Download File - Science with Mrs. Ramirez

Grade 6
Science
Unit: 09 Lesson: 02
The Process of Forming Igneous Rock
The word “igneous” is from the Latin ignis which means “of fire”. Igneous rocks are formed by the
melting of rock, turning it into magma or lava, and then cooling off again. The rate at which the molten
rock cools causes specific sizes and types of crystals.
The rocks can be classified by where they are formed and their crystal size.
• Extrusive rocks form when the lava cools and crystallizes (hardens) on the surface of the
Earth. Igneous rocks that are formed above ground are called volcanic. If magma makes it to
the surface, it is known as lava and it will erupt or explode from a volcano and then cool and
crystallize to form an extrusive rock. The crystals will be very small and may have to be seen
with the aid of a microscope. An example would be obsidian.
• Intrusive rocks form when the magma cools and crystallizes below the Earth’s surface.
Intrusive rocks become exposed after millions of years by the processes of uplift, mountainbuilding, weathering, and erosion. If the magma cools slowly and crystallizes below the
surface, it will form an intrusive rock. The crystals will be large enough to see with an unaided
eye. An example would be granite.
Igneous rocks are also classified by the size of their crystals. Magma is a mixture of molten (liquid)
rock, crystals, and gas. Magmas are less dense than surrounding rocks and will move upward.
Because cooling of the magma takes place at different rates, the crystals that form and their texture
exhibit different properties.
Igneous rocks can form under water as well as on the land. Hot magma oozes out as lava along the
rifts or cracks in the seafloor. The extremely hot lava starts changing from liquid to solid as it meets
the cold seawater and begins cooling. The Hawaiian Islands are chains of volcanic peaks that have
risen above the surface of the ocean.
Some common igneous rocks are:
• Basalt – extrusive rock that cools quickly so crystals are not visible to the unaided eye. It
makes up about 90% of the Earth’s crust and lines the ocean basins.
• Gabbro – intrusive rock that has the same chemical composition and minerals as basalt. It is
the most common rock in the oceanic crust.
• Granite – intrusive rock that cools very slowly (hundreds of thousands of years) so the crystals
are large and visible. It makes up much of the Earth’s continental crust. Granites can be found
in a wide range of colors.
•
Obsidian – extrusive rock that cools so quickly that it doesn’t have time for crystals to form. It
is sometimes called volcanic glass and has a sharp edge when broken.
•
Tuff – extrusive ash and dust ejected by a volcano. The glass-like ash forms when escaping
gas forces a fine spray of magma from the volcano. When it settles on the ground, tuff hardens
to rock as it cools.
©2012, TESCCC
11/30/12
page 1 of 2
Grade 6
Science
Unit: 09 Lesson: 02
•
Pumice – extrusive foamy volcanic glass with gas trapped inside. The gas escapes as the
rock cools. The holes that remain allow pumice to float in water. It is composed mostly of SiO 2
(silicon dioxide).
Extrusive Fast cooling;
Igneous Tiny crystals form
Rock
Intrusive Igneous Rock
Slow cooling;
Medium cooling;
Large crystals form
Medium crystals form
©2012, TESCCC
11/30/12
page 2 of 2