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Fossil The preserved remains or traces of living things.

Sedimentary rock The type of rock that is made of hardened sediment.

Mold A fossil formed when an organism buried in sediment dis- solves, leaving a hollow area.

Index fossil Fossils of widely distributed organisms that lived during only one short period.

Petrified fossil A fossil in which minerals replace all or part of an organism. Carbon film A type of fossil consisting of an extremely thin coating of car- bon on rock. Trace fossil A type of fossil that provides evidence of the activities of ancient organisms. Paleontologist A scientist who studies fossils to learn about organisms that lived long ago.

Evolution The process by which all the different kinds of living things have changed over . Extinct Describes a type of organism that no longer exists anywhere on Earth.

Relative The age of a rock compared to the ages of rock layers.

Absolute age The age of a rock given as the number of since the rock formed.

Law of The geologic principle that states that in horizontal layers of superposition . Each layer is older than the layer above it and younger than the layer below it. Extrusion An layer formed when lava flows onto Earth’s surface and hardens.

Intrusion An igneous rock layer formed when magma hardens beneath Earth’s surface. Fault A break or crack in Earth’s lithosphere along which the rocks Disconformity When there are layers missing. move. Prokaryotic Have no nucleus. Unconformity A place where an old, eroded rock surface is in contact with a newer rock layer. Ice age a glacial episode during a geological period. Index fossil Fossils of widely distributed organisms that lived during only one short period. One of the most common organisms. Radioactive decay The breakdown of a radioactive element, releasing particles and energy. Pangaea supercontinent that included all current land masses, believed to have been in existence before the continents broke apart during Half-life The length of time needed for half of the atoms of a sample the and Periods. of a radioactive isotope to decay. Speciation Creation of a new species A record of the geologic events and life forms in Earth’s his- tory. Global broiling The Earth’s atmospheric temperature was raised by thousands One of the three long units of geologic time between the theory of degrees, due to the intense heat of the blasted particles. and the . Period One of the units of geologic time into which di- vide . Mass When many types of living things become extinct at the extinction same time. Uniformitarianism The same geologic processes going on now, like faulting and erosion, were also going on in the past. Carbon-14 Method that provides objective age estimates for carbon- dating based materials that originated from living organisms Principle of As sediments settle from water, they form horizontal layers horizontality of rock. Non-horizontal layers must have suffered some

Principle of If layers of sedimentary rock are not disturbed or deformed, superposition they oldest layers lies on the bottom, with younger layers on

Principle of Layers of sedimentary rock generally extend in all directions continuity until the original sediments stopped at the edge of the basin

Angular When sedimentary rocks in horizontal layers are tilted and unconformities