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The geologic time scale begins with a very long block of time called Precambrian Time. It goes from 4.6 billion to 544 million years ago. It covers most of the Earth's history.
The rest of the geologic time scale is divided into 3 major blocks of time called Eras: Paleozoic, Mesozoic and Cenozoic.
Each era is divided into shorter blocks of time called periods.

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Earth has a very long history. Years and centuries are not very helpful for such a long history. So scientists use the geologic time scale for Earth's history.
The Geologic time scale is a record of how Earth and its life forms have changed through time.
In the GTS, time is divided into bigger blocks than years or centuries.
The scale begins when Earth formed 4.6 billion years ago and goes to the present.

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Radioactive dating is a method of dating rocks. Scientist test rocks to measure how much radioactive decay has happened.
The more decay that has happened, the older the rock is.
Using this method scientists can give rocks a rough age in years (absolute age).
Radioactive dating works well for igneous rocks not for sedimentary rocks.
Ex: Suppose that half of the potassium-40 in a rock has decayed to argon-40. The half-life of potassium-40 is 1.3 billion years, so the rock is about 1.3 billion years.

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Everything is made up of one or more pure substances. These pure substances are called elements.
Some elements break down, or decay. When an element decays, it changes into another kind of element. This process is called radioactive decay.
Elements that decay are called radioactive elements. For example:Potassium-40 decays into argon-40.
The half-life is the time it takes for half of a sample of an element to decay. Half-life of potassium-40 is 1.3 billion years.

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They tell the relative ages of the rock layers in which they occur.
The rock layers in different places can be hard to match up. It can be hard to tell which layers are older and which layers are younger.
Index fossils help scientists match rock layers in different places. An index fossil is a fossil of an organism that lived over a wide area and existed for just a short period of time.
Two rock layers in different places that contain the same index fossil are about the same age.

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The position of rock layers may change after the rock forms.
Extrusion: new melted rock that forms on top of sedimentary.
(D) Intrusion: new rock that forms inside of the sedimentary rock.
(E) Fault: break in the rock where rock layers can move and o longer line up.

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According to the law of superposition in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

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The fossil record shows that all kind of organisms have changed and many have died out, or gone extinct.

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Fossil records shows that organisms have changed slowly over time.
Evolution is the gradual change in living things that happens over a long period of time

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Paleontologist collect fossils from all over the world.

All the fossils together make up the fossil record.

The fossil record shows what past life forms were like and what past environments were like.

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Remains of organisms that died long ago.

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Remains of organisms that died long ago.

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Usually found in sedimentary rock.
Sedimentary rock forms when sediment build up, in shallow water. So most organisms that turn into fossils once lived in or near shallow water.

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Position of rock layers

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Scientists can tell how old fossils are from rocks where the fossils are found.