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 EruptionsLava - 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 VolcanoesMorphology - 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 EruptionsVesuvius - 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 RocksPluton - 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 1275°C) 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.

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Types of Rocks

Rocks are not all the same!

The three main types, or classes, of rock are sedimentary, metamorphic, and igneous and the differences among them have to do with how they are formed.

Sedimentary
Sedimentary rocks are formed from particles of sand, shells, pebbles, and other fragments of material. Together, all these particles are called sediment. Gradually, the sediment accumulates in layers and over a long period of time hardens into rock. Generally, sedimentary rock is fairly soft and may break apart or crumble easily. You can often see sand, pebbles, or stones in the rock, and it is usually the only type that contains fossils.

Examples of this rock type include conglomerate and limestone.

Metamorphic
Metamorphic rocks are formed under the surface of the earth from the metamorphosis (change) that occurs due to intense heat and pressure (squeezing). The rocks that result from these processes often have ribbonlike layers and may have shiny crystals, formed by minerals growing slowly over time, on their surface.

Examples of this rock type include gneiss and marble.

Igneous
Igneous rocks are formed when magma (molten rock deep within the earth) cools and hardens. Sometimes the magma cools inside the earth, and other times it erupts onto the surface from volcanoes (in this case, it is called lava). When lava cools very quickly, no crystals form and the rock looks shiny and glasslike. Sometimes gas bubbles are trapped in the rock during the cooling process, leaving tiny holes and spaces in the rock

 

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Also

Structural geologists use a variety of methods to (first) measure rock geometries, (second) reconstruct their deformational histories, and (third) calculate the stress field that resulted in that deformation.
[edit] Geometries
Primary data sets for structural geology are collected in the field. Structural geologists measure a variety of planar features (bedding planes, foliation planes, fold axial planes, fault planes, and joints), and linear features (stretching lineations, in which minerals are ductily extended; fold axes; and intersection lineations, the trace of a planar feature on another planar surface).

Illustration of measurement conventions for planar and linear structures
[edit] Measurement conventions
The inclination of a planar structure in geology is measured by strike and dip. The strike is the line of intersection between the planar feature and a horizontal plane, taken according to the right hand convention, and the dip is the magnitude of the inclination, below horizontal, at right angles to strike. For example; striking 25 degrees East of North, dipping 45 degrees Southeast, recorded as N25E,45SE.Alternatively, dip and dip direction may be used as this is absolute. Dip direction is measured in 360 degrees, generally clockwise from North. For example, a dip of 45 degrees towards 115 degrees azimuth, recorded as 45/115. Note that this is the same as above.
The term hade is occasionally used and is the deviation of a plane from vertical i.e. (90°-dip).
Fold axis plunge is measured in dip and dip direction (strictly, plunge and azimuth of plunge). The orientation of a fold axial plane is measured in strike and dip or dip and dip direction.
Lineations are measured in terms of dip and dip direction, if possible. Often lineations occur expressed on a planar surface and can be difficult to measure directly. In this case, the lineation may be measured from the horizontal as a rake or pitch upon the surface.
Rake is measured by placing a protractor flat on the planar surface, with the flat edge horizontal and measuring the angle of the lineation clockwise from horizontal. The orientation of the lineation can then be calculated from the rake and strike-dip information of the plane it was measured from, using a stereographic projection.
If a fault has lineations formed by movement on the plane, eg; slickensides, this is recorded as a lineation, with a rake, and annotated as to the indication of throw on the fault.
Generally it is easier to record strike and dip information of planar structures in dip/dip direction format as this will match all the other structural information you may be recording about folds, lineations, etc., although there is an advantage to using different formats that discriminate between planar and linear data.
[edit] Plane, fabric, fold and deformation conventions
The convention for analysing structural geology is to identify the planar structures, often called planar fabrics because this implies a textural formation, the linear structures and, from analysis of these, unravel deformations.
Planar structures are named according to their order of formation, with original sedimentary layering the lowest at S0. Often it is impossible to identify S0 in highly deformed rocks, so numbering may be started at an arbitrary number or given a letter (SA, for instance). In cases where there is a bedding-plane foliation caused by burial metamorphism or diagenesis this may be enumerated as S0a.
If there are folds, these are numbered as F1, F2, etc. Generally the axial plane foliation or cleavage of a fold is created during folding, and the number convention should match. For example, an F2 fold should have an S2 axial foliation.
Deformations are numbered according to their order of formation with the letter D denoting a deformation event. For example D1, D2, D3. Folds and foliations, because they are formed by deformation events, should correlate with these events. For example an F2 fold, with an S2 axial plane foliation would be the result of a D2 deformation.
Metamorphic events may span multiple deformations. Sometimes it is useful to identify them similarly to the structural features for which they are responsible, eg; M2. This may be possible by observing porphyroblast formation in cleavages of known deformation age, by identifying metamorphic mineral assemblages created by different events, or via geochronology.
Intersection lineations in rocks, as they are the product of the intersection of two planar structures, are named according to the two planar structures from which they are formed. For instance, the intersection lineation of a S1 cleavage and bedding is the L1-0 intersection lineation (also known as the cleavage-bedding lineation).
Stretching lineations may be difficult to quantify, especially in highly stretched ductile rocks where minimal foliation information is preserved. Where possible, when correlated with deformations (as few are formed in folds, and many are not strictly associated with planar foliations), they may be identified similar to planar surfaces and folds, eg; L1, L2. For convenience some geologists prefer to annotate them with a subscript S, for example Ls1 to differentiate them from intersection lineations, though this is generally redundant.
[edit] Stereographic projections
Stereographic projection of structural strike and dip measurements is a powerful method for analyzing the nature and orientation of deformation stresses, lithological units and penetrative fabrics.
[edit] Rock macro-structures
On a large scale, structural geology is the study of the three dimensional relationships of stratigraphic units to one another within terranes of rock or within geological regions.
This branch of structural geology deals mainly with the orientation, deformation and relationships of stratigraphy (bedding), which may have been faulted, folded or given a foliation by some tectonic event. This is mainly a geometric science, from which cross sections and three dimensional block models of rocks, regions, terranes and parts of the Earth's crust can be generated.
Study of regional structure is important in understanding orogeny, plate tectonics and more specifically in the oil, gas and mineral exploration industries as structures such as faults, folds and unconformities are primary controls on ore mineralisation and oil traps.
Modern regional structure is being investigated using seismic tomography and seismic reflection in three dimensions, providing unrivaled images of the Earth's interior, its faults and the deep crust. Further information from geophysics such as gravity and airborne magnetics can provide information on the nature of rocks imaged in the deep crust.

Structural geology

Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation (strain) in the rocks, and ultimately, to understand the stress field that resulted in the observed strain and geometries. This understanding of the dynamics of the stress field can be linked to important events in the regional geologic past; a common goal is to understand the structural evolution of a particular area with respect to regionally widespread patterns of rock deformation (e.g., mountain building, rifting) due to plate tectonics