Geologic Time Scale
By studying rock layers and the fossils within
them, geologists can reconstruct aspects of Earth’s
history and interpret ancient environments.
Fossils – remains, traces, or imprints of ancient
Geologic time scale – a record of Earth’s history
from its origin 4.6 billion years ago (bya) to the
Eons – largest time unit
Hadean, Archean, Proterozoic, and Phanerozoic
Precambrian – Earth was formed and became suitable for
Eras – usually 10-100 million years of time
They are defined by the different life forms found in the
Periods – 1-10 million years of time
Named for the geographic region in which rocks or fossils
Epochs – less than 1 million years of time
Scientists use geologic principles to learn the
sequence in which geologic events occurred.
James Hutton proposed an idea in the late 1700s
that tried to explain Earth’s old age using evidence
from erosion and sea-level changes.
His principle is called uniformitarianism –
geologic processes occurring today have been
occurring since Earth formed.
Principles for Determining
Scientists study the order in which geologic
events occurred to understand Earth’s history.
The principle that sedimentary rocks are
deposited in horizontal (flat) layers.
Gravity combined with wind and water spreads
sediment layers evenly.
The principle that in an undisturbed rock
sequence, the oldest rocks are at the bottom and
each consecutive layer is younger than the layer
Kaibab Limestone in the Grand Canyon is the
youngest and the Vishnu Schist at the bottom of
the canyon is the oldest.
An intrusion is a rock that formed from magma as
it solidified and cooled.
The principle states that an intrusion is younger
than the rock it cuts across.
Rock layers sometimes contain pieces of rock from
the layer next to it.
The principle states that the fragments, called
inclusions, in a rock layer must be older than the
rock layer that contains them.
Disconformity – the eroded surface between two
sedimentary rock layers
Nonconformity – when a layer of sedimentary
rock overlies a layer of igneous or metamorphic
Angular unconformity – when horizontal layers
of sedimentary rock are deformed during
mountain building, they are usually uplifted and
The matching of exposed rocks or fossils
from nearby geographic locations.
Fossils provide scientists with a record of the
history of life on Earth.
Many marine fossils are thought to have been
buried in deep sedimentary layers over time and
we are now tapping into a rich petroleum and oil
resources all across the globe.
Mold – forms when
sediments cover the
original hard shell of an
organism and the hard
part is later removed by
erosion or weathering
Cast – a mold that has
been filled with material
to create a representation
of an animal long ago.
Trace fossil – provide information about how
an organism lived, moved, and obtained food
Dinosaur tracks, worm poop, snake trails
Index fossil – fossils that are easily
recognized, abundant, and widely