Rock Cycle
The concept of the rock cycle is attributed to James Hutton (1726–1797), the 18th-century founder of
modern geology. The main idea is that rocks are continually changing from one type to another and
back again, as forces inside the earth bring them closer to the surface (where they are weathered,
eroded, and compacted) and forces on the earth sink them back down (where they are heated,
pressed, and melted). So the elements that make up rocks are never created or destroyed — instead,
they are constantly being recycled. The rock cycle helps us to see that the earth is like a giant rock
recycling machine! The concept of the rock cycle is attributed to James Hutton (1726–1797), the 18th-
century founder of modern geology. The main idea is that rocks are continually changing from one
type to another and back again, as forces inside the earth bring them closer to the surface (where they
are weathered, eroded, and compacted) and forces on the earth sink them back down (where they are
heated, pressed, and melted). So the elements that make up rocks are never created or destroyed —
instead, they are constantly being recycled. The rock cycle helps us to see that the earth is like a giant
rock recycling machine!
Three types of rock:
Igneous rocks are formed when hot magma (melted rock) is rapidly cooled, either by
hitting underground air pockets or by flowing from the mouth of a volcano as lava.
Granite, obsidian, and pumice are all common examples of igneous rocks. Pumice is a
very porous rock, because when the lava cooled, pockets of air were trapped inside.
Because of all those air pockets, pumice can actually float!
Sedimentary rocks are formed by layers of sediment (dirt, rock particles, etc.) being
mixed and compressed together for extended periods of time. Common examples of
these rocks are limestone, sandstone, and shale. Sedimentary rocks often have lots of
fossils in them because plants and animals get buried in the layers of sediment and
turned into stone.
Metamorphic rocks are a combination of rock types, compressed together by high
pressure and high heat. They usually have a more hard, grainy texture than the other
two types. Schist, slate, and gneiss (pronounced like ‘nice’) are metamorphic rocks.
These rocks change over hundreds of years in the six steps of the rock cycle:
Weathering & Erosion. Igneous, sedimentary, and metamorphic rocks on the surface of
the earth are constantly being broken down by wind and water. Wind carrying sand
wears particles off rock like sandpaper. Rushing river water and crashing surf rub off all
the rough edges of rocks, leaving smooth river rocks or pebbles behind. Water seeps
into the cracks in mountain rocks, then freezes, causing the rocks to break open. The
result of all this: large rocks are worn down to small particles. When the particles are
broken off a rock and stay in the same area, it is called weathering. When the particles
are carried somewhere else, it is called erosion.
Transportation. Eroded rock particles are carried away by wind or by rain, streams,
rivers, and oceans.
Deposition. As rivers get deeper or flow into the ocean, their current slows down, and
the rock particles (mixed with soil) sink and become a layer of sediment. Often the
sediment builds up faster than it can be washed away, creating little islands and forcing
the river to break up into many channels in a delta.
Compaction & Cementation. As the layers of sediment stack up (above water or
below), the weight and pressure compacts the bottom layers. (Try making a stack of
catalogs and watch how the bottom one gets squished as you add more on top – this is
the same idea as the compaction of layers of sediment.) Dissolved minerals fill in the
small gaps between particles and then solidify, acting as cement. After years of
compaction and cementation, the sediment turns into sedimentary rock.
Metamorphism. Over very long periods of time, sedimentary or igneous rocks end up
buried deep underground, usually because of the movement of tectonic plates. While
underground, these rocks are exposed to high heat and pressure, which changes them
into metamorphic rock. This tends to happen where tectonic plates come together: the
pressure of the plates squish the rock that is heated from hot magma below. (Tectonic
plates are large sections of the earth’s crust that move separately from each other. Their
movement often results in earthquakes.)
Rock Melting. Can you imagine ‘rock hard’ rocks melting? That’s what they do in the
depths of the earth! Metamorphic rocks underground melt to become magma. When a
volcano erupts, magma flows out of it. (When magma is on the earth’s surface, it is
called lava.) As the lava cools it hardens and becomes igneous rock. As soon as new
igneous rock is formed, the processes of weathering and erosion begin, starting the
whole cycle again.
Main Points
Molten magma may cool off and crystalize beneath the earth’s crust, forming
intrusive igneous rocks. With time, pressure may cause uplift and rocks end up
on the surface.
Molten magma may also flow to the surface by volcanic action, causing
extrusive igneous rocks as they harden and crystalize.
On the surface, they undergo weathering, erosion and transport. Sediments
are therefore carried to low lying places and into rivers and water bodies. The
pilling up of sediments cause compaction and cementation. Sedimentary rocks
form.
After a long period of pressure and heat from the overlying weight, the
igneous and sedimentary rocks buried deep inside the crust change to
metamorphic rocks, deep under the earth’s crust.
Some of the metamorphic rocks begin to melt as they get closer to the molten
magma region. Some will also undergo uplift to the surface again, in places
where volcanic activity is not common. If they melt, they get released back to the
surface through volcanic activity, especially in places with high tectonic activity.
