all about the rock...
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All About the Rock CycleNo Beginning or End
Many people think of rocks as solid and never changing. However,Earth’s rocks are always changing form. These changes are known asthe rock cycle.
Scientists class Earth’s rocks into three groups. These are igneousrocks, metamorphic rocks, and sedimentary rocks. Igneous rocks formwhen melted rock hardens. Sedimentary rocks are formed from littlebits of matter, called sediment. This sediment is generally made up oftiny pieces of rock. Metamorphic rock is created when heat andpressure change the minerals in sedimentary rocks and igneousrocks.
Over time, any kind of rock can become any other kind of rock. Therock cycle has no beginning or end.
Down to the Core
If you visited Earth’s center, you would first passthrough the rocks of the crust. The crust is made ofplates that push together, move apart, and slidepast each other. It is thinner under oceans andthicker under land. Under land, the crust isgenerally 20 to 30 miles (32–48 km) thick.
The second layer, the mantle, is about 1,800 miles(2,900 km) thick. It contains hot, melted rock.Below that lies the core. The outer core is 1,430
THINKLIKE ASCIENTISTA trip to thecenter of Earthwould be almost4,000 miles(6,440 km) long.
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Under mountains, Earth’s crust may be asmuch as 44 miles (70 km) thick. However,most of the crust under oceans is just 6miles (10 km) thick.Shutterstock.com
miles (2,300 km) thick and filled with meltedmetals. Deepest of all is the inner core, which ismade of solid metals.
Fire
Fountains
Sometimes the plates in Earth’s crust crash together. This pushes theedge of one of the plates under ground. It also produces so much heatthat some rocks melt. Melted rock, called magma, gets trapped inpockets deep under ground. Over time, magma cools and formsigneous rocks. Most rocks in Earth’s crust are igneous rocks.
Sometimes magma flows to Earth’s surface before it cools andhardens. Lava, as magma on Earth’s surface is called, escapes throughbreaks in Earth’s crust, called volcanoes. Some volcanoes throw lava,ash, and steam into the air. Lava flows slowly out of other volcanoes.Volcanoes in Hawaii sometimes shoot sprays of lava known as firefountains. These fountains can rise hundreds of feet (m) into the air.
From Hot to Cold
Redhot magma forms twokinds of igneous rocks.
It would also bevery hot. Thecore is about12,600° F(6,982° C)!
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Kilauea, in Hawaii, is one of the world’smost active volcanoes. It has beenerupting, or spilling out lava, steadily since1983.G. Brad Lewis/Stone/Getty Images
The igneous rock basalt sometimes formshexagonal, or sixsided, columns as itcools. You can see columnar basalt atIceland’s Skaftafell National Park.Shutterstock.com
Magma that cools insideEarth forms intrusiveigneous rock. Magma thathardens only after it hasflowed to Earth’s surfaceand become known as lavaforms extrusive igneousrocks.
Andesite and basalt areextrusive igneous rocks.Andesite is named for theAndes mountains, wherethe rock is common.Andesite forms from stickylava. Igneous rocks thatcool quickly, such asandesite and basalt, havesmall, smooth grains.Granite is a strong intrusiveigneous rock. Granite cools slowly, so it has large grains.
Falling Apart
Rocks on Earth’s surfaceare always breaking apart.The breaking down of rocksinto smaller pieces is calledweathering. Weatheringhappens many ways.Sometimes water fills holesin rocks and then freezes.Ice spreads to fill morespace than water does, sothe ice presses against therocks, and the rocks break.
Waves pound against rocksand move pieces of rockback and forth. Rock piecesbecome so small that they
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Waves crashing against the coast of Gozo,an island in the Mediterranean Sea, wearaway the island’s cliffs. This is one kind ofweathering.Shutterstock.com
form sand. Rivers also break down rocks as water flows. Wind blowsgrains of sand against rocks. The blowing sand wears away rocks, littleby little. Any rock that wind or water touches gets slowly worn down.
A Washout
Water and wind also carrysediment, such as brokenpieces of rocks, from onearea to another. This actionis called erosion. Erosionmoves sediment acrossEarth’s surface in severalways. Every day, erosioncarries away rocks atNiagara Falls, a hugewaterfall that lies betweenCanada and the UnitedStates. About 1 foot (.3 m)of rock is lost each year.Rain washes away bits of rock. Large ice blocks break from mountainsand carry sediment. The wind blows small bits of sediment to newplaces.
In time, all sediment gets deposited, or dropped, somewhere. Streamsand rivers deposit sediment at the bottoms of seas and lakes. Largerivers, such as the Mississippi River, deposit thousands of tons (t) ofsediment every day.
A Wet Start
Many layers of sedimentbuild up over time. Muchsediment is rock, but deadplants and the shells of seaanimals can becomesediment, too. Each newlayer of sediment buriesthe older layers deeperunder ground and adds
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Glaciers, such as these ones in the Andes,are slowmoving masses of ice. Glacierspull sediment along with them as theymove.NASA/Science Faction/Getty Images
Antelope Canyon is made of sandstone.Weathering and erosion formed thiscanyon, or very narrow valley, on Navajolands in Arizona.Shutterstock.com
more weight. The weight ofthe layers forces thesediment near the bottomto bond together. Thissediment hardens intosedimentary rock.
Sandstone is a sedimentaryrock that often forms inareas that had waterflowing through sand.Sediment made of seaanimals that lived millionsof years ago formslimestone rock. Asedimentary rock called oilshale forms when deadanimals settle at thebottoms of lakes.
Hot and Heavy
Metamorphic rocks formwhen the minerals insedimentary or igneousrocks change. Pressure andheat cause these changes.The deeper rocks areburied inside Earth, themore the pressure and heatincrease. When thepressure becomes greatenough to fold or squeezerocks, the minerals in thoserocks change.
When heat reaches certaintemperatures, theminerals in rocks startchanging. The sedimentary rock shale becomes a metamorphic rock,slate, when the temperature reaches about 400° F (204° C). The
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The metamorphic rock marble is oftenused for buildings and monuments. TheTrevi Fountain, in Rome, Italy, has manymarble statues.Shutterstock.com
metamorphic rock gneiss forms at 1,472° F (800° C). Mostmetamorphic rocks form underground, but hot lava flowing oversedimentary and igneous rocks can also form metamorphic rocks.
New Life in OldRock
We can see signs of therock cycle taking place allaround us. Have you everfelt dust in the wind? Dustsettles to the ground andhardens into sedimentaryrock. Though metamorphicrocks form deep underEarth, you can see thembeing pushed up inmountain ranges, such asthe Alps and the Andes.
Volcanoes around the worldmake igneous rocks. If youdived to the bottom of theocean, you would seeplaces where magma flows through cracks in the seafloor and formsrock ridges. This action, called seafloor spreading, happens again andagain. Seafloor spreading makes the Atlantic Ocean grow about 8inches (20 cm) wider every year.
A Piece of theAction
Earth is a rocky planet filledwith action. Volcanoeserupt. Mountain ranges,such as the Sierra Nevada,push up through Earth’scrust. The rock cycle showsus how rocks take differentforms and are always
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Many mountain ranges are still growing.The Himalayas, seen here, grow .2 inch (5mm) taller each year.Shutterstock.com
A diagram of the rock cycle.Shutterstock.com
changing.
We all need rocks. Rocksprovide blocks forbuildings. Rocks containiron and other mineralsfrom which we make tools,too. Roads, glass, andmonuments are all madewith rocks. Scientists,called geologists, study therock cycle to understandour planet. The better weunderstand Earth and therocks that form it, the moreuses for rocks we are likelyto find. The good news is that Earth will never run out of rocks!
Find Out More
The Web sites andbooks below can helpyou do more researchon your topic.
Web Sites
Earth Science Explorer:Rock Cycle
http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html
Geography 4 Kids: The Rock Cycle http://www.geography4kids.com/files/cycles_rocks.html
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Science Up Close: The Rock Cycle http://www.harcourtschool.com/activity/science_up_close/506/deploy/interface.html
Books
Bryan, Bethany. How the Rock Cycle Works. New York, NY: PowerKidsPress, 2009.
Hyde, Natalie. What Is the Rock Cycle? (Let’s Rock!) New York, NY:Crabtree Publishing, 2010.
Ostopowich, Melanie. The Rock Cycle (Earth Science). New York, NY:Weigl Publishers, 2010.
GLOSSARYash (ASH) Pieces of tiny rock that shoot out of a volcano when itblows.
contains (kunTAYNZ) Holds.
cycle (SYkul) Actions that happen in the same order over and over.
erupt (ihRUPT) To break open.
freezes (FREEZez) Makes something so cold it hardens.
inner (INnur) On the inside.
layer (LAYer) One thickness of something.
minerals (MINrulz) Natural things that are not animals, plants, orother living things.
outer (OWTur) On the outside.
pressure (PREHshur) A force that pushes on something.
ridges (RIJez) Long, narrow chains of hills or mountains.
squeeze (SKWEEZ) To force together.
surface (SERfes) The outside of anything.
temperatures (TEMpurcherz) How hot or cold things are.
Article Citation in MLA (ModernLanguage Association) format:"All About the Rock Cycle." PowerKnowledgeEarth & Space Science. Rosen Publishing. 4
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Oct. 2016,www.pkearthandspace.com/article/563/allabouttherockcycle.
Article Citation in APA (AmericanPsychological Association) format:All about the rock cycle. (n.d.). RetrievedOctober 4, 2016, fromhttp://www.pkearthandspace.com/article/563/allabouttherockcycle
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