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A Trip Through Geologic Time Test Study Guide
fossil- imprints, remains, or traces of once-living organisms that have been preserved; usually
found in sedimentary rock
conditions necessary for fossils to form- 1. must be a once-living organism 2. must be buried
quickly 3. must be protected from scavengers or microorganisms 4. must have hard parts
(teeth, shells, bones)
fossils can give information about- the environment, climate, and animal behavior as well as
dating rocks
petrified remains- are hard and rocklike; some or all of the remains have been replaced by
minerals (i.e. quartz replacing calcium in bones)
carbon films- the organism is exposed to pressure and heat which forces gases and liquids out
from the organism. A thin film of carbon is left forming an outline of the original organism.
molds and casts- holes in rock let water and air reach the shell or hard part causing it to
dissolve and leave behind a hollow part called a mold. Later the other sediments fill in the
hollow part and harden into rock to make a cast.
preserved remains- actual organisms are found trapped in amber, frozen ground, glacial ice,
or tar seeps
trace fossils- fossilized tracks and other evidence of animal activity
index fossils- lived on Earth for a short period of time, were abundant (many), and were
widespread geographically (everywhere)
law of superposition- states that for undisturbed rock layers, the oldest rocks are on bottom
and become younger and younger toward the top.
relative dating- used in geology to determine the comparative age of rocks by examining the
position of the rock layers. i.e. top layers are younger than bottom layers, bottom layers are
older than top layers, etc.
extrusion- lava that hardens on the earth’s surface; always younger than the rocks below it
intrusion- magma that cools and hardens in a mass of igneous rock below the earth’s surface;
always younger than the rock layers around and beneath it
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fault- a break in the earth’s crust
unconformity- gaps in rock layers; a place where new rock layers meet much older rock
surface beneath them
absolute dating- a method used to determine the age in years of a rock or other object. It
uses the properties of atoms in rocks and other objects to find their ages.
radioactive decay- the atoms of one element break down to form atoms of another element;
used to determine the absolute ages of rocks
half-life- the time it takes for half of the atoms of an isotope to decay (Carbon-14 = 5730
years)
Carbon-14 dating- Carbon-14 has a long half-life (5730 years). Because every living thing has
carbon in it, the amount of carbon-14 remaining in a dead organism can be measured to
determine the age of the fossil.
geologic time scale- a record of the life forms and geologic events in Earth’s history
*scientists believe the mass extinction of the dinosaurs was due to an object from space
crashing into Earth
Honors only:
invertebrates- animals without backbones
vertebrate- animals with backbones