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A Trip through Geologic Time
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Fossils:
1. Help scientists infer how Earth’s surface has changed
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How a Fossil Forms
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Fossils are normally found in sedimentary rock.
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Sedimentary rock is the type of rock that is made of hardened sediment.
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Made of remains of dead organisms and rock particles
Steps to form a fossil
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2. Sediment covers the animal
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4. Weathering and erosion eventually expose the fossil at the surface.
Types of Fossils
Fossil Types
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Petrified fossils
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Trace fossils
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Molds and Casts
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The most common fossils are molds and casts
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A cast is a solid copy of the shape of an organism.
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Petrified Fossils
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The term petrified means “turned to stone”
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Petrified fossils are fossils in which minerals replace all or part of an organism.
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Some of the original substance remains but the minerals have hardened and preserved it.
Carbon Film
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How they form:
1. Sediment buries the organism
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3. Carbon is left behind leaving behind only the carbon from the organism
Trace Fossils
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Example:
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Footprints
There are many things that trace fossil tells us:
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Behavior of an organism
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Two legs or four
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Where the organism lived
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Preserved Remains
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Some processes preserve and organism with little or no change.
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Change Over Time
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Scientists who study fossils are called Paleontologists.
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Collect information from fossils
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Arrange organisms in order of which they lived
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All the information that paleontologists have gathered about past life is called the Fossil Record.
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The fossil record also shows that different groups of organisms have changed over time.
Fossils and Past Environments
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Example coal is found in Antarctica.
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Since coal is formed from the remains of dead plants and animals, this means that at
one time in Earth’s history, Antarctica had to be warm enough to support that type of
climate.
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Scientists also use fossils to learn about changes in Earth’s surface.
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Example:
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If you find a fossil of an aquatic animal in the middle of the desert, you can infer that at
one time, there was a region of water where the fossil was found.
The Relative Age of Rocks
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The absolute age of a rock is the number of years since rocked formed
The Position of Rocks
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Geologists use the law of superposition to determine the relative ages of sedimentary rock
layers
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Determining Relative Age
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To determine relative age, geologists also study extrusions and intrusions of igneous rock
faults and gaps in the geologic record.
Clues from Igneous Rocks
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An extrusion is always younger than the rocks below it
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Below the surface where magma pushes through bodies of rocks and cools is called an
intrusion
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Clues from Faults
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A fault is a break in Earth’s crust
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The fault is always younger then the earth it cuts through
Gaps in the Geologic Record
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The geologic record is not always complete
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An unconformity shows where some rock layers have been lost because of erosion
Using Fossils to Date Rocks
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To date rock layers, geologists first give a relative age to a layer of rock at one location.
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To be useful as an index fossil it must:
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Occurs in many areas
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Index Fossils are useful because they tell the relative ages of the rock layers in which they
occur
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Radioactive Decay
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All matter, including those in rocks, is made of tiny particles is called atoms
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The majority of elements are stable. But some elements exists in forms that are unstable.
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Over time these unstable elements break down or decay by releasing particles and energy in a
process called radioactive decay
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During radioactive decay, the atoms of one element break down to form atoms of another
element
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Radioactive elements occur naturally in igneous rocks. Scientists use the rate at which these
elements decay to calculate the rock’s age
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The amount of radioactive element goes down, but the amount of the new element goes up.
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This rate of decay is the element’s half-life
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This rate of decay is the element’s half-life. The half-life of a radioactive element is the time it
takes for half of the radioactive element to decay
Determining Absolute Age
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Steps to determine absolute age using radioactive dating:
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2. Compare the amount with the amount of the stable element into which the radioactive element
decay
Potassium-Argon Dating
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Potassium-40 is useful in dating the most ancient rocks because of its long half-life
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Half-life of 1.3 billion years
Carbon-14 Dating
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A radioactive form of Carbon is Carbon-14
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Carbon-14 is very useful in dating materials from plants and animals that lived up to about
50,000 years ago.
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For this reason, it cant be used to date very ancient fossil or rocks
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The amount of carbon-14 would be to small
Radioactive Dating of Rock Layers
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Radioactive dating works well of igneous rocks, but not for sedimentary rocks
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Scientists use intrusion and extrusions to date the age of sedimentary rocks
The Geologic Time Scale
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Because the time span of Earth’s past is so great, geologists use the geologic time scale to
show Earth’s history.
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Divisions of Geologic Time
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Geologists have found that over time, major changes in life form at certain times.
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The divisions of the geologic timescale depends on the history of life on Earth
Precambrian
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Geologic time begins with a long span of time called Precambrian time.
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Precambrian Time:
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Was the time at the beginning of Earth
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Divisions of Geologic Time
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After Precambrian Time, the basic units of the geologic time scale are eras and periods.
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The time after Precambrian are divided into three units of time called eras
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Mesozoic Era
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Paleozoic Era
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The Paleozoic
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Lasted approximately 300 million years
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Mesozoic Era
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Mesozoic
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Age of the Reptiles
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Lasted 180 million years
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Cenozoic Era
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Cenozoic
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Ceno – means “recent”
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Divisions of Geologic Time
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The names of many geologic periods come from places around the world where geologists first
described the rocks and fossils of that period.