Introduction to Igneous Rocks:

Igneous rocks

      Formed from magma

            How is lava and magma different?

Two basic types of igneous rocks

intrusive

extrusive

Question:

How do these two “types” differ?

How does this affect the minerals that create the rocks?

Changes the texture of the rock

      Visual texture not physical

            “Texture” of Igneous Rocks

                  Intrusive

slow cooling

minerals grow large (can see unaided)

                  Extrusive

cools quickly

often hard to see minerals unaided

      Other differences

            Intrusive:

                  Water retained due to “sealed unit”  Creates element rich solutions

                  No evidence of flow

                  Never vesiculated

            Extrusive:

                  Water tends to degas

                  Often see flow structures…banding, jointing etc…

                  Often has vesicles

Composition of Igneous Rocks

      Based on total % of silica

            Some common rock forming minerals found in igneous rocks

                  Quartz:  SiO₂  (all silica)

Plagioclase  NaAlSi₃O₈ to CaAl₂Si₂O₈

Orthoclase:  KAlSi₃O₄

                  Augite:  Ca(Mg, Fe)Si₂O₃(Al,Fe)₂O₃

Biotite:  K(Mg, Fe)₃AlSi₃O₁₀(OH)₂

                  Olivine:  (Mg,Fe)₂SiO₄  (low silica)

Felsic:  high percentage of silica (SiO₂)   (>65%)

            Intermediate:  silica from 55-65%

Mafic:  silica from 40-55% and large amounts of magnesium (Mg) and iron (Fe)

Ultramafic: silica <40% and abundant iron and magnesium

      Based on total silica in rock…all minerals together

Viscosity of a magma/lava is controlled by the temperature and the silica content

      High silica = high viscosity

      High temperature = low viscosity

      What is Viscosity?

      Which rocks, Felsic or mafic will have the highest viscosity?

Other compositional changes

Felsic:

            High aluminum, sodium and potassium

      Ultra mafic:

            Low aluminum, sodium and postassium

Felsic:

            Low iron, calcium and magnesium

      Ultra mafic:

            High iron, calcium and magnesium

Felsic   (often light in color)
      intrusive:  Granite

extrusive:  Rhyolite

Intermediate (Salt and Pepper)

intrusive:  Dorite

extrusive:  Andesite  (Cascade Mountains)

Mafic (Dark)

intrusive:  Gabbro

extrusive:  Basalt  (Ocean floor)

Ultramafic

intrusive:  Peridotite (Mantle)

      Extrusive: Komatiites

How do we get different igneous rock types?

Why are there different compositions of magma?

Possible processes?

      In Class activity….work in teams and come up with ways that different compositions of magma can be created

Bowen’s Reaction Series
Revisited….

      Shows ideal mineral crystallization sequence

      Shows temperature minerals are stable

      Indicates which minerals are found in which rocks

      Can also use to see which order minerals melt

Peridotite (mantle)

      Rich in iron and magnesium (low in silica)

      Melting peridotite creates basalt and gabbro

      Creates ocean floor…Where does this happen?

Continental material

      Rich in silica and aluminum

      Often creates rhyolite and granite

      Found on continents

            Continental material + basalt = intermediate rocks
            OR
      Melt basalt

            Often found as volcanoes from subduction zones

Cascade mountains are primarily andesite and diorite

EXERCISE for Bowen’s reaction series

Based on temperature, which rock forms from a “hotter” magma, Granite or Gabbro?

Two magmas, one is granitic in composition and one is gabbroic in composition, which would be more viscous?

Which of the two rocks mentioned above (granite or gabbro)  have the minerals olivine and a calcium rich plagioclase in it

What minerals would the other rock have in it?

If a basaltic rock were to experience partial melting, which two minerals would be the first to melt and what would the new magma composition be?

How can we get a rock to melt?

      Add Heat

      Lower pressure

      Add water

What rock type is found at the following locations

      Divergent boundaries

Subduction zones

      Hot spots or mantle plumes

At all locations, both intrusive and extrusive rocks are found

Some Igneous rock Names

      Intrusive

            Granite

            Diorite

Gabbro

Periodotite

      Extrusive

Rhyolite

Andesite

            Basalt

Kimberlite

            Scoria

            Pumice

            Obsidian

Intrusive igneous rocks

      Granite                        (Felsic)

      Diorite                         (Intermediate)

Gabbro                        (Mafic)

Peridotite                     (Ultra mafic)

      Magma cools slowly over long period of time

      Hundreds of thousands to millions of years

      Creates larger crystals

      Magma rises by assimilation and eventually forms plutons

Pluton:  large volume of intrusive igneous rock

            Uplift, erosion and exposure create batholiths and stocks

Batholiths:  granitic

      Area exposed at surface is > 100 km2

      Examples:

Idaho Batholith

Sierra Nevada mountains

cooling time = ~130 mil years

Stocks:  silicic to intermediate composition

      Area exposed at surface is < 100 km2

      Often partially exposed batholith

Dikes

      Magma filled fractures

      All compositions of magma

      Often resistant to erosion

Sills

Magma intrudes between layers of rocks (often sedimentary)

Lacoliths & Lopoliths

Lacoliths  Bow upward in rock layers

Lopoliths bow downward in rock layers