Transcript Earth Science 12.1 Discovering Earth’s History: Geologic Time

Earth Science 12.1 Discovering Earth’s History: Geologic Time
Discovering
Earth’s History
Studying Earth’s History


For geologists today, one goal of
geology is to interpret Earth’s
history. Geologists do this by
studying the rocks of Earth’s crust;
especially the sedimentary layers.
When studying the Earth’ s history,
geologists make use of three main
ideas
 The rock record provides evidence
of geologic events and life forms
of the past
 Processes observed on Earth in
the present also acted on it in the
past
 Earth is very old and has changed
over geologic time
Studying Earth’s History


Scientists in Europe and the British
Isles began to develop these ideas
during the 1700s. They wondered
about the processes that the Earth’s
landscape features and they noticed
that sedimentary rocks were laid
down in layers. They thought about
how much time it must have taken
for these layers to form.
In the late 1700s, James Hutton
published his Theory of the Earth.
In this work, Hutton put forth the
idea of uniformitarianism which
simply states that “the physical,
chemical and biological laws that
operate today also operated in the
geologic past.”
Uniformatarianism



Uniformatarianism means that the
process that we observe today have
been at work for a very long time;
hundreds of millions of years.
To understand the geologic past, we
must first understand present day
processes.
It is important to remember that
although many features of our
landscape may seem to be unchanging
within our lives; when viewed over a
scale of millions of years, they are
actually constantly changing.
Principals of Relative Dating


In relative dating, geologists follow
several principals:
 the law of superposition,
 the principal of original
horizontality,
 and the principal of cross-cutting
relationships.
These principals help scientists
determine the sequence in which
geologic events occur.
Law of Superposition
Law of Superposition:



Nicolaus Steno, a Danish anatomist,
geologist and priest,(1636-1686)
made observations that are the basis
of relative dating.
Based on his observations, Steno
developed the Law of Superposition.
The law of superposition states that
in a sequence of sedimentary rock
layers, each layer is older than the
layer above it, and younger than the
layer below it.
Law of Superposition
Law of Superposition:


Although it may seem obvious
that a rock layer could not be
deposited unless it had
something older than it for
support, it was not until 1669
that Steno stated the principal.
Applying the law of superposition
to the rock layers exposed in a
section of the Grand canyon, we
can easily place the layers in
their correct geological order.
Law of Superposition
Principal of Original Horizontality:




Steno also developed the principal of
original horizontality.
The principal of horizontality states
that layers of sediment are generally
deposited in a horizontal position.
If you see rock layers that are flat,
they are undisturbed and are still in
their original position.
If you see layers that are tilted
vertically or are bent and folded;
they have undergone change since
they were originally formed.
Principal of Cross-Cutting relationships
Principal of Cross-Cutting Relationships:



Later in time, geologists developed
another principal used in relative
dating.
The principal of cross-cutting
relationships states that when a
fault cuts through rock layers, or
magma intrudes into other rocks and
hardens, than the fault or intrusion
is younger than the rocks around it.
Simply, a fault that splits layers of
sedimentary rock must have
happened after the layers were first
formed.
Reading the Rock Record
Reading the Rock Records:
 Today, geologists apply Steno’s
principals to interpret or read the
order of rock layer geological events.



Geologists also determine how the
rocks in one area are related to
similar rocks in other places.
Methods that geologists use to
interpret the rock record include
the study of inclusions and
unconformities.
Geologists also correlate rock layers
at different locations to compare
evidence.
Reading the Rock Record
Reading the Rock Records:
 By studying rocks from many
different places worldwide,
geologists can construct a model of
the rock record called a geologic
column.


A geologic column is made up from
rocks arranged according to their
relative edges.
The oldest rocks are at the bottom
of the column and the youngest
rocks are at the top.
Inclusions
Inclusions:




Sometimes, the study of inclusions
can help the rock dating process.
Inclusions are pieces of one rock
unit that are contained within
another rock unit.
The rock unit next to the one
containing the inclusion must have
been their first in order to provide
the rock fragments.
Therefore, the rock unit containing
the inclusions is the younger of the
two.
Unconformities
Unconformities:




Throughout Earth’s history, the
deposition of sediment has been
interrupted again and again. Nowhere
is Earth’s rock record complete.
A surface that represents a break in
the rock record is called an
unconformity.
An unconformity indicates a long
period during which deposition
stopped, erosion removed previously
formed rocks, and than deposition
resumed.
Unconformities help geologists
identify what intervals of time are
not represented in the rock record.
Unconformities: 3 Types

There are three basic types of
unconformities:
 Angular unconformities
 Disconformities
 Nonconformities
Unconformities: 3 Types

In an angular unconformity, layers of
sedimentary rock form over older
sedimentary rock layers that are
tilted or folded.
Unconformities: 3 Types

In a disconformity, two sedimentary
rock layers are separated by an
erosional surface.
Unconformities: 3 Types

In a nonconformity, an erosional
surface separates older
metamorphic or igneous rocks from
younger sedimentary rocks.
strata rest upon older, metamorphic or
igneous rocks
(Precambrian-Cambrian contact Alexander Bay, NY)
Correlating Rock layers
Correlating Rock Layers:


Interpreting unconformities helps
geologists read the rock record in
one location.
Geologists use correlation to match
rocks of similar age in different
locations.
Correlating Rock layers
Correlating Rock Layers:


Geologists often correlate layers by
noting the position of a distinctive
rock layer in a sequence of layers.
If geologists find a distinctive rock
layer in another location, they may
infer that the same layer once
covered both locations.
Correlating Rock layers
Correlating Rock Layers:



By correlating the rocks from one
place to another, it is possible to
create a more complex view of the
geographical history of a region.
For example, by correlating the data
from two sites on the Colorado
Plateau (one in southern Utah and
one in Arizona) scientists can piece
together the geologic history of the
plateau by cross examining the
information from each site.
Correlation reveals a more complete
and complex picture of the rock
record than can be found from a
single site.