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| Children's Corner Page 2 A Special Place for Junior Rock Hounds |
Igneous Rocks
The oldest type of all rocks is the igneous rock (IG nee us). The word "igneous"
comes from a Greek word for fire. Deep inside the earth, the temperature is very
high and the minerals there are in liquid form called magma. As the magma pushes
towards the earth's surface, it starts to cool and turns into solid igneous rock.
All igneous rocks do not cool the same way.
That is why they do not look all the same.
Some cool slowly, deep under the earth's surface.
These are called Intrusive or Plutonic igneous rocks.
The slow cooling formed rocks with large crystals.
Granite and Pegmatite are examples of rocks that
cooled slowly and have large crystals.
Other rocks formed when the magma erupted from a
volcano or reached the earth's surface through long cracks.
Magma is called lava when it reaches the earth's surface.
Lava cools quickly and forms rocks with small crystals. They
are called Extrusive or Volcanic igneous rocks. Basalt is an
example of this type of rock. Obsidian is another example of
extrusive igneous rock but it cooled so fast that it has no
crystals and looks like shiny, black glass.
Here are some things to look for in igneous rocks.
-Many forms of texture and composition
-Normally contains no fossils
-Usually has no layering
-Usually made of two or more minerals
-May be light or dark colored
-Usually made of mineral crystals of different sizes
-Sometimes has openings or glass like slivers
-May be fine-grained or glassy looking
Here are some photographs of common igneous rocks. Look for these same forms,
patterns and structure in the rocks you find during your rock hunting trips.
See...you are now on your way to becoming a genuine rockhound!
Common Intrusive Igneous Rocks
Common Extrusive Igneous Rocks
| Granite, Pegmatite and Gabbro all have mineral grains of nearly equal size. This is called a Porphyritic texture. |
| Rhyolite and Basalt are called Micro crystalline Alphanitic texture (the mineral grains are too small to be seen with the naked eye) These are formed when lava cools very quickly Obsidian, Pumice and Scoria are all called Glassy Alphanitic texture The Scoria's porous textures (Vesicules) are the result of bubbles formed by escaping gases in slower cooling lava. |
| SpiritRock Shop "Junior Rock Hound Club" |
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