Igneous

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Rocks

• Igneous

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• The term igneous comes to us from the Latin word
  "Ignis " which means fire. Most igneous rocks are
  produced deep underground by the cooling and
  hardening of magma. Magma is molten (melted) rock
  under the surface of the Earth. It is produced in
  the upper reaches of the mantle or in the lowest
  areas of the crust usually at a depth of 50 to
  200 kilometers.
• Magma is produced at a subduction zone. Magma is
  less dense than the surrounding rock which causes
  it to rise. When magma reaches the surface it is
  then called lava. Lava and ash produce volcanoes.
  When lava reaches the surface of the Earth
  through volcanoes or through great fissures the
  rocks that are formed from the lava cooling and
  hardening are called extrusive igneous rocks .
• Common extrusive igneous rocks are lava rocks,
  cinders, pumice, obsidian, and volcanic ash and
  dust.

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• This is the volcano Paricutin that is located in
  Mexico. It is erupting cinders and pumice which
  are examples of extrusive igneous rocks. It is a
  cinder cone.

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• Igneous rocks that formed deep in the crust are
  are now visible because mountain building has
  thrust them upward and erosion has exposed the
  much harder igneous rocks. These are called
  intrusive igneous rocks because the magma has
  intruded into pre-exiting rock layers. Types of
  intrusive igneous rocks are granite and gabbro.
• The diagram above shows you a large intrusive
  igneous body called a batholith. A batholith is
  the largest of the intrusive bodies. They are
  larger than 100 square kilometers and usually
  form granite cores.

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• The composition of igneous rocks falls into four
  main categories. They are determined by the
  amount of silica that the rocks contain. The four
  categories are acidic, intermediate, basic, and
  ultramafic.
• Acidic rocks have a high silica content (65 or
  more) along with a relatively high amount of
  sodium and potassium. These rocks are composed of
  the minerals quartz and feldspar. Rhyolite and
  granite are the two most common types of acidic
  rock. (SiAl)
• Intermediate rocks contain between 53 and 65
  silica. They also contain potassium and
  plagioclase feldspar with a small amount of
  quartz. Diorite and Andesite are the two most
  common types of intermediate rock.

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• Basic rocks are composed of less than 52 silica
  and a large amount of plagioclase feldspar and
  very rarely quartz. The two most common types of
  basic rocks are basalts and gabbros (SiMa)
• Ultrabasic rocks are composed of less than 45
  silica and contain no quartz or feldspar. They
  are composed mainly of the minerals olivine and
  pyroxene. The most common ultrabasic rock is
  periodite. Periodite is a dark green,
  coarse-grained igneous rock that many scientists
  believe is the main rock of the mantle.

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• Basalts are dark colored, fine-grained extrusive
  rock. They are the most widespread of all the
  igneous rocks. Most basalts are volcanic in
  origin and were formed by the rapid cooling and
  hardening of the lava flows.

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• This is a vertical columnar basalt formation.
  When basaltic lava cools it often forms hexagonal
  (six sided) columns. Some famous examples of
  columnar basalt formations are the Columbia
  Plateau overlooking the Columbia River near
  Portland, the Giant's Causeway in Northern
  Ireland, and the Devils Postpile National
  Monument in California (Above).

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• Pumice is a very light colored, frothy volcanic
  rock. Pumice is formed from lava that is ejected
  from the volcano and cools in the air..
• Pumice is so light that it actually floats on
  water.
• Pumice is ground up and used today in soaps,
  abrasive cleansers, and also in polishes.

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• Rhyolite is very closely related to granite. It
  has much finer crystals.
• Rhyolite is an extrusive igneous rock.
• The minerals that make up rhyolite are quartz,
  feldspar, mica, and hornblende.

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• Gabbros are dark-colored, coarse-grained
  intrusive igneous rocks, similar to basalts.
• Composed of the mineral plagioclase feldspar ,
  pyroxene and olivine.

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• Obsidian is a very shiny natural volcanic glass.
  It fractures with a distinct conchoidal
  fracture. Obsidian is produced when it cools so
  quickly that no crystals can form.
• Obsidian is a natural glass.
• Obsidian is usually black or a very dark green,
  but it can also be found in an almost clear form.
  
• Ancient people throughout the world have used
  obsidian for arrowheads, knives, spearheads, and
  cutting tools of all kinds. Today obsidian is
  used as a scalpel by doctors in operations.

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• Another sample of Obsidian.

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Write the answers to the following four questions
in complete sentences on a piece of paper.

• In your own words write a definition for magma
  and lava.
• What is the difference between intrusive and
  extrusive igneous rocks?
• What are the most common extrusive and intrusive
  igneous rocks?
• What is the difference between granite and
  rhyolite and how are they similar?