Igneous Intrusions in the Continental Crust - Report Example

Igneous Intrusions in the Continental Crust Introduction Rocks are the materials large volumes of the earth along with other planetary bodies are comprised of. There are various ways in which rocks are formed in the continental crust. The continental crust is the layer of sedimentary and granitic, along with metamorphic rocks that form the continents. A liquid rock that is formed beneath the Earth surface is known as an intrusion. The intrusions are formed are when new materials are added to the continents result in lighter material such as magma to rise. As the magma rises various features can be formed. The magma below the surface is gradually pushed up from deep within the earth into fissures, and in some instances it is even pushes the existing country rock out of its path. The process usually takes millions of years. Magma is a mixture of gas, crystals, and liquid rock. Magma is usually linked with various chemical compositions, and high temperature, as well as liquid properties. They are also less dense in comparison to the surrounding rocks explaining why they move upward. If the magma makes it to the surface it forms extrusive igneous intrusions. On the other hand, if it solidifies prior to reaching the continental surface they form intrusive igneous. It implies that the igneous intrusions are formed as a result of the cooling and solidification of the magma prior to reaching the earth surface. The igneous rocks have coarse grains. This is because of slow cooling of the magma, which also results in the formation of the phaneritic texture. The intrusive igneous rock formations are only manifested due to erosion and weathering. To that effect, drawing on a variety of sources the paper will discuss igneous intrusion on the continental crust. Discussion There are various forms of igneous intrusions on the continental crust formed as a result of cooling and solidification of the magma before reaching the earth surface. Overtime, they are often visible on the Earth surface. The magma moves through the fractures and joints, as well as between the crystals of the solid rock of the crust. When it attains the its freezing temperatures, it crystallizes. The structures formed beneath the Earth’s surface after the crystallization of magma include sills, dikes, stock, batholiths, laccoliths, phacoliths, and volcanic neck/pipe. If the magma solidifies at shallow depths it will have small to medium crystal such as those of sills, dykes, and laccoliths. On the other hand, if the magma crystallizes at depths of more than 20 kilometers the resultant feature will have large crystals. If the magma cuts across the bedding planes is usually known as discordant intrusion while if it follows the bedding planes is referred to as concordant intrusion (Hughes 39-40). Dike A dike is formed when magma pushes towards the surface via cracks. As illustrated by (Hughes 57), a dike is an igneous intrusion that vertically cuts across the layers of rocks. Dikes are less than 20 meters wide and exhibit a discordant relationship to the rocks in which they intrude. Similarly, Hughes (58) adds that the term discordant implies that they cut across the structures that are preexisting. The dikes may appear as isolated bodies or numerous features originating from an enormous intrusive body deep within the earth surface. The basic dikes rarely come singly although they often occur in swarms with a recognizable structural control on their attitude. However, the single dykes are generally quite thin, various meters wide, vertical, and discordant at the time of intrusion. The geologists intuitively conceive it as a feature of crustal ‘tension.’ A study by Anderson (1951) found out that the dikes may be indeed be emplaced along the planes perpendicular to the minimum horizontal stress. The swarms of dikes mostly occur in regions where volcanic rocks are. Even if they were once present, they now may have been eroded away. The dikes attain their highest numbers in the dike swarms. They form radial patterns. The radial dykes usually converge on igneous intrusions. Hughes (59-60) points out that swarms of basic dikes portray examples of multiple intrusions. This implies that there were successive intrusions of similar composition. Unlike other minor intrusions of other compositions, the basic dikes are commonly aphyric. A radial pattern of the dike intrusions is usually pronounced near certain central complexes and intrusions. This shows the localized stress pattern. The commonly figured examples of the radially-patterned dike intrusions include the Spanish pekas in Colorado, and Rum, as well as Inner Hebrides in western Scotland. The linear dike swarms are not only more extensive in contrast to the radial dike swarms, but also concentrated around huge igneous intrusions. For example, in the south-eastern Iceland, the succession of the Cenozoic lava is cut across by thousands of aligned dikes in a near-vertical position averaging less than 1 meter thick. Figure 1: Dike As shown in Figure 1, dikes are sheets of igneous rocks that are formed by cutting across the bedding planes. Therefore, they run perpendicular to the strata. Figure 2: Dikes as discordant The dikes are mostly very abundant. As an example, the Mull and Arram dikes are very abundant occurring in over a distance of 22km and 24 km respectively. Dikes are tubular-like structures. Huppert and Sparks (600) posit that dikes play a crucial role in the heating up of the continental crust to initiate melting. Studies have also demonstrated that some dikes weather faster than the country rock around. In the same way, it has also been evidenced that dykes cool from the edges towards the center. As they cool, they contract resulting in the formation of cooling joints. These cooling joints run in two directions at 900 parallel to the cooling surfaces. Dikes are mostly fine grained because they are formed shallowly beneath the Earth surface. However, the small dikes have fine grains while the large dikes are a bit coarse grained. Sills A sill is defined by Thorpe and Thorpe (413) as an igneous rock that cuts between the layers of rocks. It is horizontal in its direction. When sills are formed, there is a gentle dipping sheet of igneous rocks. . Sills are also small and are less than 50 meters wide. The sills are formed where the average density of the column rock that is above the sill intrusion level is less than the basic magma density at liquid temperature. Under these conditions, less work is carried out when a body of magma intrudes laterally along a particular convenient plane. As a consequence, the overlying strata are raised as compared to when it raises itself to the surface. Once the magma has crystallized and formed a sill with a density of 3 gcm-3, the rock column’s average density is enhanced and later sill igneous intrusions will occur at higher levels. After millions of years, the sills can be visible. The rocks are required to be brittle in order to create the planes along which the magma intrudes the body of the parent rock (Hughes 61). As shallow intrusions, they demonstrate a concordant correlation with the rocks that they intrude. In most instances, sills are filled by dikes, even though this may not be seen in the field. Figure 3: Sill Sills are also tabular like features, and run in the same direction as the layering in the country rock as shown in Figure 3. In other words, sills are igneous intrusions that lie parallel to the bedding planes. Sill intrusion is often constrained by the preferred bedding plane leading to concordant intrusions at either horizontal or near horizontal strata. Therefore, there is a tendency for a low-angle cross-cutting correlation to develop (Hughes 61). The sills may be originally placed in a horizontal position, even though the tectonic processes can lead to the subsequent rotation of the horizontal mills into the near-vertical direction. Proceeding further, Gerya and Burg (124) argue that intrusion of the ultramafic bodies such as sills into the lower density continental crust requires an understanding of the emplacement mechanism. The emplacement of sills in the crust lasts from a few kilo years to several hundreds of kilo years depending on the viscosity of the intruding magma. Huppert and Sparks (599) observe that when magmas are emplaced into the continental crust, melting as well as generation of silica magma is expected to occur. In this perspective, the researchers found put that basalt sills of thickness 0.0ikm to 1.5 km requires 1 to 270 years to solidify. Hughes (61) also mentions that sills are generally thicker and wider as compared to dikes. Therefore, they take a longer period of time to cool. Huppert and Sparks (600) also note that sill igneous intrusions concentrate the heat at a certain level and in the crust and they do not lose their heat over a large depth range unlike the dykes. In other words, sills are more efficient intrinsically as compared to the dykes. Another factor that contributes to the formation of the sills in the lower parts of the continental crust is the strongly layered pattern of the sills that offers a structural environment in which horizontal intrusions can easily be formed. Various sills have also been found to contain crucial ore deposits, for instance, the Stillwater igneous complex of the U.S and Great Dyke complexes of South Africa. Batholiths Batholiths, as described by Armstrong (613), are igneous rocks of solidified magma of unknown depth and size. Batholiths are comprises of a large mass of magma. The large bodies of magma that solidify underground before they reach the crust’s surface are usually known as plutons. All batholiths are plutons. The plutonic intrusive rock features are formed because of the cooling and solidification of the magma at greater depths of the Earth surface. The deep intrusions are formed more than 2km below the surface of the Erath. The magma cools and solidifies forming coarse-grained rocks. One such igneous body formed deep the Earth’s surface are plutonic racks. They are usually large, blob-shaped ingenious intrusions formed when the rising magma gets trapped within the magma (Von Huene and Scholl 613). The plutons are usually comprised of intermediate or felsic rocks such as diorite, gabbro, and granite. The pluton igneous bodies can be up to 20-30km thick. Since the continental crust lies above the sea level, it explains why the plutonic igneous intrusions are visible after erosion and with minimal reduction in the water levels if it the intrusion is located in the sea. Figure 4: Peggy’s Cove lighthouse on granite pluton, Nova Scotia, Canada. Proceeding further, the batholiths are huge, deep-seated igneous intrusions. They usually form as the thick and viscous magma progressively makes its way via various channels, but is does not reach the surface of the earth. The batholiths are more than 100 km in width. This makes them wide intrusive igneous bodies. There are extremely large that their bottoms are rarely exposed. In some instances, they are made of various smaller intrusions. However, it requires large amounts of erosion so as to expose the batholiths. The batholiths are exposed regions of the continuous plutonic rocks that cover a region of more than 40 square miles (100km2). However, if the area is less than 40 square miles they are are known as stocks. Most of the batholiths on the continental crust by means of outcropping have estimated area of more than 40 square miles. These regions are exposed to the crust surface via the process of erosion activated by the continental drift that has been occurring for over numerous tens of millions of hundreds of millions of years. On the surface of the Earth, the batholiths that are exposed are usually subjected to enormous pressure differences between their previous site deep within the continental crust and their new position near the Earth’s surface. As a consequence, over time their crystal structure expands to some extent. This is seen through the ingenious intrusion by a type of mass wasting commonly referred to as exfoliation (Glazner and Coleman 4-11). Furthermore, Plummer (61-63) asserts that this type of weathering results in convex combined with relatively thin rock sheets that marsh off the exposed areas of the batholiths. The process is hastened by frost wedging. The outcome is a fairly clean and round-faced rocks as seen in Figure 4. Another clean and round-shaped batholiths is the Half Dome, which is located in the Yosemite Valley. Figure 5: Batholith Even though the batholiths appear uniform, they are in fact features with complex compositions and histories. They are made up of plutonic masses of igneous rocks that have irregular dimensions. The batholiths can be differentiated from the adjacent igneous rocks by combining texture, age, and mappable structures. The individual plutons are crystallized from the magma that travelled in the direction of the crust’s surface from a region of partial melting around the base of the Erath crust. For a very long time, the batholiths have been regarded as a form of elevation of the buoyant magma in enormous masses known as plutonic diapirs. This attributable to the fact that diapirs are hot and in liquid form. Therefore, they have a tendency to rise through the near country rock, thus pushing it off its path in addition to melting it in part. The diapirs that do not reach the surface of the continental crust cool and solidify about 7 to 30 km underneath the crust surface as batholiths. With time, batholiths converge to form a large expanse of granitic rocks. Some batholiths are extremely large, for instance the Sierra Nevada batholiths. It is so gigantic that it forms the majority of the Sierra Nevada in California. Furthermore, a mammoth batholiths is occupies the Coast the Mountains of the western Canada extending for an estimated 1,800 kilometers to reach the southwestern Alaska. It is known as the Coast Plutonic Complex (Plummer 61-63). Examples of batholiths according to continents: Africa – Aswan Granite Batholith, Paarl Rock, and Sibebe; Europe – Leinster Btholith, Ljusdal batholiths, and Cornubian btholith; North America, Enchanted Rock, Boulder batholiths, and Idaho batholiths; Oceania – Moruya and New England batholiths; Asia – Gangdese, Karakoum batholiths; South America – Patagonian, Cordillera Blanca, and Chilean Coastal Batholith. While the dykes and sills are shallow-seated and narrow, the batholiths are deep-seated and wide as shown in Figure 6 below (Plummer 16). Figure 6: Dikes, sills, and batholiths Batholiths are irregular discordant intrusions. They cover more than 100 square kilometers in area (Thorpe 72). In the same way, the geological cross-section of the batholiths have demonstrated that the igneous intrusion extends downwards indefinitely. More to the pint, the geophysical methods together with field observations have found out that majority of the batholiths are about 10 kilometers in thickness. Numerous batholiths have sections of the former roof rocks that are often present as roof pendants. While the isolated rock masses that are trapped in the magma that forms the batholiths are termed as xenoliths, majority of the batholiths are composite and are made of numerous smaller intrusions as shown in the figure below (Dutch). Figure 7: Batholith sections (Dutch) Stock A stock is also an intrusive rock in the fissures at a deeper depth under the Earth surface. A stock is miniature batholiths of less than 100 km2 in area. The stocks are smaller igneous intrusions probably fed from within the deeper level of the batholiths. The stocks can be feeders for the volcanic eruptions. Also, extremely huge amounts of erosion are needed in order to expose either the stock. Over hundreds of millions of years, the rock above the stock may wear to expose this granite structure (Thorpe 73-74). Equally important, the stock is plutonic rock since is formed deep within the continental crust. Majority of the rocks are likely to be the cupolas of the hidden batholiths. It has been shown that the circular stocks could have been the vents that fed the earlier volcanoes (Whitov 513). Also, the spheroidal weathering results in the formation of small boulder fields, together with sheet-like exfoliative weathering. Similar to the batholiths, the stocks have smooth edges due to the process of exfoliation hastened by frost wedging. The stocks are smaller in size and are irregular discordant intrusions. Figure 8: Stock Laccoliths Laccoliths are igneous rock with a shape like a mushroom shape. Laccoliths are wide intrusions which lead to the uplift and folding of the preexisting rocks. They are often more than 100 meters width. They exhibit a concordant relationship from the rocks in which they break in. Laccoliths have a flat base, convex top, and often have a feeder pipe below. The pressure of the magma is very pushing the overlying strata upward. This gives the laccoliths the dome-shape with a planar base (Wichman and Schultz 193-194). The magma flows upward via the dike to spread out and swell upwards in a subterranean mushroom shape. Laccoliths are usually smaller as compared to the stock Figure 9: Laccoliths Hawkesworth et al. (230) shares the evidence that about 40 percent of the surface of the earth is overlaid by the continental crust. This implies that the continental crust is made up of more than 55 percent of the total volume of the Earth’s crust. The laccoliths intrude on the continental crust. They formed in relatively shallow depths also by relatively viscous magma. Underground cooling occurs very slowly resulting laccoliths having large crystals. The laccoliths usually range between hundreds of meters to a few thousand meters in thickness. The acidic rocks are more common in the laccoliths. According to Wichman and Schultz (194), the surface rocks are usually eroded completely leaving only the core mound of the igneous rock. Even though the lower parts of the laccoliths are rarely visible, they are often understood as having a slightly small feeder from the source of magma below. Due to the homogenous formation process of the laccoliths, a study by (Friedman and Huffman 1)found out that three of the well-known laccolith complexes if the southeastern Utah: Abajo Mountains, the Henry, and La Sal have the same tectonic control, petrogenesis, and hydrothermal mineralization. Besides that, the complexes are emplaced at the same nodes in an orthogonal northeast-northwest fault grid. Figure 10: Concordant relationship and laccolith. An example of a laccolith is the Barber Hill in Charlotte, Vermont. The laccolith was originally formed deep in the crust. Considering that the granite is much harder in comparison to the surrounding rocks, the laccolith has survived the erosion which has worn other rocks away. Moreover, the laccolith in Vermont has various volcanic trachyte dikes. Also visible in the intruding outcrops is molybdenite. The Solotario, which exists at the Ouchita orogeny comprises of the eroded remains of the laccolith (Wichman and Schultz 195). Lopolith The lopolith is a considered as a mega sill. It is usually made of diorite or gabbros and covers hundreds of square kilometers in area and kilometers width. The lopoliths have a concave structure in addition to being differentiated. This explains the reason why they take very long to crystallize, and the heavy minerals have the opportunity of sinking while the lighter minerals may rise. (Rudnick)P.353). Just like the laccolith, the lopolith has a roughly flat top but the base is convex-shaped and shallow. It also has a feeder dike beneath it. Generally, it is parallel to the bedding planes. Besides that, it is a large igneous intrusion that is lenticular in shape with a depressed central region. The funnel shaped rock mass bodies below the body. Typically, the lopoliths comprise of huge layered igneous intrusions that vary in age ranging from the Archean to the Eocene (Blatt and Robert 15-16). Figure 11: Lopolith(Blatt and Robert 16) The lopolith also has a feeder dike and occupies an area of not more than 100 square meters. Some of examples of lopoliths include the Bushveld ingneous complex in South Africa, Sadbury igneous Complex in Ontario, Humboldt lopolith of Nevada, and Great Dyke of Zimbabwe (Blatt and Robert 15). The large lopoliths have been reported to be comprised of mostly the basic rocks. A lot of other lopoliths are either differentiated or composite. It has been assumed that the lopolith’s feeder is somewhat small and likely to be situated centrally. At the depth, it may be connected to a larger magma chamber. Phacolith The phacolith are concordant pluton shaped like a lens. The phacolith occupies the trough of the syncline or the crest of the anticline. Young (335) illustrates that phacoliths are also deep-seated igneous intrusions like batholiths and are formed in the same direction as the bedding plane or folded rock. In some rare instances, the body of the phacolith may extend from the sill from an anticlines’ crest via the trough of the nearby syncline. As a result, the cross-section is seen as S-shaped. More to the point, in the immensely folded terrain the edges of the fold may be the regions of decreased pressure. Consequently, they are probable sites for magma emplacement in addition to migration. Figure 12: Phacolith Volcanic pipe or volcanic neck The volcanic neck is the remaining part of the solidified lava in the volcano’s throat after erosion. The volcanic neck is also a shallow intrusion. According to Thorpe (72), the volcanic neck or pipe is roughly a tubular vertical feature that can be having a feeder vent. If the igneous bodies remain below the surface of the earth they can be termed an intrusive feature. The volcanic neck is often found in the extinct volcanoes where the magma has crystallized in the vent. Over time, the soft rocks are eroded exposing the solidified magma. This results in the formation of the volcanic neck as shown in Figure 13. Figure 13: Volcanic neck in New Mexico. In the figure below, the magma chambers lie below the volcano and harden when the volcano is extinct. The Devil’s Tower as shown in Figure 14 was formed via this process. As the magma cooled, the surrounding rocks fractured and were eroded leaving the vertical column as it appears. Figure 14: The Devil’s Tower in Wyoming, United States. The magma cooled very slowly in order to form the slender-pencil shaped columns of the phonolite porphyry. Erosion eliminated both the surrounding and overlying rock. Glacial erosion can result in the exposure of the plug on either both sides or one side. This leads to the formation of a distinctive upstanding landform after the erosion of the surrounding rocks. The igneous intrusions on the continental crust have various impacts. It has been evidenced that the emplacement of the shallow-level igneous intrusions especially in the sedimentary basins affect greatly on the development of petroleum systems. As an example, the circulation of the hydrothermal fluids may decrease the permeability along with porosity of the host reservoir rocks, and associated deformation of the host rock can lead to the formation of ‘forced fold traps.” (Jackson 1). It has been evidences that igneous intrusions affect the heat transfer, petrology, and characteristics of coal quality. Conclusion In summary, the igneous intrusions to the continental crust are formed via the processes of cooling of the hot magma emanating as from the inside the earth. Magma progressively pushes from the deep within earth into any cracks it can find, but it does not reach the earth’s surface. The process often takes million of years. The intrusions include dike, sill, batholiths, pluton, stock laccoliths, lopoliths, phacolith, and volcanic pipe or volcanic neck. A sill is cuts horizontally between rock layers. A dike is an igneous intrusion that cuts vertically across rock layers. A laccolith is an igneous intrusion shaped like a mushroom. A batholith is an igneous intrusion of indefinite size and distance downward. A stock is an undersized batholith less than 100 km2. A volcanic neck is the loose ends of the hardened lava in the throat of a volcano after wearing away. The concordant intrusions run in the same direction as the strata and they include sills, lopoliths, and laccoliths. On the other hand, the discordant igneous intrusions cut across the bedding planes and include dikes, stocks, and batholiths. These igneous intrusions impact on the various processes in the continental crust such as oil and coal production. Works Cited Armstrong, L. “The persistent myth of crustal growth.” Australian Journal of Earth Sciences 38: 613–630. Blatt, Harvey and Robert, Tracy, Petrology: Igneous, Sedimentary and Metamorphic, 1996. pp. 15-16. Print. Dutch, Steven. Igneous Rocks. https://www.uwgb.edu/dutchs/EarthSC202Notes/igneous.htm>. Web. September 18, 1997. Accessed April 17, 2015. Friedman, Jules and Curtis Huffman. Laccolith complexes of southeastern Utah: Time of emplacement and tectonic setting. New York: U.S G.P.O, 1997. Print. Gerya, Taras and Jean-Pierre Burg. "Intrusion of ultramafic magmatic bodies into the continental crust: Numerical simulation." Physics of the Erath and Planetary Interiors 160.2 (2007): 124-142. 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