Download Igneous Rocks

Igneous Rocks
No one has ever seen some of the biggest volcanoes in the world. In
fact, the Mid-Ocean Ridge is the largest mountain range on Earth.
It’s almost 805 kilometers (500 miles) wide and 48,280 kilometers
(30,000 miles) long. So why don’t we see these volcanoes? They lie
2.4 km (1.5 miles) below the surface of the ocean.
Every day at least one of the Mid-Ocean’s ridge volcanoes erupts.
Hot lava comes out onto the sea floor. The ocean’s cold water
rapidly cools the lava. This cooled lava becomes igneous rocks.
Layers of the lava build up and can eventually reach the surface of the ocean. If the volcano is active
enough it can create a volcanic island. Hawaii and Iceland are volcanic islands. Like we saw in class, Hawaii
is still growing by a couple of inches every day.
Igneous (ihg nee uhs) is a rock formed by the crystallization and hardening of molten material called
magma. This magma comes from the mantel deep below our ground that we walk on every day. Magma is
a mixture of liquids, gases, and mineral crystals. When molten rock is underground we call it magma
whereas when it is above ground we call it lava, even though it is the same stuff. The name “igneous”
comes from the Latin word ignis, meaning “fire.”
Scientists classify igneous rocks into two groups—extrusive and intrusive. Extrusive rocks form when
magma flows onto the surface of Earth or floor of the ocean and then cools and hardens. Intrusive rocks
result when magma solidifies beneath Earth's surface.
Extrusive igneous rocks are formed by magma that reaches Earth's
surface along fissures (deep cracks) and at volcanic vents. This magma
that flows onto the surface is called lava. It forms broad flat sheets, or it
may build up and form a mountain shape. Most types of lavas cool
rapidly, which causes microscopic crystals to form. Some lavas cool so
quickly that they form a smooth volcanic glass called obsidian. Obsidian
does not form any mineral crystals. A porous igneous rock called pumice
results when gases bubble out of the solidifying lava.
Intrusive igneous rocks are found in mines and tunnels or at the surface where they have been exposed by
erosion or changes in the Earth’s surface. Intrusive rock formations vary from thin sheets to huge. Magma
that forms intrusive rocks freezes (solidifies) slowly, which allows bigger crystals to form.
The mineral makeup of an igneous rock is determined by the mineral and elements that are in the magma.
Each type of igneous rock has a specific mineral composition.
Felsic rocks form from magma that contains a large amount of silicon. This type of rock generally has the
light coloring. Besides silicon other minerals like potassium, feldspar and quartz can be found in Felsic
igneous rocks. Some common felsic rocks are granite, rhyolite, obsidian, and pumice.
Mafic rocks do not have large amounts of silicon in them; rather they are rich in iron and magnesium, and
feldspar. Mafic rock may also include dark-colored minerals, such as hornblende. For this reason mafic
rocks are dark in color. The mafic family includes the common rocks basalt and gabbro.
1. As you read the article, highlight important information. Then circle at least three words that you do not fully
understand. Look up their meaning in a dictionary.
a. Word: ________________________________________________
b. Definition: _________________________________________________________________________
c. Word: _________________________________________________
d. Definition: _________________________________________________________________________
e. Word: _________________________________________________
f. Definition: _________________________________________________________________________
A.
B.
Granite is the most common igneous rock on the Earth’s surface. Both A and B are granite rocks.
2. Which rock is coarse grained? ________________________
3. With coarse grains the rock must have cooled _____________________________
4. Which rock is fine grained? ___________________
5. With fine grains the rock must have cooled _______________________________
6. Which rock must have cooled deeper under ground? _____________________________ Why?
_____________________________________________________________________________
7. Which type of igneous rock would that represent? Intrusive or extrusive
C.
Basalt crystals are very tine.
You need a microscope to
see them.
D.
Pumice has many holes. But
these holes are not grains, the
holes were made by gases.
E.
Obsidian is called “volcanic
glass” because it has no
crystals.
8. Basalt grain are fine because they cooled ____________________________
9. Pumice and obsidian cooled really ____________________________ and that is why no crystals are present.
10. Basalt, pumice and obsidian are considered what type of igneous rock? Intrusive or extrusive
11. Which rocks would be
a. Felsic: ___________________________________
b. Mafic:____________________________________