What do rocks like to eat for breakfast? Cocoa Pebbles…. Bahahahahaha
Okay, okay… to be more serious…
Rock covers the entire earth. 70% of it is covered by Earth’s oceans, but still, the earth is completely covered in rock.
There are three main types of rock that you should know about.
Igneous Rocks
Do you think it is hot inside the earth? It is very hot. Between the crust and the mantle, the temperature is high enough
to melt minerals. So, some of the rock inside the earth is in a liquid state.
What happens if the liquid rock is cooled? Igneous rocks are formed. Igneous rocks are rocks that form when melted
minerals cool and harden. The word “igneous” comes from the Greek work for fire. Although igneous rocks are not
formed by fire, very high temperatures melt rock.
Melted minerals inside the earth are called magma. There is a lot of magma deep inside the earth. Sometimes, magma
rises to the upper part of the earth’s crust. The temperature of the crust is much cooler. The magma cools and hardens.
Igneous rock is formed. It may take thousands of years for igneous rock to form from magma.
Sometimes, magma forces its way to the surface of the earth. Then it is called lava. Lava cools when it has contact with
air or water. Cooling makes the lava harden into igneous rock. It does not take a long time for igneous rocks to form
from lava.
How can we look at an igneous rock and tell if it cooled slowly or quickly? Igneous rocks have crystals of different
sizes. Different speeds of cooling made different size crystals.
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Slow cooling forms rocks with large crystals. This happens inside the earth and forms an intrusive igneous rock
with a coarse-grain texture.
Rapid cooling forms rocks with small crystals. This happens in the surface of the earth and forms an extrusive
igneous rock with a fine-grain texture.
Extra-fast cooling forms rocks with not crystals. This happens on the surface when the lava enters extremely
cold water or air and forms an extrusive igneous rock with a glassy texture.
Sometimes the magma cools slowly for part of the time and quickly for part of the time. This would give the
rock some large crystals and some small crystals. This results in a porphyritic texture.
Sometimes the magma or lava has a lot of gas bubbles trapped in it when it cools. This results in the rock having
a lot of little or big holes in it giving it a vesicular texture.
Sedimentary Rocks
The tallest mountain in the world is Mount Everest, in Asia. People have risked their lives trying to climb it. But
someday, Mount Everest will be completely worn away. It is wearing away right now, a little bit at a time. It will take
millions of years for Mount Everest to wear away, but it will happen.
Forces in nature, such as wind and water, keep breaking rocks and soil into smaller and smaller pieces. These broken
pieces are called fragments. Pebbles, gravel, sand, silt, and clay are five different sizes of rock fragments, which are also
the 5 types of sedimentary rock textures. Pebbles are the largest texture and clay is the smallest size texture.
Fragments are moved about by water, wind, and large ice masses called glaciers. The fragments settle in a new place.
The settled fragments are called sediments. As time passes, older layers of sediments are buried by new layers. As more
and more sediments are added to the layers, the lower layers become tightly packed. The lower layers of sediment
harden into solid rock.
Rock that is formed from hardened sediments is called clastic sedimentary rock.
sedimentary rock in two ways:
Sediments can harden into
1.) From the pressure of its own weight, or
2.) By cementing. Minerals dissolved in water “glue” the sediment together.
Most sediment builds up under water.
Some sedimentary rocks form from the remains of living things and shells. These are called biochemical sedimentary
rocks. Soft coal, for example, was formed from layers of dead plant matter.
Some sedimentary rocks also form from minerals dissolved in water. These are called chemical sedimentary rocks. For
example, rock salt is a sedimentary rock made up of the mineral halite.
Metamorphic Rocks
Many things we use are changed over from what they were to begin with. For example, glass, plastic, and synthetic
fabrics do not look like the raw materials they came from. Many of the things we use were changed. Several forces can
cause change. Two of these are heat and pressure.
Heat and pressure can change many things. They can even change rocks and the minerals in them. The name for
changed-over rocks is metamorphic rocks. Metamorphic comes from the Greek words meaning “change” and “form.”
Metamorphic rocks are formed deep in the earth where there is high temperature and great pressure. The heat and
pressure change one kind of rock into another kind of rock. The new rocks become harder and more dense than the old
rocks. They also look different. Sometimes the mineral in the rocks change too.
Some metamorphic rocks also are formed when other rocks come in contact with magma. This results in contact
metamorphism. The heat from the magma changes the minerals in the rock.
There are many kinds of metamorphic rocks. Slate is a metamorphic rock. Slate is changed-over shale. Marble is
another metamorphic rock. Marble is changed-over limestone.
The original rock, such as limestone, is called the parent rock.
being exposed to heat and pressure.
Here, the parent rock, limestone, becomes marble after
Metamorphic rocks can be found with different textures too. Some metamorphic rocks result in layering of the minerals.
This is called a foliated texture. If there is no foliation, the texture is said to be granular.
Rock Cycle
Rocks do not remain the same forever. In fact, any rock can change to another kind of rock. The endless change from
one form to another is called the rock cycle.