Igneous RocksUnit Essential Question:

Why is there an essentially fixed amount of rock on Earth?

Presentation Objectives: § Define rock, viscosity, crystallization, catastrophism, and uniformitarianism. § Explain the main differences between magma types based on composition. § Explain how magma forms and then how crystals/minerals form upon cooling. § Classify igneous rocks based on place of formation, texture, and composition. § Identify the three main volcano shapes.

Geology Rocks!• A rock is a naturally occurring combination of minerals

and/or mineraloids. • Mineraloid are the substances that are mistaken as minerals, but

don’t fit the definition.

Opal Amber

• There are three rock types that are based on how the rock formed. • Igneous - formed from the cooling of magma or lava. • Sedimentary - formed from the cementing of mineral and rock fragments. • Metamorphic - formed when rocks experience extreme heat and/or

pressure.

Q1

Brief History Lesson!• It was originally believed that the Earth was created on

October 22, 4004 B.C. • In addition, many believed in the idea of Catastrophism.

• The features seen on Earth are the result of catastrophic events.

• James Hutton is considered the father of modern geology. • Proposed the Theory of Uniformitarianism.

• The Theory of Uniformitarianism states that the current geologic processes have always occurred, at about the same rate, and can account for all of Earth’s features. • “The present is the key to the past.”

• By understanding what happens today, we can figure out what has happened...

These ideas inspired Charles Lyell, who inspired Darwin, who developed the theory of Evolution.

Q2 It’s All About Magma and Lava!• Magma is a slushy mix of molten rock, gases,

and mineral crystals. • The main ingredient of magma is Silica (SiO2).

• There are three types of magma which vary in composition and therefore viscosity.• Viscosity is the ability of a fluid to flow.

• Molasses has a high viscosity while water has a low viscosity.

• The viscosity of a magma is influenced by the amount of silica and the amount of dissolved gases in the magma.

• The viscosity of the magma determines its explosiveness!

Type Silica Content Gas Content Viscosity Explosiveness

Basaltic 50% 1-2% Low Least

Andesitic 60% 3-4% Medium Intermediate

Rhyolitic 70% 4-6% High Greatest

Q3

Composition

Igneous Rock Type Type of Magma Color

Felsic Rhyolitic Light

Intermediate Andesitic Medium

Mafic Basaltic Dark

Ultra Mafic Upper mantle? Green

Q4

Basalt - MaficGranite - Felsic Andesite - Intermediate

• Since there are different types of magma we can classify igneous rocks by their composition.

Ultra Mafic ExamplesUltra mafic rocks contain only olivine and pyroxene, so they

have a greenish, black look to them.

Pyroxenite Dunite

What Do You Think?Is obsidian felsic, intermediate, or mafic?

Q5 Magma vs. Lava• Igneous rocks are formed from the cooling and crystallization

of magma or lava. • Two types of based on Location of Formation:

• Intrusive: Igneous rocks that cool slowly from magma beneath the Earth’s surface.

• Extrusive: Igneous rocks that cool quickly from lava on the surface.

• Two igneous rocks can have the same composition, but one can be intrusive while the other is extrusive.

Extrusive Intrusive

Q6

How Does It Feel?• The texture of an igneous rock is dependent on the size of the

crystals, which is often referred to as grain size. • Grain size is dependent on the rate at which the magma/lava cools.

Cools Quickly

• There are three main textures: • Fine-grained: crystals are too small to see without a microscope.

• Coarse-grained: crystals are large enough to see with your eye or with a hand-lens.

• Porphorytic: has large crystals in a matrix of small crystals.

• Small Crystals

Q7

Cools Slowly• Large Crystals

Now For The Weird Ones...• Glassy - formed so quickly that no crystals form.

• Vesicular - igneous rocks that have holes in it.

Obsidian

Scoria Pumice

Q8

Linking Them Together• Where the rock forms will determine its texture.

Let’s see if you can link them together!

Making Magma• Magma is formed from melted rocks.

• Magma forms from a process known as partial melting. • Since different minerals have different melting points, not all

of them will melt at the same time.

Q8

• As temperature increases, more minerals melt, changing the composition of the magma.

800-1400°C

Making Rocks• When magma or lava cools, it begins to solidify.

• This process is known as crystallization.

• Because crystallization depends on temperature, we get a predictable sequence of mineral formation known as the Bowen’s Reaction Series.

Q9 Where Can You Find Them?Since igneous rocks form from lava and magma, they are found

near active, dormant, and extinct volcanoes!

• There are about 1500 active volcanoes currently in the world.

• The United States ranks third after Indonesia and Japan, in the number of historically active volcanoes.

• Volcanoes will take on different shapes based on their level of explosiveness. • Shield • Cinder Cone • Composite

Q10

Quiet eruption

Explosive eruption