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The Rock Cycle
Directions: Read, highlight, and then go to schoology.com to answer the questions.
The fun and danger of watching volcanoes can be an intriguing job for many
geologists. As we saw in our web quest, sometimes getting samples can be a
little traumatic.
So why the interest in gaining a sample of magma from a volcano? Geologists
study the earth both inside and out. Although dangerous, there is much to
learn from sampling lava fresh from the mouth of a volcano.
Magma is a mixture of melted or semi-melted rock. Its chemical composition
determines the type of rock that will form. So knowing this information tells a
scientist the types of minerals present inside the magma.
Once the magma cools and forms rock, it begins the never ending cycle of formation, destruction, and
reformation called the Rock Cycle. The Rock Cycle refers to a never ending series of changes as earth’s
inner liquid (magma) cools and changes into a variety of different forms.
Igneous Rock
Most rocks on earth began as igneous rocks. I
say most since sometimes meteors or asteroids
slam into earth bringing rocky material with
them.
Igneous rocks are formed from magma. Magma
is a hot liquid made of melted minerals. As the
magma cools, it solidifies into rock making
crystals.
If igneous rock forms underground, the magma
cools slowly. This is called intrusive formation.
If igneous rock forms above ground, the magma
cools quickly. This is called extrusive formation.
Extrusive formation happens when volcanoes
erupt.
When magma pours out on Earth's surface,
magma is called lava. Yes, the same liquid rock
matter that you see coming out of volcanoes.
Sedimentary Rock
Over the course of millions of years, igneous rocks are weathered down by wind and water (called
erosion). Fine particles of eroded rock dirt begin to cover the landscape. Often, these small rock particles
end up being suspended in water, and find their way to the bottom of lakes, streams, and the ocean.
Usually, the rock pieces, called sediments, drop from the wind or water to make a layer. When something
drops to form a layer scientists say it has precipitated. The layer can be buried under other layers of
sediments. After a long time the sediments can be cemented together to make sedimentary rock. In this
way, igneous rock can become sedimentary rock.
There are several types of sediments:
 clastic sediments are made of particles of various sizes and are carried by wind, water (rain) or ice
(glaciers). This type of sediment is like dust piles that form different layers over time. Sand is an
example of a clastic rock.
 chemical sediments precipitated from water (rivers, oceans, streams). For this type of sediment the
chemical is dissolved in the water and then settles out. This is similar to how we made crystals in class.
Halite (salt) is an example of a chemical rock.
 organic sediments are precipitated from materials that were alive at one point. Many plants and
animals drop to the bottom of oceans, rivers, and streams leaving organic sediments behind.
Limestone is an example of organic sediment and so is coal.
Metamorphic Rock
All rock can be heated. If it is re-melted turning back into magma, it will return to earth’s surface as
igneous rock. But what happens if the temperature doesn’t melt the rock? And where does the heat
come from? Inside Earth there is heat from pressure (push your hands together very hard and feel the
heat). There is heat from friction (rub your hands together and feel the heat). There is also heat from
radioactive decay (the process that gives us nuclear power plants that make electricity).
The heat from inside the earth bakes the rock kind of like a giant oven. Baked rock does not melt, but it
does change. It forms crystals. If it has crystals already, it forms larger crystals. This is how the Naica
Crystal Caves were formed.
Because this rock changes, it is called metamorphic. Remember that a caterpillar changes to become a
butterfly. That change is called metamorphosis. Metamorphosis can occur in rock when they are heated
to 300oC to 700oC.
The Cycle Continues
Of these three rock types, the most common near the earth’s surface is sedimentary rock. This is due to
the many rivers, lakes, and seas that have covered the surface at one time or another as well as the
ongoing erosion of earth’s surface. This layer of sedimentary rock is very thin, however, extending
downward only a mile or so in depth.
Below this sedimentary rock and also mixed with it in many locations is igneous rock. The bulk of the
Earth’s crust is made up of both igneous rock and metamorphic rock.
Remember to go to schoology.com to answer the questions!