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Fossils: Rock’s Timekeepers
Outcome: develop a chronological time scale of major events in Earth’s history
Fossils: Rock’s Timekeepers
 Usually when an animal dies, its decaying flesh is consumed by microorganisms,
scavenger animals and birds, as well as insects. This breakdown is aided by sun
and run until nothing remains.
 Fossils form when these agents (sun and rain) can not act on the organism
 Fossils are rocklike casts, impressions, or actual remains of organisms that were
buried after they died.
 Animals or plants can be preserved or fossilized in a number of ways
 The organism can be preserved entirely. For example the Wooly
Mammoths found in frozen Siberian mud
 This type of fossilization is fairly recent since the Ice Age
 Some, or hard, parts of the original organisms are left. Los Angeles tar pits
have preserved some bones of trapped organisms
 Plant life can also be fossilized
 Leaves falling into fine mud are well-preserved due to lack of
oxygen, and quick burial by more mud
 A very detailed impression of the leaf remains including carbon
from the original leaf
 Petrifaction is another form of fossilization by which once living substance,
like bone, is buried in sedimentary rock
o Water seeps through the rock, dissolving minerals from surrounding
rock one molecule at a time
o The mineral water changes the molecular structure of the bone into a
new mineral substance such as silica
o This type of fossilization that gives geologists the best window into
pre-Ice Age Earth
o Can also be covered by volcanic ash, mud or quick sand in which it
will eventually become sedimentary rock
Age of Fossils
 To determine the age of rocks, geologists use fossils
o Both absolute age (the time in years) and relative age (the order of
events)
o Absolute age is rarely obvious therefore geologists must use evidence,
such as that taken from fossils, to determine relative age
o Because every period on Earth had its own characteristic set of life forms,
the fossils found in layers of sedimentary strata provide vital information
about the age of the host rock
 Paleontologists are scientists who study ancient life forms
o They are experts at recognizing and identifying fossils and using them to
determine the relative age of rocks, and to estimate their approximate
absolute age
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