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The Relative Age of RocksEarths HistoryWhat does the fossil record show?

What does the fossil record show?Evidence of Gradual ChangeOrganisms have changed significantly over time. In rocks more than 1 billion years old, only fossils of single-celled organisms are found. At 550 million years old, fossils of simple, multicellular animals. At 500 million years ago, ancient fish without jawbones surface.At 400 million years ago, fish with jaws are found. At 350 million years ago, reptiles At 300 million years ago, mammals At 230 million years ago, and birds.At 150 million years ago, as the rocks become more and more recent, the fossils look like the animals we observe today.

Fossils . MoldPALAEOZOIC: late Ordovician Phylum: PROTOCHORDATACommon name: graptolites

Petrified Fossil Phylum: ARTHROPODALate middle CambrianCommon name: trilobite

Carbon FilmExtremely thin coating of carbon on rock.When sediment buries an organism, some gases escape from the sediment , leaving carbon behind.

Index FossilsTrilobite. Cambrian Period

SummaryA hollow area in sediment in the shape of an organism is a A solid copy of the shape of an organism is a .What type of fossils provide evidence of the activities of ancient organisms..Which type of substance is NOT able to preserve entire organisms? a)icec)amber b)waterd)air Relative Age of RocksThe age of the rock is compared to the ages of other rocks.Ex:Sandstone in an area is older than limestone.

Absolute Age The absolute age of an igneous rock can be best determine by the sizes of crystals found in the upper and lower parts of the rock.

Absolute Age of RocksIt is number of years that passed since the rock formed.Geologists often use both absolute and relative ages to determine the age of a rock.Geologists use the law of superposition to determine the relative ages of sedimentary rock layers.For example: The sandstone is 300 million years old.

Two different Trash Pits

What can we say and learn from these excavations? Relative age of trash layers Because of the shape of the pits the oldest layers of trash occur below younger layers . The inhabitants of the area likely deposited the trash by throwing it in from the top, eventually filling the pits. Thus the relative age of the trash layers is, in order from youngest to oldest.:

5.25" Disk Layer - Youngest Al Cans Layer Tin Cans Layer Ceramic Cups Layer Stone Tools Layer - Oldest

Rock LayersFossils are most often found in layers of sedimentary rock.Geologists use the law of superposition to determine the relative ages of sedimentary rock layers.According to the law of superposition, in undisturbed horizontal sedimentary rock layers the oldest layers is at the bottom. Each higher layer is younger than the layers below it. Law of Superposition

ExtrusionMagma that reaches the surface is called lava.Lava that hardens on the surface and forms igneous rocks is called an extrusion.Extrusion

IntrusionMagma may push in bodies of rock below the surface. There the magma cools and hardens into a mass of igneous rock called an intrusion.A mass of igneous rock below the Earths surface is called intrusion.

Intrusion

Fault A fault is a break in Earths crust. Forces inside Earth cause movement on the rock on the opposite sides of a fault.A fault is always younger than the rock it cuts through.To determine the age of a fault, geologists find the relative age of the youngest cut by the fault.

FaultSan Andres Fault

Index FossilThey help geologists match rock layers . Index fossils tell the relative ages of the rock layers in which they occur.To be an index fossil, the fossil must be widely distributed an represent an organism that existed for a geologically short period of time.Index Fossils

Index FossilsWhat is it?Any animal or plant preserved in the rock record of the Earth that is characteristic of a particular span of geologic time or environment. A useful index fossil must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata. In marine strata, index fossils that are commonly used include the single-celled Protista with hard body parts and larger forms such as ammonoids. In terrestrial sediments of the Cenozoic Era, which began about 65.5 million years ago, mammals are widely used to date deposits. All of these animal forms have hard body parts, such as shells, bones, and teeth, and evolved rapidly.Index Fossils

How can rock layers change?Most of Earths geologic record has been lost to erosion.Gaps in the geologic record and folding can change the position in which rock layers appear.Because of erosion most of the geologic record of sedimentary rock layers has been lost.Gaps in the Geologic RecordWhen rock layers erode away, an older layer surface may be exposed. Then deposition begins again building new rock layers.UNCONFORMITYIt is a gap in the geologic record. It shows where rock layers have been lost due to erosion.UNCONFORMITY

FoldingSometimes forces inside Earth fold rock layers so much that the layers are turned over completely.In this case, the youngest rock layers must be on the bottom.Two ways of rock layers can change are folding and unconformity.Folded rock layers are not longer flat and , sometimes older layers end up on top.Folding

Geologic Folding

Law of superpositionAccording to the law of superposition ,in horizontal sedimentary rock layers the oldest layer is at the bottom and the youngest layer is at the top.Law of superposition

Law of superposition

Radioactive DecayMost elements usually do not change, but some elements can break down, or decay, over time. These elements release particles and energy in a process called radioactive decay.These elements are RADIOACTIVE.During Radioactive decay, the atoms of one element break down to form atoms of another element.Radioactive Decay

Half- LifeHalf- life of a radioactive element is the time it takes for half of the radioactive atoms to decay.Half-life

Radioactive DatingRadioactive elements occur naturally in igneous rocks.Scientists use the rate at which these elements decay to calculate the rocks age.As a radioactive element within the igneous rock decays, it changes into another element. So the composition of the rock changes over time.The amount of the radioactive element DECREASES and the amount of the new element INCREASES.Radioactive Dating

Determinig Absolute AgesIn radioactive dating, scientists first determine the amount of a radioactive element in a rock. Then they compare that amount with the amount of the stable element into which the radioactive element decays.Potassium-Argon DatingPotassium-40 is an element scientists use to date rocks.Potassium-40 decays to stable Argon-40 and has a half-life of 1.3 billion years.Potassium-40 is useful in dating the most ancient rocks because of its long half-life.Potassium-Argon Dating

Carbon -14 DatingCarbon-14 is a radioactive form of carbon. All plants and animals have Carbon and some Carbon-14.After an organism dies, the Carbon-14 in the organisms body decays.It changes to Nitrogen-14.

Carbon -14 DatingTo determine the age of a sample, scientists measure the amount of Carbon-14 that is left in the organisms remains.Carbon-14 has been used to date fossils such as frozen mammoths and the skeletons of prehistoric humans.Carbon-14 half-life is only 5,730 years.Carbon -14 Dating

Radioactive DatingTo determine Earths age scientists use radioactive dating and evidence from rocks on Earths and the moon.

Using fossils, evidence from rock layers, and radioactive dating, scientists can infer about events in Earths history.

Geologic Time ScaleIt is a record of the geologic events and the evolution of life forms as shown in the fossil record.

Scientists have placed Earths rocks in order by relative age.

Then, they divided geologic time into parts, based on time in Earths history when there were major changes in life forms.Geologic Time ScalePrecambrian time is the earliest part of the Geologic time.This part is the longest of the geologic time ended 542 million years ago.Scientists divided the time between Precambrian time and the present time in three eras:Paleozoic ERAMesozoic ERACenozoic ERA

Velociraptor

Geologic Time ScaleThe Paleozoic ERA is made up of six periods.The Mesozoic ERA is made up of three periods.The Cenozoic ERA is made up of three periods.The Quaternary Period is the last and continues to the present time.

Tyrannosaurus Rex

UNIFORMITARIANISMScientists use the principle of uniformitarianism to explain Earths past.Allosaurus

UNIFORMITARIANISMThis principle states that geologic processes that operate today are the same ones that operated in the past.Natural processes such as weathering, erosion, and plate tectonics have shaped Earths surface .The distribution of land and water on Earths surface has changed over time.

Theropod ( beast-footed )