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"Solid as a rock." Have you heard someone say that before? Rocks have a
reputation for being solid, hard, and indestructible. Rocks line river beds and
jut above the landscape as mountain peaks; they are fun to collect and sometimes
are very beautiful. Each rock is different - some are smooth and round, some are
sharp and dangerous. They come in all colors: pink, green, orange, white, red.
They are everywhere, and we take their presence for granted and assume that they
But rocks are not unchangeable! Just like the
water cycle, rocks undergo changes of form in a rock cycle. A metamorphic rock can become an
igneous rock, or a sedimentary rock can become a metamorphic one. Unlike the
water cycle, you can't see the process happening on a day-to-day basis. Rocks
change very slowly under normal conditions, but sometimes catastrophic
events like a volcanic eruption or a flood can speed up the process. So
what are the three types of rocks, and how do they change into each other?
Keep reading to find out!
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
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
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
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.
delta in Louisiana deposits lots of sediment in the Gulf of Mexico!
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
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
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 over again!
See if you can find sedimentary, igneous and metamorphic rocks where you
live. As you study them, think about how they have undergone many slow
changes to become what they are. Draw a picture of the rocks you find and then
draw a diagram of the whole rock cycle. Next, do some rock cycle