A ROCKY HISTORY Millions of years ago Earth looked very different than it does today Over time rocks have been formed worn down and formed again This is called the rock cycle A cycle is a series of events or actions that repeat regularly There are several cycles in nature The seasons are a cycle The way water moves around Earth is called the water cycle Scientists can find clues to Earth’s past by studying how its rocks have changed or moved For example rocks Though rocks seem to be an unchanging and permanent part of Earth they are actually part of an endless cycle of change 4
Traces of extinct sea creatures such as this trilobite are a sign that land was once underwater can show us that large areas of land were once covered by thick layers of ice Layers of rock can show us how ancient earthquakes shaped mountains and valleys The tiny fossils of ocean creatures found in rocks can show us how some areas that are now dry land were once covered by water In fact the history of Earth can be told in rocks Vocabulary Fossils are the remains or traces of plants and animals that lived long ago 5
WHAT ARE ROCKS Layers of soil organic material plants topsoil subsoil Rock or stone is a hard material made up of one or more minerals Minerals are inorganic substances meaning that they do not come from an animal or a plant Rock makes up the outer layer of Earth called the crust At its thickest the crust is about 19 miles 31 kilometers thick The lower parts of this layer broken rock bedrock Soil is made up of topsoil subsoil and broken rock Beneath is solid bedrock 6 2013 Encyclopaedia Britannica Inc
The sedimentary rock shale forms in muddy lagoons floodplains and similar areas are solid rock or bedrock Broken bits of rock lie on top The tiniest bits of rock make up sand and soil Underneath Earth’s crust is another layer of mostly solid rock called the mantle Pools of melted rock or magma form in the upper part of the mantle and the lower part of the crust Scientists divide rock into three main types These are igneous sedimentary and metamorphic rock Each of these types of rock forms in a different way Compare and Contrast What do the mantle and crust share in common In what ways are they different 7
