Interpreting Geologic History

1
Interpreting Geologic History

• Unit 13

2
Relative Dating

• The determination of the age of a rock or event
  in relation to other rocks or events.
• Differs from absolute age.

3
Principle of Superposition

• When viewing layers of sedimentary rock or some
  igneous rock, always assume that the lowest layer
  is the oldest.
• Exceptions to the rule may occur when deformation
  of layers occur

4
Dating Intrusions, Extrusions, and Inclusions

• Intrusion- magma squeezes into preexisting rock
  and hardens. The intrusion is younger than the
  surrounding rock.
• Extrusion- lava flow, volcanic eruption. Younger
  than any rock before it and older than any rocks
  deposited above it.
• Inclusion- rock falls into magma but doesnt melt
  before the magma solidifies.

5
Dating Rock Features

• Rocks are older than any fault
• Sedimentary rocks- sediments are older than the
  cement
• Igneous rocks- mineral crystals vary in age.
  Usually older crystals are bigger.
• Veins- watery mineral solution fills a crack in a
  rock. What is older?

6
Correlation

• Show that rocks or events from different places
  are the same or similar in age.

7
Exposed Bedrock

• Correlation can be determined by looking at the
  continuity of the layers.

8
Index Fossils

• One of the best methods.
• Fossils- remains or evidence of former living
  things
• Index fossils- must have lived over a large
  geographical area, life form must have lived for
  only a short time.

9
Volcanic Eruptions

• Volcanic ash is spread out over large areas.
  Each eruption has unique characteristics

10
Geologic Time Scale

• Based on fossil evidence, geologists have been
  able to divide time into divisions.
• Eons, eras, periods, and epochs. Listed from
  smallest to shortest.
• Precambrian- composed of the earliest eons,
  represents 88 of time.

11
Unconformities

• When reading the rock record, we find buried
  eroded surfaces.
• Indicates uplift occurred in the past, which led
  to erosion. Later, sinking occurred causing new
  layers to be deposited on top.
• Rocks in an unconformity are older than the ones
  above them and younger than the ones below them.

12
Uniformity of Process

• One of the basic principles used to interpret
  history.
• Based on the idea that geologic processes today
  also occurred in the past.

13
Absolute Dating of Rocks using Radioactive Decay

• Ages given in years before present.
• Element- substance consisting of atoms that are
  chemically alike.
• Isotope- an element that exists in several
  varieties. The only difference is in the mass of
  its atoms.
• All mass is generally located in the nucleus.
  Most nuclei are unstable.

14

• Unstable atoms emit particles and EE. This
  process is called radioactive decay. Eventually
  a stable atom will form.

15
Uranium-238

• One of the most important isotopes used. Passes
  through a series of decays and eventually ends up
  as lead-206.

16
Half-Life

• The time required for half the atoms in a given
  mass of an isotope to decay
• Each isotope has its own characteristic
  half-life.
• Not affected by environmental factors.

17
Carbon-14 Dating

• Uranium-238 is used to determine ages of rocks
  that are billions of years old.
• When dealing with younger rocks, carbon-14 (with
  a half-live of 5,700yrs), is used.

18
Evolution of Earth and Life

• The study of fossils and rock layers can tell us
  a lot about the history and evolution of life.

19
Variations in Fossils and Environments

• Chances for fossilization are low and only a
  small percentage of sedimentary rocks are
  studied, therefore many life forms of the past
  probably have not been identified.
• Comparing fossils to similar life forms today we
  can make inferences about past environments. EX.
  Ancient corals have been found in western NY,
  today we know that coral grow in warm shallow
  water, so we can assume that a portion of NY was
  under the ocean and in a tropical environment.

20
Environment Evolution

• Major reason for changes, the movement of plates.
• As they move the latitude changes, therefore
• Colliding plates can cause local changes in
  environments, how?

21
Environmental Evolution and Rock Type

• As environments change the types of rocks that
  can form also change.

22
Fossils and the Evolution of Life

• Species- similar group of organisms that can
  interbreed and produce fertile young.
• Not all members are exactly alike.
• Organic evolution- variations in the genetic code
  give some individuals a favorable trait which
  gives them a better chance for survival.

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• These favorable traits are passed down to other
  generations.
• Eventually organisms with the weaker traits will
  die out and become extinct.
• Fossil record provides evidence for evolution.
  Sometimes fossils from adjacent intervals of
  geologic time show a gradual transition from an
  older species to a newer one.

24
Early Evolution of Earth

• Evidence from the radio active decay of moon
  rocks and meteorites show that Earth formed about
  4.6 billion years ago.Earth heated up and melted
  due to the heat from impact events.
• Earths zones formed due to density
• First known crust formed about 4.2 billion years
  ago. Earths first atmosphere formed.
• Earth cooled and the oceans were formed.

25

• 3.8 billion years ago, single-cell organisms
  existed.
• 3.5 b.y.a. bacteria formed and release oxygen
• Most oxygen reacted with iron to form rust and
  the surface may have looked like Mars does today.
• 2.8 b.y.a. early ozone formed.
• Life began to evolve ever since.