Relative Dating
Geologists attempt to determine the
order in which events happened in
Earth’s history. They use ___________ and
________________ as their clues.
_________________ Dating – Determining
whether an object or event is older or
younger than other objects or events.
The Principle of Superposition:
In layers of rock, the layers on the
________________ were deposited first,
therefore they are __________. The layers
at the top were deposited __________,
therefore they are _______________.
Superposition – A principle that states
that younger rocks lie ___________ older
rocks in undisturbed sequences.
“Younger over older”
Original Horizontality – A principle that
states that sediments are deposited
in flat, horizontal layers.
The Geologic Column:
Scientists have combined data from
rock sequences all over the world into
one continuous sequence.
____________________________ – An ideal
sequence of rock layers that contains
all the known fossils and rock
formations on Earth, arranged from
oldest to youngest.
Scientists ________________ real rock
sequences to the geologic column. This
allows scientists to interpret and date the
sequences, especially sequences that
may have been disturbed.
Disturbed Rock Layers
Geologists often find features that cut
through existing rock layers.
These features have to be ______________
than the rock layers because the rock
layers had to be present before the
features could cut across them.
Fault
Intrusion
A break in the Earth’s crust
along which blocks of crust
slide relative to one another.
Molten rock from the Earth’s
interior that squeezes into
existing rock and cools.
If rock layers are not ___________________,
then something must have disturbed
them (because of original horizontality).
The events that disturbed the rock
layers must have occurred _____________
the rocks were formed.
Folding
Tilting
When rock layers bend and
buckle from Earth’s internal
forces.
When internal forces in the
Earth slant rock layers
without folding them.
Gaps in the Record – Unconformities:
Missing rock layers create gaps in rocklayer sequences called _________________.
Unconformity – A break in the geologic
record created when rock layers are
eroded or when sediment is not
deposited for a long period of time.
Unconformities essentially represent
“missing time”.
Unconformities can occur because
sediment was never deposited
(________________________) or sediment
was removed (_______________).
Types of Unconformities:
Disconformity – Exists where part of a
sequence of parallel rock layers is
________________. While often hard to
see, a disconformity is the most
common type of unconformity.
Nonconformity – Exists where sedimentary
rock layers lie on top of an _____________
surface of nonlayered igneous or
metamorphic rock.
Angular Unconformity – Exists between
horizontal rock layers and rock layers
that are _____________ or _______________.
The tilted or folded layers were eroded
before horizontal layers formed above
them.
Nonconformity
Angular Unconformity