GeoClassroom Physical Geology Historical Geology Structure Lab Mineralogy Petrology


Introduction to Igneous and Metamorphic Petrology

Volcanoes and Igneous Rocks

Magma: Molten material consisting of liquid, gas, and a few xls.

1) Basaltic magma: 50% SiO2

    temp @ 900-1200 C
    Ca-rich plagioclase feldspar & pyroxene
    dark - > 40% dark minerals
tends to be highly fluid, low viscocity

2) Granitic magma: 65-77% SiO2

    <800 C temp
    quartz, orthoclase feldspar, Na-rich plagioclase
    light colored - < 15% dark minerals
    highly viscous - silica tetrahedrons link together to resist flow

Magma which reaches the surface and erupts from a volcano is termed extrusive, such as lavas and ash clouds. That which xlizes underground is called intrusive or plutonic.


    Cooling causes xls to form like ice on a pond.
    Rate of cooling controls xl size and growth.
      Fast cooling means there is not much time for xls to grow, thus they are small.
      Rapid cooling as a magam reaches the surface quenches the magma into a glass called obsidian.
      Slow cooling causes the xls to grow larger.

    Generally, the Si-O tetrahedra are the first things to form, then the other 8 abundant elements, and many other accessory elements, combine with these tetrehedra to form the basic rock-forming minerals.

    These minerals don't all form at once, they form at different times and under diferent conditions. And they depend on the intial chemical comositon of the magam itself.

Bowen's Reaction Series:

    Illustrates when certain minerals will xlize based on the temp and comp of a magma.

    When 50% of a magma has xlized, the remaining magma will be depleted in Fe, Mg, Ca because those elements would have gone into the Ol, Pyx, Amph, Ca-plag.

    Al, Na, K, Si becomes enriched in the remaining magma.
    The left side of the series is termed a discontinuous series because each mineral has a different xline structure.
    The right side is called a continuous series because Ca-plag becomes progressively Na-rich as the mineral xlize w/ decreasing temp. But the mineral is still a feldspar - it's just that Na replaces for Ca more and more as the temp decreases.


Basaltic eruptions:

    -most common volcanic activity. Produce BASALTS.
    -generally erupted from fissures in the earth's crust, usually at the MOR.
    -not explosive, generally erupts lava which flows freely down slopes.
      Aa: little gas, thick, slow moving, outer surface cools quickly crust breaks into jagged pieces as the flow continues to move.
      Pahoehoe: smooth wrinkly skin, fast moving, thin, low viscosity, ropy textured skin

    -gases can be 70% water, 15% CO2, 5% N,S, and then some other compounds of Cl, H, and Argon.

    -when pressure in a magma chamber is lessened, gases like CO2 come out of solution, like a COKE can, and cause the volcano to erupt.
    -in addition to lava flows, these basalitic eruptions produce splatter cones from the accumulation of droplets and blobs of lava blown out of the vent. This lava quenches into a glass and forms a small cone.

    Basaltic lava flows form Shield volcanoes.

      Slightly domed structures form from the successive build up of lava flows.
      Can be very large, like Mauna Kea in Hawaii. Largest mountain on earth. Slopes do not exceed 15%

Silicic eruptions:

    -produce granites and associated igneous rocks
    -cooler than basaltic eruptions, but more violent
    -produce much more in the way of PYROCLASTICS.
      -defined as particles and material ejected into the air from a volcano, as opposed to a lava flow which hugs the ground.
        -welded tuff
        -also produce thick, viscous lava flows.

    -Violent because there are lots of gases in silicic magmas. Gas is prevented from escaping by the viscous magma, and pressure builds. When released, the euption tends to be violent.

    - As ash falls to the ground, it compresses into a hot ASH-FLOW TUFF. These hot ash clouds and gases can race down the side of a mountain at near 125 mph wiping out everything in its path. These are called Nuee Ardentes. When ash becomes saturated with water, by rain, it can flow down valleys. These are called Lahars.

    -Volcanoes with alternating layers of ash and lava flows are termed stratovolcanoes or composite volcanoes. Mt. St. Helen's is an example.

Igneous Rocks

    -magma which never reaches the surface, but xlizes underground instead.

    -generally coarser grained rx.

    Batholith and stock:

    Stock is the same except that parts of the rocks are exposed to the surface with and outcrop of less than 10 km2.
      Sill and laccolith

Distribution of igneous activity:

    ->600 active volcanoes in the world today. There is usually one eruption per month, although they don't necessarily make the news.

    1) Divergent plate boundaries:

      -basaltic-type volcanoes
      -greatest # of volcanoes and amount of material.
      -basalt makes up the entire oceanic crust

    2) Convergent plate boundaries:

      -silicic-type volcanoes
      -Cascades Mountains, Andes Mtns.
      -when decending plate reaches 100 km, it partially melts to create a silicic magam, which rises to the surface because of the density difference.

    3) Hot spots.

      -mantle plumes
      -plates move over stationary plumes, creating a chain of volcanoes like Hawaii.
      -basalitic rocks in Hawaii
      -intermediate rx in Yellowstone because the orginal basaltic magma has incorporated some silicic continental crust.

Metamorphism and Metamorphic Rocks


    - transformation of pre-existing rx into other rx by heat, pressure, and chemically-active fluids.
    - can metamorphose igneous, sedimentary, or even other metamorphic rocks.
    - changes are textural as well as mineralogical.

- keep in mind that rocks are capable of plastic deformation

    - like silly putty
    - takes great heat & pressure & time.
    - easily done in lab experiements.

- when huge areas of rx are affected during mountain building, it's called regional metamorphism. When 2 plates collide.

- when a hot igneous body pushes up into the crust, the crust is affected by contact metamorphism. Forms halo (aureole) of metam. rx around intrusion.

1) Heat is the most important factor

    - increases with depth. Can reach >750C.
    - causes minerals which are in equilibrium at the surface at surface temps to melt and recrystallize or just change their crystal structure to form another mineral which is stable at the new higher temp.
    - specific minerals form at different temps (kyanite, andalusite, sillimanite), so we can tell how hot a rock got by which mins it contains.

2) Pressure also increases with depth.

    - changes the texture of a rock by aligning & the mineral grains to form foliation.
      - micas and chlorite do this
      - forms a book structure
    - makes the rock deform plastically, allowing rocks to fold as they are subject to forces.

3) Chemically-active fluids

    - water is the most important agent, especially if it is contaminated with various elements.
    - as temps increase, this water aids in chemical reactions during mineral recrystallizations.
    - water is found in the spaces between grains in many rx (pore fluid).
    - water is a medium of transport to take away unwanted elements as minerals change into other minerals.
    - pressure forces the water out of the pore spaces, taking the unwanted elements with it.


    - slaty cleavage
      - fine-grained mica minerals aligning to give the entire rock a layered look, like slate.
    - schistosity
      - medium-grained mica xls align to form rx with a shiny scaly appearance.
    - gneiss
      - high temps and press cause light and dark mins to segregate into separate layers.

All Pages Copyright © GeoClassroom. All Rights Reserved.