Geologists have developed a
set of principles to compare
the age of rock layers.
 Relative age is the age of rocks and geologic features
compared with other rocks and features nearby.
 There is order in rock formations.
 This principles states that in undisturbed rock layers,
the oldest rocks are on the bottom.
 Unless some force disturbs the layers after they were
deposited, each layer of rock is younger than the layer
below it.
 Most rock-forming materials are deposited in
horizontal layers.
 Sometimes rock layers are deformed or disturbed after
they form.
 Even though they might be tilted, all the layers were
originally deposited horizontally.
 This principle of relative-age says that sediments are
deposited in large, continuous sheets in all lateral
directions.
 The sheets continue until they thin out or meet a
barrier.
 This is a piece of an older rock that becomes part of a
new rock is called an inclusion. In the following
picture magma intrudes into the rock layers forming a
dike. The dike contains inclusions from the rock
layers.
 The dike is younger
than the rock layers.
 The fault line is younger
than the dike.
 Sometimes forces within Earth’s crust cause rock
formations to break or fracture.
 If one geologic feature cuts across another feature, the
feature that does the cutting is younger, and the
feature being cut is older.
 An unconformity is a surface where rock has eroded away;
producing a break, or gap, in the rock record.
 An unconformity indicates that the sediment deposition
was not continuous.
 An unconformity does represent a gap in time. It could
represent a few hundred years, a million years, or even
billions of years.
 Matching rocks and fossils from separate locations is
called CORRELATION
 The rock layers correlate if there are similarities in the
rock types or similarities in the fossils they contain.
 Index fossils represent species that existed on Earth for a
short length of time, were abundant, and inhabited many
locations.
 Fossils that lived on Earth for long periods of time are not
helpful. The most useful fossils represent species, like
certain trilobites, that existed for only a short time in many
different areas on Earth.