Transcript Interpreting Geologic History

Interpreting Geologic History
Unit 13
Relative Dating
• The determination of the age of a rock
or event in relation to other rocks or
events.
• Differs from absolute age.
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
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 doesn’t
melt before the magma solidifies.
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?
Correlation
• Show that rocks or events from different
places are the same or similar in age.
Exposed Bedrock
• Correlation can be determined by
looking at the continuity of the layers.
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.
Volcanic Eruptions
• Volcanic ash is spread out over large
areas. Each eruption has unique
characteristics
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.
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.
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.
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.
• Unstable atoms emit particles and EE.
This process is called radioactive decay.
Eventually a stable atom will form.
Uranium-238
• One of the most important isotopes
used. Passes through a series of
decays and eventually ends up as lead206.
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.
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.
Evolution of Earth and Life
• The study of fossils and rock layers can
tell us a lot about the history and
evolution of life.
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.
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?
Environmental Evolution and
Rock Type
• As environments change the types of
rocks that can form also change.
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.
• 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.
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.
• Earth’s zones formed due to density
• First known crust formed about 4.2 billion
years ago. Earth’s first atmosphere formed.
• Earth cooled and the oceans were formed.
• 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.