Transcript EARTH AND SPACE SCIENCE

EARTH AND
SPACE SCIENCE
Chapter 9 A View of Earth’s
Past
9.1 Geologic Time
9.1 Geologic Time Objectives
• Summarize how scientists worked
together to develop the geologic
column.
• List the major divisions of geologic time.
Introduction
• The Earth’s surface is constantly changing –
mountain chains are formed and eroded,
oceans rise and fall, and continents move
about.
• The changes in the Earth’s surface affect life
living on that surface.
• Evidence of change in life on Earth over time
is recorded in the layers of sedimentary rock.
• Scientists have developed a geologic time
scale to describe the sequence of changes in
life on Earth.
The Geologic Column
• The law of superposition states that in a column of
sedimentary rock, the lower the rock unit, the older it
is in relation to the others.
• As one ascends through the rock units, the rock units
get progressively younger.
• In the 19th century, scientists were able to determine
relative ages of sedimentary rock layers around the
world by applying the law of superposition.
• No single area on Earth contains the entire segment
of geologic time, so observations must be combined
to create a composite idealized arrangement of rock
units.
The Geologic Column
• A geologic column is an
ordered arrangement of
rock layers that is
based on the relative
ages of rocks and in
which the oldest rocks
are at the bottom.
• Rock layers in a
geologic column may
be distinguished by the
type of rock from which
they are made and the
types of fossils that they
contain.
The Geologic Column
• A geologic column could be viewed as a
timeline of Earth’s history.
• Fossils found in the sedimentary rock layers
of the geologic column follow the law of
faunal succession.
• The law of faunal succession states that as
fossil fauna become progressively younger, it
becomes more familiar.
• The older the fossil fauna, the more dissimilar
it is when compared to modern fauna.
The Geologic Column
• When first developing the geologic rock column,
some scientists would attempt to apply average
depositional rates to estimate ages.
• Later, radiometric dating methods allowed scientists
to determine much more accurate ages for rock units.
• Rock layers that cannot be dated using radioisotopes
are assigned ages based on their position in the
geologic column.
• So when a fossil is found in a particular sedimentary
layer, a relatively close absolute date can be
assigned to the fossil based on the rock unit from
which it was collected.
Divisions of Geologic Time
• The geologic time scale is divided into eons,
eras, periods, and epochs.
• The units of time are not divided evenly.
• The Earth’s history is marked by changes in
Earth’s surface, climate, and organisms.
• The divisions of geologic time are marked
primarily by changes in life on Earth.
Divisions of Geologic Time
• The Eon is the largest geologic unit of time.
• There are four Eons.
–
–
–
–
Hadean
Archean
Proterozoic
Phanerozoic
• The first three eons are commonly lumped
into the designation Pre-Cambrian time – the
first 4 billion years of Earth’s history.
• Very few fossils exist in Pre-Cambrian rocks.
Divisions of Geologic Time
• Eons are divided into eras.
• The Phanerozoic Eon is divided into three
eras.
– Paleozoic (292 million years)
– Mesozoic (183 million years)
– Cenozoic (65 million years)
• Paleozoic rocks contain an abundance of
marine (and later terrestrial) fossils.
• Eras are divided into periods which are
usually named after the location where typical
fossils were first discovered for this age.
Divisions of Geologic Time
• Each period can be
further subdivided into
smaller time units called
epochs.
• Epochs may be divided
into even smaller units
called ages.
• Ages are defined by the
occurrence of distinctive
fossils found in rocks of
that particular age.
References
• Bardstown Reef – Ashley Allen photo
• Grand Canyon Superposition http://www.calstatela.edu/faculty/acolvil/ge
o_time.html
• Geologic Correlation http://rst.gsfc.nasa.gov/Sect2/Sect2_1b.ht
ml
• Eocrinoids – Ashley Allen photo
References
• Elrathiella alabamensis – Ashley Allen photo
• Hardouinia bassleri - Ashley Allen photo
http://www.squali.com/fossili/squalicorax.
php
• Carcharodon carcharias – Ashley Allen photo
• Geologic Time Scale http://gator.gasd.k12.pa.us/~dpompa/Mini
%20Lecture.html
References
• Archimedes – Ashley Allen photo
• Geologic Time Scale (Ages) http://geowords.com/histbooknetsca
pe/k09.htm
• Dating Fossils Using Relative Data and
Absolute Dating Techniques http://www.talkorigins.org/faqs/datin
g.html