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Earth started as a ball of dust, rock, and ice in space. Gravity pulled the masses together

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Eras are subdivided into units of geologic time called periods

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Geologists divide the time between Precambrian time and the present into three long units of tie called eras

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It covers about 88 percent of earth's history

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It's a record of the geologic events and the evolution of life forms

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When rock layers erode away, an older rock surface may be exposed. Then deposition begins again, building new rock layers. The surface where new rock layers meet a much older rock surface beneath them is called an unconformity. An unconformity is a gap in the geologic record. It shows where layers have been lost due to erosion

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Fossils are the preserved remains of organisms that were once living

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Only 10% of fossils will become fossils. The other 90% is destroyed before that happens.

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Nearly all fossils are found in sedimentary rock

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There are 6 main types of fossils.

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Molds- The hollow space left by the organism

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Casts- creates an exact copy of le organism

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Petrified fossils- organism is turned to stone

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Carbon Films- an organism leaves behind a thin layer of carbon. Very rare.

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5 I'm Seriously Running Out Of Names For These Types

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5 I'm Seriously Running Out Of Names For These Types

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Trace fossils- organisms leave clues behind

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Perserved remains- tar, ice, or amber

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Archaeologists study the history of humans through ancient artifacts.
Paleontologists study former life of animals that lived millions of years ago.

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When they died, the were covered by mud. It turned to rock and eventually, that rock became mountains.

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The relative age of a rock is its age compared to the ages of other rocks.

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The absolute age of a rock is the number of years that have passed since the rock formed.

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Fossils 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 that oldest layer is at the bottom. Each higher layer is younger than the layers below it.

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Lava that hardens on the surface and form igneous rock is called an extrusion. An extrusion is always younger than the rocks below it. Magma may push into bodies of rock below the surface. There, the magma cools and hardens into a mass of igneous rock called an intrusion. An intrusion is always younger than the rock layers around and beneath it.

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More clues come from the study of faults. A fault is a break in the Earth's crust. Forces inside Earth cause movement of the rock on opposite side of a fault/ A fault is always younger than the rock it cuts through. To determine the relative age of a fault, geologists find the relative age of the youngest layer cut by the fault.

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To date rock layers, geologists first find the relative age of a layer of rock at one location. Then they can match layers in other locations to that layer. Certain fossils, called index fossils, help geologists match rock layers. To be useful as an index fossil, a fossil must be widely distributed and represent an organism that existed for a geologically short period of time. Index fossils are useful because they tell the relative ages of the rock layers in which they occur. Scientists infer that layers with matching index fossils are the same age.