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Chapter 17
How we Study the Earth
Geologic Time
• Geologic Time is the time in terms of the
Earth’s history of the Earth. Scientists now
believe that the Earth is 4.54 billion years old.
• The information found in fossils, rocks, and ice
cores have given scientists insight into the
Earth’s history.
• Fossils are the recognizable remains or
impressions of an organism that lived in the
past.
Geologic Time (CONT)
• The Geologic Timescale is a calendar of Earth’s
history. It is based mostly on fossil evidence
found in rocks and ice cores. The Geologic
Timescale covers a very long time period and it
shows the diversity of Earth’s past and the
organisms that have lived and died over different
periods based on the fossil evidence of mass
extinctions. Scientists use tools and dating
techniques to construct a picture of the past and
to study climate, landforms, geologic events and
organisms.
Geologic Timescale
Era
Period
Millions of Years Ago
Major Evolutionary
Events
Cenozoic
Quaternary
Tertiary
2
65
Early Humans
First placental
mammals (rats)
Mesozoic
Cretaceous
Jurassic
Triassic
135
180
230
Flowering plants
First birds/mammals
First dinosaurs
Palezoic
Permian
Carboniferous
Devonian
Silurian
Ordovician
Cambrian
286
320
405
425
505
545
Conifers dominant
First reptiles
First amphibians
Land plants/animals
First fishes
Invertebrates
2000 Life diversifies
3000
3500
Eukaryotes
Prokaryotes
Live evolves
Precambrian
Divisions of Time
• Eons: 2 or more periods. The largest division
of time in the Geologic Timescale
• Era: 2 or more periods
• Period: 2 or more epochs
• Epoch: smallest division of time in the
geologic timescale.
Extinction
• Scientists will sometimes define eras based on
the extinction of a species. Extinction occurs
when the last organism of a species dies.
• Causes of extinction: increased competition
for resources, human interaction, climate
change, loss of food, increased predation,
disease or habitat loss.
• Extinction: can be a slow or fast
Extinction (cont)
• A mass extinction occurs when a large
percentage of the organisms on Earth go
extinct. These are usually a fast process
(couple of 1000 of years). Last one occurred
about 65 million years ago and led to the
destruction of the dinosaurs.
The Fossil Record
• Fossils can be also useful in studying geologic
time. A fossil is the recognizable remains or
body impressions of an organism that lived in
the past. The fossil record refers to all fossils
that have been found since the study of fossils
began.
• Some fossils are recognized as unique to a
certain time period, this is called an index
fossil.
The fossil record (Cont)
• When an index fossil is found in sedimentary
rock, the age of the rock is assumed to be the
same as the age of the fossil.
• Formation of fossils: most fossils come from
organisms that have hard body parts like bones
and shells. These bones and shells will fossilize
easier than soft body parts. Fossils are more
likely to form when animals are buried very
quickly. Only occasionally are conditions right to
form fossils.
Formation of fossils cont
• Catastrophic events like floods, mudslides,
earthquakes, and ask deposition from
volcanoes usually lead to the formation of
fossils. It is believed that fossils only make up
about .1% of all organisms that have lived on
the planet.
Types of fossils
• 1. Mold: When all that is left is an organismshaped hole in the rock.
• 2. A cast or internal mold is formed when
sediments or minerals fill the internal cavity of
an organism.
Types of fossils (Cont)
• 3. Body fossils: when you have a fossil of the
whole body usually happens with insects
found in amber. This is a very rare type.
• 4. Trace fossils: footprints, bite marks, and
excrement left by an organism. This is the
most common type.
The Age of Geologic Events
• Scientists use techniques that allow them to
“read” the rock deposits in the sedimentary rock
layers.
• How and when a geologic event occurred can be
determined in a number of ways. Relative dating
and absolute dating are 2 techniques used to
determine the age of rocks. Scientists use the
techniques along with ice core data, volcanic
eruption and climate changes to put the geologic
events in a logical sequence.
Relative Dating
• When it come to rock strata (layers of
sedimentary rock), each layer is older than the
layer above it. Layers get older as you go
downward. This kind of dating is called relative
dating because it allows you to know the order in
which the layers were formed but not the actual
age.
• Law of superposition: states that each
sedimentary rock unit is older than the rock unit
above it and younger than the unit below it.
Relative Dating (CONT)
• Sometimes the layers of rock and sediment
are not laid down continuously. A break in the
sedimentary geologic record is known as
unconformity.
Absolute Dating
• In absolute dating, scientists use the natural
decay of radioactive isotopes (special types of
atoms that fall apart on predictable timescales).
The unstable isotopes, called radioactive parents,
decay to form other more stable elements called
daughter products.
• The calculation of the ratio of parent to daughter
products is called radiometric dating.
• The time it takes for one half of the original,
unstable atom to decay into a daughter product is
called a half-life.
Absolute Dating (Cont)
• Example of half-life
– Let’s say a rock layer contains a million atoms of
the parent isotope with a half-life of 5 million
years. At this point the number of daughter
products is zero because decay has not occurred.
At the end of 5 million years, half of the parent
isotopes will decay or change into daughter
products. So you will have 500,000 parent
isotopes and 500,000 daughter products. After 5
million more years, how many parent isotopes and
daughter products will you have?
Ice Cores
• The glaciers and polar ice caps contain the
majority of fresh water on Earth. Massive
amounts of snow fall on Antarctica each year
and the snow brings with it particles of matter
contained within the Earth’s atmosphere at
the time of precipitation. This means that ice
sheets contain another sort of geological
record.
Ice cores (CONT)
• Ice cores are taken to extract information
contained within the ice sheets. They are long
cylinders of ice obtained from glaciers and ice
sheets by a cutting device. Scientists looks for
trends and changes in temps in the ice cores.
They measure the levels of certain atmospheric
components. They use this data to make
conclusions about climate patterns, geological
events and the more recent effects of human
activity. They compare the data collected from all
around the world.
Volcanic Eruptions
• Sulfates found in ice cores are used to determine
the intensity and timing of volcanic eruptions.
Volcanic eruptions spew ash and sulfuric gases
into the atmosphere. This blocks out sunlight
and lowers temps worldwide. Ice cores are one
way that scientists can determine the occurrence
and impact of volcanic eruptions. Another way is
to identify the layer of volcanic ash in the rock
layers with the fossils and look for changes in the
fossil record.
Climate Variability
• Climate change is a term to describe significant
long-term changes in Earth’s climate. Earth’s
climate has changed many times over the past.
Studying past climates can help scientists
determine how humans may be influencing the
climate today.
• Global warming is part of a natural Earth
warming that is called the greenhouse effect.
The greenhouse effect traps solar heat within the
Earth’s atmosphere.
Greenhouse effect (cont)
• Humans have altered this natural warming
cycle. Global warming is the term used to
describe the measured rise in the Earth’s
temps. It describes the changes in modern
climate. It comes from a rise in greenhouse
gases.