IGNEOUS ROCKS

Rocks - Aggregates of minerals

Igneous Rocks - Rocks formed by crystallization from a melt (magma)

  1. Extrusive (volcanic) - produced when magma flows on the earth's surface

  2. Intrusive (plutonic) - produced when magma solidifies at depth beneath the earth.

Classification of Igneous Rocks

Process-oriented. Based on the rate of cooling of the igneous rocks and their resultant grain size.

Texture - size, shape and arrangement of mineral grains in a rock.

Coarse grained - Individual mineral grains can be seen which the naked eye. Rock must have cooled slowly to allow large crystals to develop.

Fine grained - Mineral grains are present but are two small to be seem with the eye. Cooled rapidly before crystals had a chance to grow.

Vesicular - Rock containing vesicles (gas holes). Always light weight. Example pumice.

Glassy - Not composed of minerals at all but a true glass. Glasses are not crystalline!

All typical classification schemes rely on a combination of texture, particularly grain size, and mineralogy. But, keep in mind they are process-oriented. Coarse grained are plutonic, fine grained are volcanic. See Figure below that depicts a typical classification. Stress similar mineralogy of granite vs. rhyolite, just differ in grain size. Compare granite to gabbro which have the same grain size, but different mineralogy. Notice from figure how the three comon fine-grained rocks, rhyolite, andesite and basalt differ in their chemistry. Rhyolite is very rich in silica while basalt has less silica, but more iron and magnesium. Andesite is intermediate.



Volcanoes

Volcano - Cone shaped feature with a pit or depression at the summit.

Crater - The pit or depression at the top of the volcano.

Caldera - A destructive feature that marks the site of collapse of a volcano's summit. Form when magma chamber beneath the volcano is emptied.

Anatomy of a Volcanic Eruption

  1. Magma is generated at depth. Cause of magma generation is a combination of factors including; geothermal gradient, radioactive heating and friction along plate boundaries. Temperature of typical magma 600-1400 degrees C. Depth of generation 50-100 km based on geophysics.

  2. Because magma is less dense it begins to rise. Lithostatic pressure drops as magma rises and it begins to boil. This releases gas which exerts outward pressure.

  3. At depth of 3-5 km magma reaches gravitational equilibrium. Boiling continues. If outward pressure of gas exceeds lithostatic pressure an eruption occurs. Obviously greater the volatile content (water) the more potential for a destructive eruption we have. Viscosity of the magma also an important consideration.

Products of Volcanic Eruptions

Lava - Magma which flows on the surface

  1. Pahoehoe - Ropy, fast moving low viscosity lavas

  2. AA - Blocky, slow moving higher viscosity lava

Pyroclastics - Airborne material

  1. Dust - Fine fragments carried into upper atmosphere. Can remain suspended for weeks or years.

  2. Ash - Fragments of angular glass <.5 cm in diameter

  3. Cinders - Slag sized fragments .5-2.5 cm in diameter

  4. Lapilli - Fragments >2.5 cm

  5. Blocks - Very large angular fragments

  6. Bombs - Large rounded masses

Classification of Volcanoes

Morphology - appearance, size and shape

  1. Shield Volcano - Built up by repeated lava eruptions from a central vent. Very large with broad, mound- shaped, sides. Slopes 5-10 degrees. Typical example is Kilauea. Few in number and in center of plates.

  2. Composite (stratovolcano) Built from a combination of lave flows and pyroclastic material. Have smaller size, diameter 3-30kms and steeper slopes (10-30 degrees). Occur along plate margins. Many examples Vesuvius, Cascade volcanoes.

  3. Cinder Cone - Small feature a few thousand meters in diameter or less with very steep sides (30-40 degrees). Very numerous. Example Paracutin in Mexico.

Distribution of Active Volcanoes - Most lie in a belt around the Pacific termed Ring of Fire. Also occur in Southern Europe, Atlantic, Central Africa.

Comparison of Mt. St. Helens and Kilauea

Mt. St. Helens

Kilauea

violent eruption

quiescent eruption

mostly pyroclastics

mostly lava

sticky viscous lavas

low viscosity lava

rhyolite

basalt

composite cone

shield volcano

at plate margins

center of a plate

Some Volcanic Eruptions

Vesuvius - 79 AD Was a dormant volcano called Mt. Somma. Erupted with no warning in August. Eruption was so sudden inhabitants of Pompeii and Herculeaneum were buried where they lie. Eruption was believed to be a nuee ardente (fiery cloud) traveling at velocities of 150-200 km/hr. Prior to 79 AD last eruption believed to have occurred about 10,000 BC when Mt. Somma was formed. 79 AD eruption blew top off Mt. Somma and cone of Vesuvius was born. Since that time periodic eruptions have occurred to the present. Initial history one of repeated violent pyroclastic explosions. Since 17th century we are in a period of quiet eruptions accompanied by lavas.

Mt.St. Helens - May 18, 1980. Eruption preceded by numerous of small earth tremors and steam venting. Last previous eruption was 1831. Summit blown off removing upper 400 m in one blast of rock and ash. One of a chain of volcanoes from southern B.C. into northern California (Cascade Range).

Hawaiian Islands - Part of a 2400 km long chain of volcanic islands stretching across the central Pacific. Oldest islands in the chain are the most heavily eroded and have the oldest rocks. Lie to the northwest. Youngest islands are still active and lie at the southeast end of the chain. Postulated that the islands overlie a mantle hot spot. Movement of the Pacific lithospheric plate to the northwest over the stationary hot spot has caused the observed relationships.

Iceland - Fissure Eruptions occur as lava flow from a long linear fissure rather than a central volcano. Seem to be associated with spreading centers at constructive plate margins. Erupted lavas consist of voluminous low viscosity basaltic lava.

Intrusive Igneous Rocks

Pluton - Body of magma which has solidified beneath the earth. Classified based on whether they are concordant (i.e. they are parallel to layering of host) or discordant (cross cut host). Also if they are tabular (table-like) or massive (equi-dimensional football-shaped). (Figure)

  1. Sill - Tabular concordant pluton

  2. Dike - Tabular discordant pluton

  3. Laccolith - Massive concordant pluton

  4. Batholith - Massive discordant pluton


Magma Crystallization

By the end of the 19th century it was recognized that all igneous rocks formed from the crystallization of a magma. A fundamental question that followed was "why do we get so many different types of igneous rocks if we had one primordial starting material". Use the analogy of baking a cake. N.L. Bowen conducts the first systematic study of the crystallization of igneous rocks.

Publishes Bowen's Reaction Series (Figure) which shows that the minerals in igneous rocks crystallize in an orderly sequence. Discontinuous Series so named because as temperature falls we change from one new mineral to another (Ex. olivine alters to pyroxene). Continuous Series in which plagioclase feldspar merely changes composition from Ca-rich at high temperature to Na-rich at low temperature. Does not involve the formation of a new mineral, just a compositional change. This does not really help us understand why we have different igneous rocks, but it does seem to show that there is some order in nature. To more closely examine this order let's look only at the plagioclase feldspars. Why? Because plagioclase occurs in most igneous rocks. So if we can understand how and why feldspars form we may have some understanding about how different rocks form.


Figure (Phase Diagram for Plagioclase) Explain how the diagram works. Plot of temperature vs. composition. Upper line is liquidus. Separates liquid field from liquid + crystals field. Lower line is the solidus which separates the liquid + crystals field from the solid field. We can begin by examining the crystallization path of a liquid of composition X0. It cools to temperature X1 and at that point the first crystals begin to form. To determine their composition we project a horizontal line to the solidus and find they have the composition C1 or about 85% Ca plag. As temperature continues to fall liquid composition shifts along liquidus to X2 and solid crystals shift in composition along the solidus to C2. At the completion of crystallization, (about 1275C) the final solid has exactly the same composition as the starting liquid. This is an example of equilibrium crystallization.


Now let's look at what happens when we remove some of the crystals from the liquid as they form rather than allowing them to remain in contact with the liquid and change composition as they did in the example above. Result would be a series of fractions of crystals of different composition (Fractional Crystallization).

Theoretically, fractional crystallization seems possible, but how could it occur in nature? By the process of gravitative settling, in which the early formed crystals in a magma sink to the bottom of the chamber due to their greater density and as such are shielded from reacting with the magma. Result is a series of layers of crystals of differing composition. Where can we find such a phenomenon in nature? Figure shows layering in the Palisades Sill that has occurred as the result of gravitative settling and fractional crystallization.


Return to Bowen's Reaction Series and show the result of plotting the various major igneous rocks on the diagram (Figure). We could form each of these rocks as the result of fractional crystallization. The problem with fractional crystallization, however, is that it is not very efficient. Even under the best of circumstances we can form only 5% granite by fractional crystallization. Continents are 60% granite so where did all of it come from? Answer is there must be another mechanism involved. Go back to Plagioclase Phase Diagram and look at what happens if we take a solid of 50% Na plagioclase and 50% Ca plagioclase and heat it just enough to partially melt the solid. Liquid that forms is very Na-rich. Because it is a liquid it rises out of the system, eventually to crystallize higher in the crust. The solid that forms has the very same Na-rich plagioclase as the composition of the liquid. Thus if we partially melt a solid we can generate a liquid of very different composition which eventually recrystallizes as a rock of very different composition. This mechanism of forming rocks of different composition is termed Partial Melting and is thought to be the dominant mode of formation of the various different rocks.


Partial melting leads to the following:

peridotite ---> basalt

basalt ---> andesite

andesite --> granite (rhyolite)

Mantle of the earth thought to be peridotite. This conclusion ts based on the velocity of seismic waves and samples of peridotite found in diamond pipes. If we partially melt a peridotite (3-8%) the magma we generate has the composition of a basalt. Figure shows the typical result of partial melting of mantle peridotite at a divergent plate boundary such as the Mid- Atlantic Ridge. The crust is pulled apart and a basaltic magma is produced and then rises upward and emplaces itself on the sea floor as a pillow lava. Beneath the pillow lavas are diabase dikes, gabbro and peridotite.


The situation is different for the formation of granites at subduction zones. In order to form a partial melt at realistic depths we need water. This is because water dramatically lowers the melting point of rocks. The water comes from sediments carried down the subduction zone at convergent plate boundaries (Figure). Water lowers melting point of sediments and surrounding igneous rocks, thus forming a partial melt at 30-50km.


So the following occur:

At divergent plate boundaries peridotite mantle partially melts to give basalt magma.

At convergent plate boundaries water is carried down subduction zones causing partial melting and the formation of granitic magmas.