isotope and a stable daughter isotope.
Radiocarbon Dating
The process of radiometric dating used for fossils using carbon-12 and carbon-14 as the parent isotope and nitrogen-14 as the daughter isotope.
Potassium-Argon Dating
A radiometric dating process often used to date igneous VOLCANIC rock. It uses potassium-40 as the parent (radioactive) isotope and argon and calcium as the daughter isotopes.
Uranium-Lead Dating
A radiometric dating process used to determine the age of igneous rocks which are between 100 million and billions of years old. Uses uranium-238 as the parent (radioactive) isotope and lead-206 as the daughter isotope.
Meteorites
Small, rocky bodies which have traveled through space and fallen to Earth's surface.
Index Fossils
The trace remains of an organism which lived during a relatively short geologic time span. As a result, it can be used to estimate the absolute age of rock layers in which they are found. They can then be used to determine the age of rock layers that cont
Ammonite Tropites
Index fossils which lived between 230 and 208 million years ago.
Trilobite
Index fossils, related to the horseshoe crab, spiders and scorpions which lived between 405 and 360 million years ago.
Phacops Rana
A trilobite that lived between 405 and 360 million years ago.
Composition
For sedimentary rock, this shows the source of the sediment that makes up the rock. For example, whether the sedimentary rock was made from minerals or organic material.
Texture
For sedimentary rock, this shows the environment in which the sediment was carried and deposited.