Fossils and the Rock Record

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Joan Crawford
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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
organisms.
Geologic time scale – a record of Earth’s history
from its origin 4.6 billion years ago (bya) to the
present.
Time Divisions

 Eons – largest time unit
 Hadean, Archean, Proterozoic, and Phanerozoic
 Precambrian – Earth was formed and became suitable for
modern life
 Eras – usually 10-100 million years of time
 They are defined by the different life forms found in the
rocks
 Periods – 1-10 million years of time
 Named for the geographic region in which rocks or fossils
were found
 Epochs – less than 1 million years of time
Relative-Age Dating

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
Relative Age

Scientists study the order in which geologic
events occurred to understand Earth’s history.
Original horizontality
Superposition
Cross-cutting relationships
Inclusions
Unconformities
Correlation
Original Horizontality

The principle that sedimentary rocks are
deposited in horizontal (flat) layers.
Gravity combined with wind and water spreads
sediment layers evenly.
Superposition

The principle that in an undisturbed rock
sequence, the oldest rocks are at the bottom and
each consecutive layer is younger than the layer
beneath it.
Kaibab Limestone in the Grand Canyon is the
youngest and the Vishnu Schist at the bottom of
the canyon is the oldest.
Cross-Cutting Relationships

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.
Inclusions

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.
Unconformities

Disconformity – the eroded surface between two
sedimentary rock layers
Nonconformity – when a layer of sedimentary
rock overlies a layer of igneous or metamorphic
rock
Angular unconformity – when horizontal layers
of sedimentary rock are deformed during
mountain building, they are usually uplifted and
tilted
Correlation

The matching of exposed rocks or fossils
from nearby geographic locations.
Fossil Remains

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.
Fossil Vocabulary

 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.
more vocab…

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
distributed geographically.
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