Chapter 5.1

Before, your learned:  Living things exchange materials with their environments  Living things have different characteristics that they pass on to their offspring

Now, you will learn:  How one type of rock can change to another  What ages of rock layers reveal about Earth  How the relative and absolute ages of rocks are determined

Changes in rocks provide a record of the past  Relative age – is the age of an event or object in relation to other events or objects.  Geologists reconstructed Earth’s story based on the relative ages of different clues such as rocks, fossils, and other natural evidence

The three main types of rocks  Igneous Rock  Sedimentary Rock  Metamorphic Rock

Igneous Rock  Forms when molten rock cools and becomes solid.  This type of rock can form inside or on the surface

Sedimentary Rock  This types usually forms when pieces of older rocks, plants and other loose material get pressed or cemented together.  The loose material, called sediment, is often carried by wind or water.

Metamorphic Rock  Forms when heat and pressure causes existing rocks to change in structure, texture, or mineral composition

Layers of sedimentary rock show relative age  Sedimentary rock layers contain information about relative ages of events and objects in Earth’s Past

Principle of Superposition  It states that when horizontal layers of sedimentary rock are undisturbed, the youngest layer is always on top.

Index Fossils  Fossils contained within sedimentary rock can offer clues about the age of the rock  Fossils of organisms that were common, the lived in many areas, and that lived only during specific spans of time are called index fossils. Arnioceras semicostatum, an organism that lived between 206 million and 144 million years ago

Radiometric dating  Absolute age is the actual age of an event or object  Radiometric dating or radioactive dating is used to determine the absolute ages of rocks  Atoms make up everything on Earth and some are unstable that breakdown over time into another form.  This breakdown – called radioactivity – is useful in radiometric dating.

Half-life  Half-life is the length of time it takes for half of the atoms in a sample of a radioactive element to change from an unstable for into another form

 Different elements have different half-lives and it is not useful tool for measuring ages of rocks the elements with very short half-lives.  Uranium 235 has a half- life of 704 million years is used by scientists to date the age of the Earth.

Radiometric Dating  Radiometric dating works best with igneous rocks.  Carbon 14 can be used to find ages of the remains of some things that were once alive.  Carbon 14 has a half-life of 5730 years.

Absolute and Relative Age  Radiometric dating of igneous rocks reveals their absolute age  Interpreting layers of sedimentary rock shows the relative order of events  Fossils help determine ages of sedimentary rock layers