Rocks Igneous rocks make up the majority of the Earth s crust. Sedimentary rocks dominate the Earth s surface. Igneous Rocks Sedimentary Rocks Metamorphic Rocks Igneous Rocks All rocks that form from cooling of a mass of molten rock (melt or magma). Includes crystalline rocks (interlocking mineral crystals) and glasses (lacking crystalline minerals). First order classification: based on average crystal size (termed texture). Coarse-grained: 1 mm or larger. Fine-grained: less than 1 mm. Phaneritic: mineral grains can be seen with the unaided eye. Aphanitic: mineral grains cannot be seen with the unaided eye. Coarse-grained igneous rocks are formed as intrusive rock bodies: They crystallize relatively slowly within the Earth s crust. Fine-grained igneous rocks are formed as extrusive rock bodies: They crystallize relatively quickly at or very near the surface of the Earth. In general, the size of the crystals depends on the rate of cooling; the slower the rate of cooling the larger the crystals that form. 1
Magma that is extruded to the Earth s surface is called lava. Many lavas crystallize so quickly that there is no time for the organized structure of crystals to develop. The texture of such rocks is termed glassy and the rocks lack discrete minerals. Gas trapped in the magma when it cools quickly forms bubbles that remain after cooling and solidification; the resulting void spaces in the rock are termed vesicules Obsidian is an igneous rock that cooled very quickly at the Earth s surface and displays a glassy texture and conchoidal fracture. Pumice is a glassy igneous rock that is characterized by many small vesicules. Additional terms related to crystal size: Pegmatite: very coarsegrained igneous rock with crystals exceeding 2.5 cm in size. The world s largest crystals: gypsum crystals found in caves near a zinc and silver mine in Mexico. Porphyritic: large crystals set in a matrix of finer crystals. The large crystals are termed phenocrysts. 2
Igneous rocks are classified more precisely on the basis of the relative proportions of their minerals. Silicic or Felsic rocks: white, grey or pink in colour; rich in quartz, potassium feldspars and sodium plagioclase feldspars and biotite/muscovite. Intermediate rocks: salt and pepper for coarsegrained rocks, dark grey for fine-grained rocks; rich in amphiboles and calcium plagioclase feldspars. Mafic rocks: dark grey to black in colour; rich in calcium plagioclase feldspars and pyroxene. Ultramafic rocks: green to black in colour; rich in olivine. Igneous rock names based on texture and composition. Coarse-grained Fine-grained Silicic or felsic Granite Rhyolite Diorite Andesite Intermediate Gabbro Basalt Mafic Ultramafic Peridotite Komatiite 3
The Earth can be considered as being made up of a series of concentric spheres, each made up of materials that differ in terms of composition and mechanical properties. The temperature increases from the surface of the crust to the centre of the core (7000 degrees C). Crystallization from Magma Magmas begin deep within the crust or the upper mantle where temperatures are high enough to melt rock. Geothermal Gradient: the rate of increase in temperature with depth beneath the Earth s surface. On average: 3 C per 100 m depth. the world's deepest mine, 3,585 m below surface at the East Rand mine, SA. The melting or crystallization temperature depends on: The pressure exerted on the material (which depends on the depth of burial). The amount of water that is present within the magma. Melting/Crystallization temperature increases with depth beneath the Earth s surface (if the rocks are dry) due to the increase in pressure with depth. Melting of dry rocks will normally not occur beneath continents because temperature do not become sufficiently high. The chemical composition of the magma. 4
Water, under pressure, substantially reduces the melting temperature. The greater the pressure that is exerted on the water the lower the melting temperature. In the presence of water melting will take place beneath continents. Melting/crystallization temperature varies widely depending on the composition of the magma. Complete melting of a mixture of potassium feldspar (Kspar) and quartz occurs at a minimum of 1000 C when there is 42% Quartz and 58 % K-spar. The melting temperature increases with decreasing quartz to 1300 C for pure K-spar. The melting temperature increases with decreasing K-spar to over 1500 C for pure Quartz. The melting or crystallization temperature depends on: The pressure exerted on the material (which depends on the depth of burial). The amount of water that is present within the magma. The chemical composition of the magma. 5
Some magmas begin within the mantle as semisolid masses. Even though the temperature is very high the extreme pressure inhibits melting. These masses may slowly rise towards the crust due to convection within the mantle. At a depth of about 50 km from the Earth s surface pressure is low enough to allow melting to form a magma. The rising plume of magma remains hotter than the ambient mantle, retaining heat from greater depths. The plume rises into the overlying crust and continues to migrate upwards. It continues to cool as it moves through the crust. Higher in the crust temperatures are lower and the magma cools and crystallizes into a body of igneous rock. If it doesn t cool within the crust it reaches the surface to form a volcano. 6
The temperature at which a mineral crystallizes from a magma depends on its composition. Bowen s Reaction Series describes the sequence in which minerals will crystallize with decreasing temperature in the magma or melt. The rock type that forms from the crystallization of a magma depends on: The initial composition of the magma. The stage at which the minerals crystallized. When a rock heats up the minerals melt in the reverse order to Bowen s Reaction Series. The wide variety of igneous rocks is due to three primary processes: 1. Crystal settling and magmatic differentiation. 2. Assimilation of host rock. 3. Magma mixing. 1. Crystal settling and magmatic differentiation. Time 1 While the magma body is first emplaced into the crust it has an initial composition. The first igneous rocks may be mafic rocks with abundant iron and magnesium. 7
Time 2. As the first crystals begin to form in the magma (olivine) they remove iron and magnesium from the magma, changing the composition of the magma as the crystals settle to the bottom of the magma chamber. Time 3 As successive minerals crystallize, following Bowen s series, the composition of the magma continues to change or differentiate. Over the period of crystallization of the magma the types of igneous rock change due to the changing chemical composition of the magma. The last rocks to form with have a felsic or silicic composition, reflecting the composition of the differentiated magma. 2. Assimilation of host rock: if the rock into which the magma has intruded is melted by the high temperatures, its inclusion in the magma will change its composition; the rock type that forms will similarly change. 3. Magma mixing: if two magmas with different compositions become mixed, the resulting magma will have a different composition and different rocks will crystallize from it. 8
Igneous Structures Volcanoes are structures that are produced by extrusive igneous activity (when magma is extruded to the surface). Plutons are solitary masses of igneous rock within the crust. Over millions of years the surface of the crust is eroded away. If the surface rocks are softer than the igneous rocks of the pluton it will form a topographic high as it resists erosion. When many plutons are emplaced into the crust they coalesce to form a larger structure called a batholith. Exposed batholiths form broad uplands when they are exposed by erosion. Batholiths form extensive masses of igneous rock that may become exposed at the surface following erosion of the land surface. 9
Mt. Rushmore is likely the best known batholith! Mt. Evans Batholith, Colorado Smaller intrusive structures commonly extend away from major bodies such as batholiths and plutons. Dikes cut across layered strata that they intrude. Sills intrude along planes that are parallel to associated strata. A vertical dike forms a resistant ridge of igneous rock that intruded softer sedimentary rocks. Image by Dr. Roger Bain. http://enterprise.cc.uakron.edu/geology/natscigeo/lectures/igneous/volcano2.htm#intrusions 10
Sedimentary Rocks Sedimentary rocks include those that are made up of discrete particles of minerals or rock fragments (termed clastic sedimentary rocks) and those made up of interlocking crystals (termed chemical sedimentary rocks). Individual grains in clastic rocks are surrounded by cement (normally of calcite, dolomite or quartz) Igneous Rock Clastic Sedimentary Rock Thin sections are 30 micron (30/1000 mm) thick slices of rock through which light can be transmitted. Click here to see how a thin section is made. http://faculty.gg.uwyo.edu/heller/sed%20strat%20class/sedstratl1/thin_section_mov.htm Weathering, transport and deposition of sediment Sedimentary rocks: composed of the products of weathering of source or parent rocks. Weathering: the process by which a rock breaks down when exposed at or near the Earth s surface. Physical or mechanical weathering involves the physical breakdown of the source rock. Frost wedging, unloading expansion, thermal expansion, biological activity. Frost wedging produced the scree or talus at the base of this mountain in the Northwest Territories. Solid particles are produced. 11
When a granitic pluton is deep within the crust it is compressed by the great weight of overlying rock. When erosion of the land surface exposes the pluton the weight is removed and it expands. Tree roots can grow into the fractures in rocks. As they grow they exert considerable pressure and cause the fractures to expand. Eventually the roots may break the surface rocks entirely into large boulders. As it expands it exfoliates like the skin of an onion into sheets of rock. Chemical weathering takes place when the source rock undergoes chemical reactions with surface water in contact with it. Chemical weathering produces: Solutions. The resistance of igneous minerals to weathering is similar to the Bowen s Reaction Series. The most stable minerals are those that crystallize last (quartz, k- spar and muscovite). Minerals that crystallize under high temperature are more prone to chemical weathering. Stable mineral grains (e.g., quartz) as detrital grains. New minerals grains (e.g., clay minerals, oxides). 12
Solid grains may be transported by: Rivers Wind Glaciers Ocean currents Volcanic explosions Solutions are transported largely by rivers. Clastic sediment: made up of the solid products of weathering. Deposition takes place when medium ceases to move the particles. Clastic sedimentary rocks include: Sandstone Conglomerate Shale Clastic sediment becomes a sedimentary rock following compaction and cementation. Compaction involves the pushing together of the particles by the weight of overlying sediment that is subsequently deposited. Cementation involves the precipitation (crystallization) of minerals that are in solution in waters flowing through the sediment. The precipitate forms a cement in the void spaces between particles and binds them together. Calcite and quartz are common cements in sedimentary rocks. Chemical sediment: made up of material that is transported in solution. Chemical sediment is deposited when material in solution is precipitated Precipitation may take place: Due to changes in water chemistry. Due to evaporation (e.g., halite). Due to shell production by organisms. Many limestones are made up of calcite produced by organisms. 13
Clastic sedimentary rocks are classified on the basis of their average grain size. Conglomerate is made up of well-rounded gravel. Sediment Name Rock Name Average Grain Size (particle shape) Gravel Conglomerate (rounded) > 2 mm Breccia (angular) Sand Sandstone/Arenite 2 0.0625 mm Breccia is characterized by angular gravel. Silt Siltstone/Lutite 0.0635-0.004 mm Clay Claystone/shale <0.004 mm The shape of the gravel indicates that it has not traveled far from where it formed. Sandstone: Individual grains can be seen with the naked eye. Shale is smooth to the touch and weathers into thin flat slabs. Siltstone is very fine-grained but feels gritty to the touch. 14
Chemical sediments are classified on the basis of their chemical composition. Gypsum formed in a playa lake. Halite (NaCl) and Gypsum (CaSO 4 +H 2 0) form by precipitation of salt water. Ions dissolved in water form crystals as the water evaporates. Halite accumulations in Death Valley Halite hopper crystal Gypsum rosettes Limestone (CaCO 3 ) forms most commonly by the accumulation of whole and/or broken shell material. Dolomite (MgCO 3 ) commonly forms from limestone when a magnesium ion replaces the calcium ion bonded to the carbonate ion. The reaction of limestone to hydrochloric acid. CaCO 3 + 2H + Ca 2+ + CO 2 (gas) + H 2 O Limestone and dolomite are commonly very fossiliferous. 15
Primary Sedimentary Structures Many clastic rocks, limestones and dolomites display structures that formed at the time that the sediment was deposited. Sun cracks: formed when previously wet muds dry out to form a polygonal pattern of cracks. Modern sun cracks Sun cracks on an ancient sandstone Wave ripples are straight-crested, symmetrical mounds of sand that form when waves act on the water above a deposit of sand. They indicate that the sediment was laid down in an environment that was influenced by waves (a lake or sea). Wave ripples on a vertical rock face. Current ripples are asymmetric in cross section and have short, curved crests. The upstream side has a gentle slope whereas the downstream side is steep. Metamorphic Rocks Metamorphic rocks form when pre-existing rocks are subjected to high temperatures and/or pressure and interaction with chemically active fluids. Original minerals may not be stable under the changed P/T conditions so new minerals form that are stable. Metamorphic Grade: a measure of the degree to which a rock has changed during metamorphism. 16
Metamorphic Grade is reflected by Index minerals: minerals that form under a limited range of pressures and temperatures. Types of Metamorphism Burial metamorphism: occurs when rocks become buried within the crust due to subsequent deposition. As they are buried deeper the temperature and pressure increases. Metamorphism begins at temperatures above 200 C (about 8 km depth). Contact metamorphism: takes place when an igneous intrusion heats up the rocks into which it intrudes. Only rocks near the intrusion are affected. The zone of contact metamorphism is termed a metamorphic aureole. 17
The type of rock that forms with contact metamorphism varies with the composition of the original rock and the distance from the intrusion (cooling away from the intrusion). Sandstone Quartzite Limestone Marble Regional Metamorphism: The most common metamorphic rocks; formed over extensive areas due to high temperatures and pressures associated with the interaction between tectonic plates. Unlike burial metamorphism, pressures have a preferred direction. Foliation: the tendency in regional metamorphosed rocks to have minerals that are preferentially oriented parallel to each other. A directed pressure may align minerals into an orientation that is perpendicular to the applied force. 18
Under a directed pressure crystals that grow will grow more readily in the direction that is perpendicular to the applied force. Pressure and temperature ranges for different types of metamorphism. Burial and Contact metamorphic rocks are not foliated and include: View pictures of the metamorphic rocks described below at: http://www.gpc.edu/~pgore/geology/geo101/meta.htm A site created by Pamela J.W. Gore of Georgia Perimeter College Marble: metamorphosed limestone. 19
Foliated rocks formed by Regional Metamorphism (in order of increasing metamorphic grade): Quartzite: metamorphosed sandstone. Slate: produced by low grade metamorphism of shale. Phyllite: characterized by a silky sheen due to the presence of very fine grained muscovite. Hornfels: a general term for low pressure metamorphic rocks. Pamela Gore points to the similarity of the sheen to frosted eye shadow and notes that many cosmetics have ground up muscovite to produce such a sheen. Schist: a metamorphic rock with abundant large micas minerals up to several millimetres across. Folded gneiss on Greenland Gneiss: (pronounced "nice") - a banded rock characterized by alternating layers of dark and light minerals. The dark layers commonly contain biotite, and the light layers commonly contain quartz and feldspar. Migmatite: a very high grade metamorphic rock that is intermediate between metamorphic and igneous rocks (i.e., they have undergone partial melting). 20
Igneous Rocks The geologic cycle Sedimentary Rocks Metamorphic Rocks A concept that relates the three rock types through processes that act in their formation. Involves: Cooling of magma (to form igneous rocks). Heat and pressure inside the earth (metamorphism or melting to form a new magma). Uplift of buried rocks by tectonic processes (e.g., mountain building). The geologic cycle. Weathering: the breakdown of a rock exposed at the Earth s surface. Transport of weathering products (e.g., by rivers). Deposition of transported material (as loose sediment) to where it can no longer be transported. The geologic cycle. Burial and compaction: covered by subsequent deposition and pushed into close contact due to the weight of overlying sediment. Cementation: the binding together of sedimentary particles by minerals that act as a cement. 21
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