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A Trip through Geologic Time. Chapter 8. How a Fossil Forms. Fossils are the preserved remains or traces of living things Most fossils form when living things die and are covered by ___________________. As it slowly hardens into rock, it preserves the shape of the organisms.
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A Trip through Geologic Time Chapter 8
How a Fossil Forms • Fossils are the preserved remains or traces of living things • Most fossils form when living things die and are covered by ___________________. As it slowly hardens into rock, it preserves the shape of the organisms. • This is why fossils are usually found in ____________________ rock. • Remember, sediments are particles of rock and soil that are moved around by __________________. sediment sedimentary erosion
How a Fossil Forms Most fossils form near quiet water where sediment builds up easily, such as a lake, pond, or shallow sea. When an organism dies, the soft parts decay or are eaten quickly. That is why it is usually only the hard parts of an organism that leave fossils behind.
How a Fossil Forms For a fossil to form, the remains of an organism must be protected from decay. Types of fossils found in rock include: molds and casts, petrified fossils, carbon films, and trace fossils. Other fossils form when an organism is preserved in a substance such as: tar, amber, or ice.
Molds and Casts • These are the most common types of fossils • Both copy the shape of ancient organisms • A mold is a ______________ area in sediment in the shape of an organism This forms when the hard part of an organism is buried in sediment and later decays, leaving the shape of the organism behind hollow
Molds and Casts • A cast is formed when water seeps into a mold and deposits minerals and sediment there • These harden and form a cast, a solid copy of the organism
Petrified Fossils • A petrified fossil occurs when the remains of an organism have been petrified, which means _____________________________. • In these fossils, minerals replace all or part of an organism • This happens when water with minerals enters the remains of an organism. • Eventually the minerals harden, filling in the spaces Some of the original organism is left, but it is preserved in a hardened mineral shell “turned into stone”
Carbon Films • This is an extremely thin coating of carbon on rock • Remember: all living things are made of ________________ • As organisms decay, gases evaporate, leaving carbon behind. • Eventually, only a thin film of carbon remains • This can preserve delicate structures, such as leaves or insect bodies carbon
Trace Fossils • Most fossils preserve the shapes of ancient animals and plants • Trace fossils provide evidence of ancient organism’s ____________________. • An example of this is a footprint. • When the footprint was made, it become buried in layers of sediment, which hardened over many years, leaving the shape behind These fossils can provide clues to animal behavior. behavior
Preserved Remains • Some processes preserve the remains of animals with little or no change. • ______________ is a sticky oil that seeps from the Earth’s surface. • In some areas, water used to cover this substance. Animals that came to drink the water could become stuck and die. The tar then soaks into their bones, preserving them from decaying Tar
Preserved Remains • Ancient organisms have also been preserved in _______________, which is hardened tree sap. An insect can land on a tree and become trapped. More tree sap then covers it, keeping it from decaying. • Freezing can also preserve remains, such as wooly mammoths in Siberia and Alaska Freezing stops the decay so much it has even preserved hair and skin of these animals. amber
Change Over Time paleontologists • Scientists who study fossils are called ____________________. • They gather fossils from all over the world trying to learn about the past. • They also ________________ organisms by grouping similar ones together and putting them in order from when they lived, earliest to latest. • Together, all the information they have gathered about past life is called the ___________________________. • This provides evidence about the history of Earth and how different organisms have changed over time. classify fossil record
Fossils and Past Environments • Fossils also provide evidence about the past climate of an area. • For example, ______________ has been found in Antarctica, but it only forms in warm, swampy areas. This indicates that Antarctica was once much warmer than today. coal
Change and the Fossil Record • The fossil record reveals a surprising fact, fossils occur in a particular _________________. Older rocks contain fossils that are simpler, whereas younger rocks contain fossils of more complex organisms. order
Change and the Fossil Record The fossil record provides evidence to support the theory of evolution. • Evolution is the gradual change in living things over long periods of time. • The fossil record shows that millions of types of organisms have evolved, but many others have become extinct. • An animal is extinct if it no longer exists on Earth.
The Position of Rock Layers • The relative age of a rock is its age compared to the ages of other rocks. • The ________________ age of a rock is the number of years since the rock formed. It may be impossible to know a rock’s exact age but sometimes geologists can determine a rock’s age to within a certain number of years. absolute
The Position of Rock Layers Using the law of _______________________, geologists try to determine the relative age of rocks. According to this law, in horizontal sedimentary rock layers, the ________________ layer is at the bottom. Each higher layer is younger than the one below it. superposition oldest
Determining Relative Age • There are other clues to determining the age of rocks Scientists also study extrusions and intrusions of igneous rock, as well as faults and gaps in the geologic record.
Clues from Igneous Rock Igneous rock forms when magma or lava cools and hardens • Lava that hardens on the surface is called an ____________________. • It is always younger than the extrusion below it. • Beneath the surface, magma may push into a body of rock, forming an ___________________. • It is always younger than the rock layers around and beneath it. Geologists study intrusions and extrusions to determine the relative age of the rocks around them. extrusion intrusion
Clues from Faults break • A fault is a _________________ in the Earth’s crust. • Forces inside the Earth cause movement of the rock on opposite sides of a fault. • A fault is always ___________________ than the rock it cuts through. • To find the relative age of a fault, geologists find the relative age of the youngest layer cut by a fault. Movement along faults can make it difficult for geologists to determine age of rocks because the layers shift. younger
Gaps in the Geologic Record The geologic record is not always complete, it sometimes has gaps. • Deposition lays down sediment, but erosion can wear away layers of rock, exposing older rock. • Then new rock is laid down, leaving a gap in the geologic record. The surface where new rock layers meet a much older layer is called a _____________________ unconformity
Using Fossils to Date Rocks • To date rock layers, geologists give a relative age to a layer of rock at one location, and then they can give the same age to matching layers of rock at other locations. • Certain fossils, called ___________________________ help geologists match rock layers. These types of fossils must be wide spread and have only existed for a short period of time to be useful. index fossil
Using Fossils to Date Rocks Geologists use certain organisms as index fossils because they are widespread, make good fossils, and had lots of different species that only lived a short time. • One example is an _____________________. • Geologists can identify the species and use the time period it was known to live in to show the time period of fossils around it. ammonite
Radioactive Decay Rocks are a form of matter; All matter is made up of atoms. When all the atoms in a particular type of matter are the same, the matter is an __________________. Most are stable; They do not change under normal conditions. • Some are unstable, however, and over time they break down by releasing particles and energy in a process called radioactive decay. These elements that are unstable and can break down are called radioactive. element
Radioactive Decay • Radioactive elements occur naturally in igneous rock, scientists use the rate at which they decay to tell the rock’s age. • The rate of decay of a radioactive element is constant, and specific to each element. • The amount of time it takes for half of the radioactive atoms to decay is called the _____________________. Half-life
Determining Absolute Ages Radioactive dating • Geologists use _______________________ to determine the absolute age of rocks. • Scientists first determine how much radioactive element is left in a rock. • Then they compare it to how much radioactive material is in the stable element at the end of its decay.
Potassium-Argon Dating Scientists often date rocks using _______________________. • This type of potassium decays to a stable argon-40 and has a half life of _______________________________. This is a useful radioactive dater because it has such a long half life. Potassium-40 1.3 billion years
Carbon-14 Dating • All plants and animals contain carbon, some of it is in the radioactive form carbon-14 • As plants and animals grow, carbon is added to their bodies, including carbon-14. When a plant or animal dies, the carbon starts decaying into _____________________. Nitrogen-14
Carbon-14 Dating • To determine the age of a sample, scientists measure how much carbon-14 is left. • This determines __________________________. • Carbon-14 has been used to date skeletons and pieces of wood. • This is used for dating materials that lived up to _____________________ years ago. Carbon-14 has a very short half-life of only 5,730 years, so material that is older does not have enough left to accurately age it. Absolute age About 50,000
Radioactive Dating of Rock Layers • Radioactive dating does not work well for _____________________ rock because the layers are made from particles of rock from a lot of different sources. To age sedimentary rock, scientists age the __________________ rock intrusions or extrusions in the layers. sedimentary igneous
The Geologic Time Scale • The time span of the Earth is so long that geologists use the _________________________________ to show Earth’s history. • Scientists developed this scale by studying rock layers and index fossils all over the world. They then placed the rocks and fossils in order of age Geologic time scale
Divisions of Geologic Time • As geologists studied the fossil record they found major changes in life forms at certain times. • They used these changes to make divisions in the geologic time scale. • Therefore these divisions depend on big changes in the history of life on Earth. • The first time frame in the geologic time scale is the ___________________________ Era. This is a very long period of time, covering 88% of the lifespan of the Earth and it ended 544 million years ago. Precambrian
Divisions of Geologic Time After this time period the units of time are split into _________________ and _____________________. • Geologists divide the time between the Precambrian and the present into three long units of time called eras. • They were the _______________________ Era, the ___________________________ Era and the ______________________________ Era. eras periods Paleozoic Mesozoic Cenozoic
Divisions of Geologic Time • Each era is divided into smaller units of time called periods. • For example, the Mesozoic Era is divided into the ______________________, _________________________, and _______________________ periods. • The names of many of the geologic periods describe where geologists first found the rocks and fossils in that time period. • For example, the name Cambrian refers to the old Roman name for Wales. Triassic Jurassic Cretaceous
