Transcript The Principle of Superposition

Relative Dating: Which Came First?
Chapter 3 Section 2
p. 64 - 69
Rank the following pictures
from oldest to youngest:
Vocabulary:
1. relative dating
2. superposition
3. geologic column
4. unconformity
The Principle of Superposition p. 64
Geologists try to determine the order in which events have
happened during Earth’s history. The rely on rocks and
fossils to help them in their investigation. Determining
whether an object or event is older or younger than other
objects or events is called relative dating.
Layers of sedimentary rock, like the ones
shown here, are stacked on top of
each other. As you move from top to
bottom, the layers get older. The principle
that states that the younger rocks lie above the older rocks in
undisturbed sequences is called superposition.
Disturbing Forces p. 65
Not all rock sequences are arranged with the oldest layers on
the bottom and youngest layers on top. Some rock sequences
are disturbed by forces within the Earth. These forces can
push other rocks into a sequence, tilt or fold rock layers, and
break sequences into movable parts. Sometimes geologists
even find rock sequences that are upside down!
To make their job easier, geologists combine data from all the
known undisturbed rock sequences around the world. Using
this information, they create a geologic column, which is an
ideal sequence of rock layers that contains all the known fossils
and rock formations on Earth, arranged from oldest to youngest.
Constructing the Geologic Column
How Rock Layers Become Disturbed
Geologists use the relationship between rock layers and the features that cut across
them to assign relative ages to the features and the layers. They know that the
features are younger than the rock layers because the rock layers had to be present
before the features could cut across them. Faults and intrusions are examples of
features that cut across rock layers. Folding and tilting are two types of events that
also disturb rock layers.
Missing Evidence p. 67
Missing rock layers create breaks in rock-layer sequences
called unconformities. An unconformity is a surface that
represents a missing part of the geologic column.
Unconformities also represent missing time – time that was not
recorded in layers of rock.
Most unconformities form by both erosion and nondeposition.
Geologists place unconformities into three major categories:
disconformities
nonconformities
angular unconformities
Disconformities
Disconformities are found where part of a sequence of parallel
rock layers is missing. A disconformity can form in the following
way: A sequence of rick layers is uplifted. Younger layers at the
top of the sequence are removed by erosion and the eroded
material is deposited somewhere else. At some future time,
deposition resumes, and the sediment buries in the old erosion
surface. Disconformities represent thousands to many millions
of years of missing time.
Nonconformities
Nonconformities are found where horizontal sedimentary rock
layers lie on top of an eroded surface of older intrusive
igneous or metamorphic rock. When those rocks are raised
to Earth’s surface, they are eroded. Deposition causes the
erosion surface to be buried. Nonconformities represent
millions of years of missing time.
Angular Unconformities
Angular unconformities are found between horizontal layers of
sedimentary rock and layers of rock that have been tilted or
folded. The tilted or folded layers were eroded before
horizontal layers formed above them. Angular unconformities
represent millions of years of missing time.
Quiz Time!
1. What does the Principle of Superposition state?
Oldest layers are on the bottom and the youngest layer
is at the top.
2. Tell two ways that geologists use the geologic column.
1. – to interpret rock sequences
2. – to identify layers in puzzling rock sequences
3. What is an unconformity?
A surface that represents a missing part of the geologic
column
4. Name three types of unconformities.
disconformity, nonconformity, and angular unconformity
5. Molten rock that squeezes into existing rock and cools is
called a(n):
A. fold
B. fault
C. intrusion
D. unconformity
C - intrusion