Igneous Rock

Scientists tell us that the Earth began as a huge ball of molten material. As it cooled it solidified. The first that formed was Igneous Rock -- meaning formed from fire. It took a long time for the molten material to cool. As it was cooling, it was still hot enough for atoms to move around. These atoms combined with other atoms forming compounds. These compounds formed minerals.

A rock is an Aggregate (mixture) of minerals.

There are 3 groups of rocks

  1. Igneous
  2. Sedimentary
  3. Metamorphic
Rock cycle
The Rock Cycle

After the Igneous rock formed it weathered (eroded) into fragments. The fragments eventually became another rock called Sedimentary (formed from a sediment)

Weathering worn away by wind and water Lithification Adding pressure and a cementing agent; converts sediment into rock

The Sedimentary rock is baked and forms Metamorphic rock. If there is too much heat the rock melts into magma. It will cool and form Igneous rock again.

Crystallization-- cooling; sometimes under pressure.

We end up with what we started with. We have gone around in a circle. This called the Rock Cycle.

In this lesson we will only talk about Igneous Rock.

Igneous Rock

Harden or solidified from liquid rock

Igneous rock comes from two sources --- Magma and Lava. Both are liquid rocks.

  • Magma: liquid rock containing many dissolved gases (found inside the earth).
  • Lava: liquid rock without the gases (found on the surface of the earth)

The magma rises up from the mantle region of the earth; the temperature there is 1400ºC.

Radioactive rocks are present at the crust-mantle boundary. The radioactivity gives enough heat to melt the rock.

Earth layers

Magma moves upward along cracks in the crust. If it overflows onto the earths surface, it is called lava.

Texture: the size of the mineral grains in the Igneous Rock

The magma cools slowly. The atoms come together to form compounds and minerals. A combination of minerals gives rise to different Igneous rocks.

Since these Igneous rocks formed inside the earth, they are called Intrusive Rocks. Intrusive: igneous rock that never reaches the earth’s surface

  • cools slow
  • minerals grow large
  • coarse grains
  • Example: Granite
Grey and Pink Granite

When the magma reaches the Earth's surface as Lava, it cools fast. The atoms don't have the time to move around forming various minerals. These Igneous rocks are formed on the Earth's surface and are called Extrusive Rocks.

Extrusive: magma that reaches the earth's surface

  • Cools fast
  • small grains or no grains
  • Example: Obsidian
Obsidian1 Obsidian2
Obsidian and Granite have the same chemical composition, but different textures.


two different textures; two different grain sizes due to cooling rate. A magma chamber may start to cool down and grains begin to form. Then the material in the chamber starts to erupt as a volcano. The lava cools fast, no grain, but it's mixed with grains that already formed from the magma chamber. The resulting rock shows characteristics of both intrusive and extrusive Igneous rocks.

Essential Mineral: a mineral whose presence is necessary to name the rock. Example: Granite - must have quartz or feldspar.

Accessory Minerals: a mineral whose presence is not necessary to name the rock. Example: Granite may contain muscovite, biotite, garnet, or other minerals but are not necessary to name the rock.

Volcanic Ejecta (or, Pyroclastic Material)

These are fragments thrown out of the volcano

The size of the fragments vary in size: dust to volcanic bombs (very large)

Most of this material owes its transportation to the kick given by the escaping gases at the time of the eruption, and assisted by the wind. Such ejecta falls out of the air in order of decreasing size and density.

Ejecta deposits decrease in thickness and in grain size away from the vent of the volcano. The wind will blow the fine material miles or hundreds of miles from the volcano. The heavy material can't travel as far.

3 Type of Volcanoes

  1. Cinder Cone
    Cinder cone

    This type of volcano has a thick lava due to the high amount of silica (quartz) in it. The lava is thrown high into the air. As it fall, it cools, So when it piles up, the material does not stick together. Because of this, this type of volcano erodes easily (except for the vent where the lava comes out -- it is hard).

  2. Shield Volcano
    Shield volcano

    The Shield Volcano is also known as the Hawaiian Volcano. The Hawaiian Islands are all Shield Volcanoes. The lava is called Basic lava since it has a low silica content that makes it fluid. You don't have violent eruptions with this type of lava. Since this type of lava flows well, the base of these volcanoes is very large --about 300 miles.

  3. Strato Volcano
    Strato volcano

    High and Steep. Made up of alternate layers of lava that was thrown up into the air, which solidifies before it hits the ground, and lava that flows onto the earth's surface. The flowing lava also solidifies but becomes much harder. Very explosive and dangerous.

Types of Lava

  1. AA
    • Acid lava (high silica content)
    • rough and jagged surface
    • cools fast
  2. Pahoehoe
    • Basic lava (low silica content)
    • cools slowly
    • called 'Rope Lava'
  3. Vesicular Lava
    • Contains many small holes
    • Holes produced by escaping gases
  4. Amygdaloidal Lava
    • The holes caused by the gas have been filled in by a secondary mineral, like calcite or quartz.
  5. Pumice
    • An excessively cellar glassy lava Sometimes it will float in water
  6. Obsidian
    • Volcanic glass Lava that has cooled fast
  7. Pillow Lava
    • Formed when the volcano erupts under water. Blobs of lava roll down the side of the volcano, like a log.

Volcanic Disasters

  1. Tambora
    • Erupted in 1815
    • Put 50 cubic miles of ejecta into the atmosphere
    • Located in the East Indies
  2. Krakota
    • Erupted in 1883
    • Was an island of 3 cones
    • One vent got plugged and the island blew up
    • It produced a tidal wave that killed 30,000 people on Java
    • Largest explosion known to occur on the earth's surface
  3. Mt. Peele
    • Erupted on May 8, 1902
    • Located on the island of Martinique in the Caribbean Sea
    • The volcano blew out its side producing a Nuee Ardente
    • The temperature of the cloud was 1,500ºF
    • 40,000 people were killed in St.Pierre: 2 people lived
  4. Mt. Vesuvius
    • Erupted in 79 A.D.
    • Located in Naples, Italy
    • Destroyed Pompeii
  5. El Chichon
    • Erupted in April 1982
    • Killed 2,000 people
    • Located in Mexico
    • 20 million tons of volcanic ejecta
  6. Mt. Pinatubo
    • Erupted in June 1991
    • Located in the Philippines
    • 30 Trillion tons of volcanic ejecta
    • Largest eruption of the century

Nuee Ardente

A super heated steam with particles of lava hanging in the steam.

This cloud forms at night and quietly flows down the side of the volcano.

Many towns and villages are built at the foot of the volcano. They are at times directly in the path of the Nuee Ardente.

The Nuee Ardente forms quietly at the vent of the volcano. Since the steam is embedded with particles of hot lava, the cloud becomes very dense. The density is high enough that the cloud can not rise but slides down the side of the volcano. As it moves through the village, and due to its great density, it pushes all of the air out of the village. The villagers suffocate.

The Nuee Ardente is one of the most deadly things associated with a volcano.

Gases Released During Volcanic Explosions

  1. Water: steam that is super heated
  2. Carbon Dioxide
  3. Sulfur Dioxide and Hydrogen Sulfide
  4. Hydrocloric Acid: HCl
  5. Hydrofluoric Acid: HF
  6. Methane: natural gas

Fumaroles: small vents on the side of a volcano where gases escape.

Mt. Katmai: a volcano in Alaska that produces 1,250,000 tons of HCl per year.

Bowen's Reaction Series

In a chamber of magma all of the minerals do not crystallize out at the same time. The order of crystallization is called the Bowen's Reaction Series.

Bowens reaction series

Think of the magma chamber initially as being made up only of quartz. All the other minerals are dissolved in this quartz. As the chamber cools, its temperature is gradually going down. As the temperature decreases, different minerals solidify. Sometimes more then one mineral solidifies at a time. As each mineral is formed certain atoms are used up. Consequently, the remaining minerals must be formed out of different atoms giving the mineral a different composition. After all the minerals have formed, the chamber only has quartz in it which is the last to form. The first minerals to form do so at high temperatures. Quartz forms at the lowest temperature.