Earth

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Earths History in Fossils

• P.Lobosco

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Fossils

• A fossil if the remains or evidence of a living
  thing. A fossil can be the bone of an organism
  or the print of a shell. A fossil can be a
  tunnel lefts by a worm. The most common fossils
  are bones, shells, pollen grains and seeds.
• Most fossils are buried in sediments. Plants and
  animals that live near water are preserved more
  often. Most fossils are found in sedimentary rock.

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Fossil Identification

• Fossils are identified according to the process
  by which it was formed.
• Petrification
• Molds and Casts
• Imprints
• Preservation of Entire Organism
• Trace Fossils

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Petrification

• Petrification means turning into stone. When the
  dinosaurs died, the soft parts of their bodies
  decayed quickly. Only the hard parts, the bones,
  were left. Many of these bones were buried in
  sediments of mud and wet sand. The water
  dissolved minerals in the mud and as the water
  flowed through pore in the bones, minerals were
  deposited. The water evaporated and the bones
  were turned to stone.

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Molds and Casts

• Two types of fossils are formed when an animal or
  plant is buried in sediments that harden into
  rock. If the soft parts of the organism decay
  and the hard parts are dissolved by chemicals an
  empty space will be left in the rock. The empty
  space, called a mold, has the same shape as the
  organism. Sometime the mold is filled in by
  minerals and they harden to form a cast in the
  same shape as the organism.

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Imprints

• Sometimes a fossil is formed before the sediments
  harden into rock. Thin objects, such as leaves
  and feathers, leave imprints or impressions in
  soft sediments such as med. When the sediments
  harden into rock, the imprints are preserved as
  fossils.

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Preservation of Entire Organism

• Freezing prevents substances from decaying. Some
  animals have been preserved by freezing. Wooly
  mammoths have been found in large blocks of ice.
  A furry rhinoceros was found in the loose frozen
  soil in the arctic. When the resin form
  evergreen trees hardens it forms amber. Insects
  have been trapped and preserved in amber. Some
  animals have been found in tar pits.

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Trace Fossils

• Trace fossils are fossils that reveal much about
  an animals appearance without showing any part
  of the animal. Trace fossils area the marks or
  evidence of animal activities. Tracks, trails,
  footprints, and burrows are trace fossils.

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Interpreting Fossils

• Fossils indicate that many different life forms
  have existed at different times throughout the
  Earths history. Some scientists believe that
  for every organism that exists today, 100 have
  become extinct. When fossils are arranged
  according to age, they show that living things
  have evolved. Fossils also indicate for the
  Earths surface has evolved. Fossils give clues
  about past climate.

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A History in Rocks and Fossils

• The Law of Superposition states that in a series
  of sedimentary rock layers, younger rocks
  normally lie on top of older rocks. This idea
  was proposed by James Hutton in the 18th century.
  The processes of weathering, erosion and
  deposition have always acted on the Earths
  surface. Scientists use this law to determine
  whether a fossil or layer or rock is older or
  younger than another fossil or layer of rock.

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Index Fossils

• The law of superposition helps scientists put in
  order the record of Earths past for one
  particular location. Index fossils are fossils
  of organisms that lived during only one short
  period of time. Scientists assume that index
  fossils of the same type of organism are all
  nearly the same age. So a layer of rock with one
  type of index fossil in it is close in age to
  another layer of rock with the same type of index
  fossil in it.

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Unconformities

• Sedimentary rock layers and the fossils found
  within them may be disturbed by powerful forces
  within the Earth. The rock layers may be folded,
  bent or twisted. Sometimes older layer are
  uplifted to the Earths surface. At the surface,
  the exposed rocks are weathered and eroded.
  Sediments are then deposited on top of the eroded
  surface of the older rocks. The deposited
  sediments harden to from new horizontal
  sedimentary layers. The old eroded surface
  beneath the newer rock layer is called an
  unconformity. There is a wide gap in the ages of
  the rock layers above and below an unconformity.

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Faults

• A break or crack along which rocks move is called
  a fault. The rock layers on one side of a fault
  may shift up or down relative to the rock layers
  on the other side of the fault. Because faults
  can occur only after rock layers are formed, rock
  layers are always older than the faults they
  contain. The relative age of a fault may be
  determined form the age of the sedimentary rock.
  Scientists can determine the forces that have
  changed the Earths surface by examining the
  faults in rock layers.

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Intrusions

• The relative ages of igneous rock formations can
  also be determined. Magma often forces its way
  into layers of rocks. The magma hardens in the
  rock layers and forms an intrusion. An intrusion
  is younger than the sedimentary rock layers it
  passes through.

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Extrusions

• Sometimes magma reaches the surface of the Earth
  as lava and hardens. Igneous rock that forms on
  the Earths surface is called an extrusion.
  Extrusions are younger than the rock layers
  beneath them.

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Radioactive Dating

• The discovery of radioactive elements in 1896 led
  to the development of an accurate method of
  determining the absolute age of rocks and
  fossils. An atom of radioactive element has an
  unstable nucleus, or center, that breaks down or
  decays. During radioactive decay, particles and
  energy called radiation are released by the
  radioactive element. As some radioactive
  elements decay, they form decay elements. A
  decay element is stable. This breakdown occurs
  at a constant rate.

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Half-Life

• Scientists measure the decay rate by a unit
  called a half-life, the time it takes for half of
  the radioactive element to decay. If certain
  radioactive elements are present in a rock or
  fossil, scientists can find the absolute age of
  the rock or fossil. Many different radioactive
  elements are used to date rocks and fossils. One
  radioactive element used to date the remains of
  living things is carbon-14. Nitrogen is the decay
  element of Carbon 14.

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Other Methods of Absolute Dating

• There are other methods of absolute dating.
• Varve
• Splitting or fission of the nuclei of radioactive
  atoms

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Varve

• A varve is sediment that shows a yearly cycle.
  These sediments are often deposited in glacial
  lakes. Each year a light-colored sandy layer is
  deposited in the summer and a dark-colored layer
  is deposited in the winter. Be counting the
  pairs of layers it is possible to tell haw many
  years a a glacier was in a particular place
  before it disappeared.

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Splitting of Radioactive Atoms

• The splitting or fission of the nuclei of
  radioactive atoms is another method of dating.
  The high-energy particles that are split off
  leave microscopic tracks that can be counted.
  The older the rock, the greater the number of
  tracks. This method can be used to date moon
  rock samples and meteorite samples as well as
  samples of Earth rocks.

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The Age of the Earth

• Scientists use radioactive dating to determine
  the age of rocks. By finding the age of rocks,
  the can estimate the age of the Earth.
  Scientists have found some rocks in South Africa
  that are more than 4 billion years old.
  Radioactive dating of moon rocks brought back by
  the Apollo missions show them to be 4 to 4.6
  billion years old.

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