3.2 Igneous Rocks. Text pp 70- 74. Different kinds of igneous rocks form when magma – or lava – cools and hardens. Formation of Igneous Rocks. When magma hardens beneath the Earth’s surface, intrusive igneous rocks form. This is because they intrude into the existing rocks.
Text pp 70- 74
Different kinds of igneous rocks form when magma – or lava – cools and hardens.
The SLOW cooling of the magma allows larger crystals to grow in intrusive rock, such as in this sample of granite.
Intrusive rock results when magma solidifies under the Earth’s surface. The magma can force itself into the cracks of existing rocks.
We see intrusive rock at the earth’s surface because of erosion or weathering.
Intrusive rock crystals do not have to be the same size. A finely textured matrix may form around larger crystals which cooled more slowly.
When lava hardens above the ground, the igneous rocks that form are called extrusive rocks. (They EXIT the Earth.)
Basalt, fresh broken surface.
Extrusive igneous rocks form when lava flows onto the surface of the earth or floor of the ocean through deep cracks or fissures and at volcanic vents, and then cools and hardens.
Syenite igneous rock
The result is large crystals surrounded by a matrix of fine-grained materials. This is called a porphyritic texture.
feldspar mineral labradorite