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You know that rocks are made of minerals. Now it is time to find differences between various rocks. Use that information to group rocks into the three basic types.

 
 
Let's take a quick trip though the rock cycle, and like the water cycle, there is no end!
 

Igneous rocks

It's Hot, Hot, Hot!

You feel so hot on a summer day; you think you are melting. What is the temperature? 100ºF? Inside the Earth the temperature reaches over 2500ºF! That is enough to melt rock! This is where we will start in the rock cycle. Igneous rocks are called fire rocks because they are created from magma. Melted rock inside the Earth is magma. It hardens back into a rock in pockets under Earth's surface, or it spews out onto the surface.

 

Intrusive Igneous Rock

When magma hardens into a rock under the surface, it is call intrusive igneous rock. Granite is a very common example. Intrusive igneous rocks cool slowly; so, often you see large crystals in them.
 
You can make your own granite (igneous rock) by following the recipe below.
 

DON'T eat this rock!

Materials:
  • One tablespoon of pink aquarium gravel
  • One teaspoon of white aquarium gravel
  • One-half teaspoon of black or any dark colored gravel
  • White glue
  • Paper cup
  • Craft stick or a plastic spoon
 
Procedure:
  1. Mix the pink, white, and black gravel in a paper cup.
  2. Add the glue and mix well with the craft stick or spoon.
  3. Allow the mixture to dry for 24 hours.
  4. Peel the cup away and compare your sample of granite to a sample of the real thing.
 

Extrusive Igneous Rock

Some magma is forced, or extruded, onto the Earth’s surface. This is called extrusive igneous rock. Lava and basalt are good examples. Unlike intrusive rocks, you don’t notice as large of crystals. These rocks cool more quickly. You may see holes and bubbles.

 

Try It!

Making your own extrusive igneous rock is easy! Heat some chocolate or chocolate chips in the microwave and let them cool back down. That's it, and it tastes better than the granite too!

 

Sedimentary rocks

 
Breaking Up and Coming Together

All rocks erode.

They break down into smaller and smaller pieces over time. Small particles of rock, called sediment, are deposited in the bottoms of lakes and seas. Over years and years, layers pile up. The layers cement together forming sedimentary rock.

 
Sedimentary rocks are easy to spot. You can see the particles of sediment using a magnifying glass. The particles are rounded from weathering. You may notice the layers they were deposited in, and might even find a fossil. These rocks are relatively soft. Sometimes you can rub off grains. Sandstone and shale are common sedimentary rocks. Sometimes larger rock pieces are cemented together, like this one, called a conglomerate.
 

Metamorphic rocks 

 
Morphing
 
Metamorphic rocks are rocks that morphed, or changed. Animorphs® change from humans to animals. Metamorphic rocks change from igneous, sedimentary, or even a type of metamorphic rock to another. This change requires heat, pressure, and lots of time.
 
Metamorphic rocks are very hard. You may notice layers or crystals. Marble is a metamorphic rock you may have seen.
 
Metamorphic Waffles
 
Materials:
  • Pancake/Waffle mix that needs only water
  • One bag M & M's® candy or other "sediments"
  • Large mixing bowl
  • Waffle maker
  • Measuring cups
  • Vegetable Oil or Cooking Oil
  • Syrup
  • Forks
  • Plates
 
Procedure:
Follow the steps below to make your own tasty rocks. (Make sure you get an adult's permission and help!)
  1. Heat and oil your waffle maker. This represents the heat and pressure from the earth.
  2. Mix up the batter.
  3. Pour in the M & M's® candy and mix well. This represents sediments or magma. (You can add other types of sediments you think would taste good such as coconut, walnuts, chocolate chips, etc.)
  4. Place the batter on the griddle. Make waffles.
  5. Notice how the heat and pressure changed your rock. Sedimentary to metamorphic through heat and pressure.
  6. Enjoy!!
 
Keep On Rockin'
The rock cycle never ends, and it doesn’t go in order. A rock can start out as sedimentary and get melted to become igneous. It can get melted again and become another igneous. Metamorphic rocks can be melted down and become igneous, or weathered down to become sedimentary.
 
 

To help you remember the three types try this:

Igneous: “Ignite” from fire.
Sedimentary: “settle down” from sediments.
Metamorphic: “morph” or change.

 

Which of the following rocks looks like it is sedimentary? 

 

Which of the following rocks looks like it is igneous?

 

Which of the following rocks looks like it is metamorphic?

 

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Updated October 24, 2008 by: Glen Westbroek

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