Felsic minerals are usually light in color and have specific gravities less than 3. Common felsic minerals include quartz, muscovite mica, and the orthoclase feldspars. The most common felsic rock is granite , which represents the purified end product of the earth's internal differentiation process.
The density of the rock is obtained from. This method does poorly if the rock is unconsolidated or porous. Unconsolidated rocks tend to break apart when immersed in water. Pycnometer measurements are generally used on porous rocks and sediments. Once a rock has been placed in the chamber, the volume of the chamber is changed, resulting in a change in gas pressure around the rock.
The skeletal density of the rock is therefore:. Gamma-gamma density logging is used to continuously record the bulk density of rock formation along a borehole. For these measurements, gamma rays are emmitted by a radioactive source.
While transmitting through the adjacent rock, the gamma rays interact with electrons and undergo Compton scattering. A detector within the borehole measures the intensity of the scattered gamma rays. Gamma rays experience a higher level of Compton scattering in denser materials.
Therefore, the intensity of scattered signal can be used to obtain the bulk density of the rocks within the borehole. Each rock type is defined by a range of density values not a specific value. The densities for some common rocks, minerals, and other materials are summarized below.
Shallow intrusions like dikes and sills are usually fine grained and sometimes porphritic because cooling rates are similar to those of extrusive rocks. Coarse grained rocks, formed at deeper levels in the earth include gabbros, diorites, and granites.
Note that these are chemically equivalent to basalts, andesites, and rhyolites, but may have different minerals or different proportions of mineral because their crystallization history is not interrupted as it might be for extrusive rocks see figure 6. Pegmatites are very coarse grained igneous rocks consisting mostly of quartz and feldspar as well as some more exotic minerals like tourmaline, lepidolite, muscovite. These usually form dikes related to granitic plutons.
Igneous activity is currently taking place as it has in the past in various tectonic settings. These include diverging and converging plate boundaries, hot spots, and rift valleys. Divergent Plate Boundaries. At oceanic ridges, igneous activity involves eruption of basaltic lava flows that form pillow lavas at the oceanic ridges and intrusion of dikes and plutons beneath the ridges.
The lava flows and dikes are basaltic and the plutons mainly gabbros. These processes form the bulk of the oceanic crust as a result of sea floor spreading. Magmas are generated by decompression melting as hot solid asthenosphere rises and partially melts.
Subduction at convergent plate boundaries introduces water into the mantle above the subduction and causes flux melting of the mantle to produce basaltic magmas.
These rise toward the surface differentiating by assimilation and crystal fractionation to produce andesitic and rhyolitic magmas. The magmas that reach the surface build island arcs and continental margin volcanic arcs built of basalt, andesite, and rhyolite lava flows and pyroclastic material.
The magmas that intrude beneath these arcs can cause crustal melting and form plutons and batholiths of diorite and granite Hot Spots. As discussed previously, hot spots are places are places where hot mantle ascends toward the surface as plumes of hot rock.
Decompression melting in these rising plumes results in the production of magmas which erupt to form a volcano on the surface or sea floor, eventually building a volcanic island. As the overriding plate moves over the hot spot, the volcano moves off of the hot spot and a new volcano forms over the hot spot. This produces a hot spot track consisting of lines of extinct volcanoes leading to the active volcano at the hot spot. A hot spot located beneath a continent can result in heat transfer melting of the continental crust to produce large rhyolitic volcanic centers and plutonic granitic plutons below.
A good example of a continental hot spot is at Yellowstone in the western U. Occasionally a hot spot is coincident with an oceanic ridge. In such a case, the hot spot produces larger volumes of magma than normally occur at ridge and thus build a volcanic island on the ridge.
Such is the case for Iceland which sits atop the Mid-Atlantic Ridge. Rising mantle beneath a continent can result in extensional fractures in the continental crust to form a rift valley. As the mantle rises it undergoes partial melting by decompression, resulting in the production of basaltic magmas which may erupt as flood basalts on the surface. Melts that get trapped in the crust can release heat resulting in melting of the crust to form rhyolitic magmas that can also erupt at the surface in the rift valley.
An excellent example of a continental rift valley is the East African Rift. In the past, large volumes of mostly basaltic magma have erupted on the sea floor to form large volcanic plateaus, such as the Ontong Java Plateau in the eastern Pacific. Such large volume eruptions can have affects on the oceans because they change the shape of ocean floor and cause a rise in sea level, that sometimes floods the continents. The plateaus form obstructions which can drastically change ocean currents.
These changes in the ocean along with massive amounts of gas released by the magmas can alter climate and have drastic effects on life on the planet. Examples of questions on this material that could be asked on an exam. Classification of Igneous Rocks Igneous rocks are classified on the basis of texture and chemical composition, usually as reflected in the minerals that from due to crystallization.
Physical Geology. Magmas and Igneous Rocks. Magma and Igneous Rocks Igneous Rocks are formed by crystallization from a liquid, or magma. Types of Magma Chemical composition of magma is controlled by the abundance of elements in the Earth. Viscosity of Magmas Viscosity is the resistance to flow opposite of fluidity. Higher SiO 2 content magmas have higher viscosity than lower SiO 2 content magmas Lower Temperature magmas have higher viscosity than higher temperature magmas.
Intermediate or Andesitic. Origin of Magma As we have seen the only part of the earth that is liquid is the outer core. Melting of dry rocks is similar to melting of dry minerals, melting temperatures increase with increasing pressure, except there is a range of temperature over which there exists a partial melt.
Melting of rocks containing water or carbon dioxide is similar to melting of wet minerals, melting temperatures initially decrease with increasing pressure, except there is a range of temperature over which there exists a partial melt. Three ways to Generate Magmas From the above we can conclude that in order to generate a magma in the solid part of the earth either the geothermal gradient must be raised in some way or the melting temperature of the rocks must be lowered in some way.
Decompression Melting - Under normal conditions the temperature in the Earth, shown by the geothermal gradient, is lower than the beginning of melting of the mantle.
Thus in order for the mantle to melt there has to be a mechanism to raise the geothermal gradient. Once such mechanism is convection, wherein hot mantle material rises to lower pressure or depth, carrying its heat with it. Chemical Variability of Magmas The chemical composition of magma can vary depending on the rock that initially melts the source rock , and process that occur during partial melting and transport.
Initial Composition of Magma The initial composition of the magma is dictated by the composition of the source rock and the degree of partial melting. Magmatic Differentiation But, processes that operate during transportation toward the surface or during storage in the crust can alter the chemical composition of the magma.
Now let's imagine I remove 1 MgO molecule by putting it into a crystal and removing the crystal from the magma. Now what are the percentages of each molecule in the liquid? If we continue the process one more time by removing one more MgO molecule Thus, composition of liquid can be changed. Igneous Environments and Igneous Rocks The environment in which magma completely solidifies to form a rock determines: The type of rock The appearance of the rock as seen in its texture The type of rock body.
Magma mixing in granitic rocks of the central Sierra Nevada, California. Earth and Planetary Science Letters 66 , — Bateman, P. Geological Society of America Bulletin 90 , — Partial Melting Because the mantle is composed of many different minerals, it does not melt uniformly. Crystallization and Magmatic Differentiation Liquid magma is less dense than the surrounding solid rock, so it rises through the mantle and crust. Fractional crystallization occurs in the diapirs in the crust.
Source: Woudloper Fractionation or fractional crystallization is another process that increases the magma silica content, making it more felsic [ 9 ]. If magma at composition A is ultramafic, as the magma cools it changes composition as different minerals crystallize from the melt and settle to the bottom of the magma chamber. In section 1, olivine crystallizes; section 2: olivine and pyroxene crystallize; section 3: pyroxene and plagioclase crystallize; and section 4: plagioclase crystallizes.
The crystals are separated from the melt and the remaining magma composition B is more silica-rich. Source: Woudloper.
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