|Children's Corner Igneous Rocks
A Special Place for Junior Rock Hounds
|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.
"Junior Rockhound Pages"
|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
lled 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