Rock is the hard material that most of the earth’s crust is made up of. Mountains, coastlines, valleys and plains are made up of different kinds of rock. In some places soil lies over rock formations. They are often eroded and made smaller during the course of the earth’s history. Sometimes broken parts of rock lie on top of the earth’s surface. Tiny bits of them make up sand or soil.
Most rocks contain crystals of two or more different minerals. Granite for example contains grains of quartz and feldspar.
People use rocks for many different things. They play an important role in the construction industry. Concrete, for example, is made up of stones, sand and gravel and mixed with cement. Rocks are also used to make medicine, weapons and cosmetics. Various types of rock have been used to make historic monuments, like the ancient Egyptian pyramids.
Rock that contains metals is called ore. Iron, copper, lead, gold, and uranium are among the most important ores. Aluminum is produced out of a rock called bauxite. Other rocks contain valuable minerals, like diamonds or other gems.
Geologists are scientists who study the origin and composition of rocks. This gives them more information on the history of our planet. Rocks can also reveal how life evolved and which events shaped the Earth and the solar system.
Rocks are often classified by their hardness. They may be soft like talc, which you can scratch with your fingernail. On the other side of the scale diamonds are the hardest known rocks on earth. They are used to cut other minerals or in oil drilling.
Geologists often identify rocks by studying their formations. These often contain fossils that show scientists how old the rock may be.
Rock layers in the Grand Canyon
Types of rock
Igneous rock is hot molten rock that comes from below the earth’s surface. It forms when magma comes to the surface from cracks in the Earth’s crust. When it emerges through pipes and erupts volcanoes are formed. Such rock hardens quickly when it cools down. Dark colored igneous rocks are called basalts. They are formed either on the ocean’s floor or where lava flows occur.
Granite is a gray or white igneous rock that crystallizes slowly below the earth’s surface. It is often used as building materials and in homes.
Sediments are small pieces of rock that are transported by wind, water or glaciers. In the course of their movement they break apart into smaller pieces.
Sedimentary rock is formed when a river carries pieces of broken rock along with it and slowly deposits them onto its bed. On the ocean floor, dead plants and animals are deposited during the course of millions of years. These organic sediments harden and turn into rock.
Sandstone and limestone are two of the most important and widespread sedimentary rocks. These sediments contain fossil fuels, like oil and gas. Limestone is made up of shells and skeletons of living things. It forms in shallow ocean water. The white cliffs of Dover are the most famous natural limestone areas on earth. Sandstone is made up of quartz that has been pressed over the ages. It is often white or yellowish.
Metamorphic rock is rock that changes through heat and pressure in the deeper layers in the earth. The minerals in these rocks are chemically changed. Granite for example is turned into gneiss. Limestone turns into marble and sandstone can turn into quartzite.
Folding in a Metamorphic Rock
Rock is formed, destroyed and reformed in a cycle. It breaks into pieces through erosion. These pieces settle down onto the floor of oceans and rivers and become sedimentary rock. When such rock sinks deeper and deeper it turns into metamorphic rock.
Pressure can bring such rock to the surface where it can erupt in the form of volcanoes. On the surface rock breaks down again through erosion and is transported in rivers and streams. The cycle then starts from the beginning.
Such a rock cycle takes millions of year to complete.
The Rock Cycle
- Amasia - The Earth's New Supercontinent
- Volcanoes and Volcanic Activity
- Geologic Eras of the Earth
- Plate Tectonics and the Continental Drift
- Israel Wants to Make Oil Out of Rock
- Rock - Fill in the missing words
- Rock - Matching Exercise
- Rock - Match the sentence parts
- Rock - Crossword
- Types of Rock - Fill in the missing words
- bauxite =soft material out of which you can make aluminum
- bed = bottom of a river or lake
- classify = put into categories and groups
- cliff = large area of rock or mountain with a very steep side that falls to the ocean
- coastline = where the sea meets land
- complete = finished
- composition = the way in which something is made up of different parts
- construction = building
- contain = to be made up of
- copper = soft reddish brown metal that allows electricity to pass through easily; it is used to make wires and heating pipes
- crack = thin line
- crust = hard outer layer of the earth
- deposit = to put down, leave
- during the course of = as time goes on; slowly
- emerge = come up
- erode = break into small pieces
- erosion = the process by which wind, water and the sea slowly destroy rock and breaks it apart
- erupt = break out
- evolve = grow, develop
- feldspar = grey or white mineral
- fossil = an animal or plant that lived a long time ago and whose bones have been preserved in rock
- fossil fuels = form of energy , like coal, oil or gas that is produced over millions of years by dead plants and animals
- gem = beautiful stone that has a special shape and is very valuable
- glacier = large mass of ice that moves slowly down a mountain valley
- gneiss = rock that is formed out of granite
- grain = little bit, particle
- granite = very hard, grey rock, often used as a building material
- harden = to become hard
- identify = recognize , name, classify
- lead = soft, heavy grey metal that melts easily and is poisonous
- magma = hot melted rock below the surface of the earth
- marble = hard rock that becomes smooth when polished ; it is used for statues, monuments and floors
- molten = rock that is heated and made into a liquid form
- occur = happen
- oil drilling = to make a hole into the layers of the earth in order to bring up oil
- ore = rock from which you can get metals
- organic = material that comes from something living
- origin =where something comes from
- pipe =tube
- plain = large area of dry , flat land
- pressure = force ; weight
- quartz = hard mineral substance used in making electronic watches and clocks
- quartzite = hard metamorphic rock that was originally sandstone
- reveal = show
- scale = range
- scratch = rub with your nails
- settle down = move downwards slowly
- shell = hard skin
- shallow = not deep
- skeleton = structure consisting of all the bones of an animal or human
- soil = top layer of the earth on which plants grow
- surface = the top layer of an object
- talc = very soft white or grey mineral
- tiny = very small
- valley = area of lower land between two hills or mountains; normally a river flows through it
- various = different
- weapon = object you use to fight, like a gun, knife or bomb
- widespread = common