Explain the differences between Basaltic, Rhyolitic and Andesitic volcanic eruptions with reference to processes, which lead to the eruptions, the type of volcano produced and the reasons for the differences between the material erupted by these volcanoes.
Volcanic eruptions are caused by a red-hot liquid rock, called magma inside the earth’s core. Volcanoes erupt when magma rises to the surface of the earth, which causes bubbles of gas to appear in it. This gas can cause pressure to build up in the mountain, and it eventually explodes.
When the magma bursts out of the earth, it is called lava. The type of magma in the earth can create different volcanoes. If the magma is quite thin, the gas can escape easily and there will not be an explosion. The magma just comes out of the mountain and flows down the sides, like Volcanoes in Hawaii and Mount Etna. But, if the magma is thick and sticky, the gas cannot escape, so it builds up and up until it explodes.
This can cause landslides and sends out huge clouds of burning rock and gas, which devastate everything around them, like the famous eruptions at Mount St Helens and Montserrat. Volcanic eruptions have many effects they can change the weather. They can cause rain, thunder and lightning. Volcanoes can also have long-term effects on the climate, making the world cooler; this happened when Mt. Pinatubo erupted in 1991, global temperatures dropped by 0.5 Celsius the following year. Also, Fast-moving lava known as pyroclastic flows can kill people and falling ash can make it hard for them to breathe. They can also die from famine, fires and earthquakes, which can be related to volcanoes. People can lose their possessions as volcanoes can destroy houses, roads and fields. Lava can kill plants and animals too. The Mount St Helens volcano in 1980 killed an estimated 24,000 animals including 11,000 hares, 6,000 deer, 300 bobcats, 200 black bears and 15 mountain lions.
The theory for plate tectonics, which causes the formation of volcanoes, was that Alfred Wegener did the jigsaw theory in 1912. His theory showed that Pangaea a Super Continent existed 300 million years ago. He said it split between north and south. The northern half was called Laurasia and the southern half Gondwanaland. Then Laurasia broke apart becoming north America, Eurasia. The southern continent, split up into Africa, South
America, Australia, India and Antarctica. India moved northwards and collided with Eurasia. (The Himalayas).
Alfred Wegener produced proof that this is what happened. For example: South America fit together like a jigsaw with Africa. Also, Coal deposits and sedimentary deposits from 290 million years ago are found in India, Antarctica, South America and Southern Africa; which proves that they were once linked together. Rock formations in Scotland are identical to ones found in Canada. Biological evidence includes – Fossils of reptiles identical in Australia, India, South America and Africa and the fossilized remains of tropical forests. The problem is he had no idea why the continents moved and Alfred Wegener was disregarded or laughed at.
There are many types of Plate boundaries such as Constructive plate boundaries. They are found beneath the ocean along the mid oceanic ridges in the Atlantic, beneath the pacific and Indian oceans. Also, there is one in northeast Africa where a rift valley has formed as currents are pulling Africa apart and this is also happening in the Dead Sea.
Also, they are Oceanic Ridges, which run down the Atlantic from North to South across the Indian Ocean and the Pacific. These are the places where new rock is constantly being formed and they are found where the currents in the mantel diverge underneath the crust. This causes the oceanic plate to pull apart from one another. Leaving space for the upwelling of basalt and this creates the ridges. The height of the ridges depends on the rate at which the ocean floors are moving apart; if the rate is slow (1 – 1.5cm a year) the ridges build up high above the ocean floor. If the rate is fast (upto 9cm a year) you get a smooth low crest. Fast occurs in the Pacific and slow in the Atlantic. Furthermore, there are plate boundaries Beneath Continents that sometimes-divergent currents occur beneath continents in which the continent breaks apart, producing a rift valley e.g. the Great African Rift Valley in East Africa. In a rift valley you find lakes and volcanic activity e.g. Mount Kilimanjaro. In time the rift valley will develop into an ocean of its own.
Lastly, there are Destructive Plate Margins and conservative plate margins. Destructive Plate Margins are the places where the Earth’s crust is being destroyed at the same rate as they are being formed at the constructive plate margins. Oceanic Convergence is where oceanic and continental plates meet. As oceanic plates are denser they go down toward the Mantel; this is subduction. Sediments that have accumulated at the bottom of the ocean form land mass as they rise as they melt due to the magma, through the continental crust and then re appear as the continent. Also, there is oceanic convergence. This is where two oceanic plates meet; one is forced under the other. Oceanic trenches are formed and island arcs; (Hawaii). Lastly, there are continental convergence where two continent plates meet and sediment is forced up to form mountains. And, Conservative Margins are where two plates slide over each other and there is neither creation nor destruction takes place. There is no subduction or volcanic activity. However, the plates move at different speeds creating friction, which cause shadow earthquakes.
Then Vulcanicity is normally associated with plate margins, however Hawaii is not connected with a plate boundary so it is believed that it was formed by a hot spot. This is where a concentration of radioactive elements causes the hot spot. Magma rises to eat through the plate but when lava breaks the surface, active volcanoes occur above the hot spot. The hotspot is stationary so as the Pacific plate moves over it, a line of volcanoes is created. The one above the hotspot is active and the rest form a chain of islands of extinct volcanoes.
Basaltic lava is formed from magma that is low in silica. This makes for a more fluid magma that allows gas bubbles to expand on the way up to the surface, preventing sudden explosive activity. Rhyolitic and Andesitic lavas are formed from silica-rich magma that is very viscous. This often solidifies before reaching the surface, leading to a build-up of pressure and, ultimately, to a violent explosion.
Basaltic lava flows erupt primarily from shield volcanoes, fissure systems, scoria cones, and spatter cones. The main types of extrusive volcanic
landforms where basaltic lava flows is in a lava plateau and shield volcanoes. Lava plateau are formed from fissure eruptions. The extensive lava flows are basaltic in nature, so they flow great distances along the ground. Shield volcanoes are also formed from free-flowing lava. The resulting volcanoes have gentle sides and cover a large area like in Mauna Loa, Hawaii. The Hawaiian shield volcanoes are the most famous examples. Shield volcanoes are almost exclusively basalt, a type of lava that is very fluid when erupted. For this reason these volcanoes are not steep (you can’t pile up a fluid that easily runs downhill). Eruptions at shield volcanoes are only explosive if water somehow gets into the vent, otherwise they are characterized by low-explosivity fountaining that forms cinder cones and spatter cones at the vent, however, 90% of the volcano is lava rather than pyroclastic material. They are explosive in Hawaii near the hot spots as the volcanoes are underground with water in them. Shield volcanoes are the result of high magma supply rates; the lava is hot and little changed since the time it was generated. Shield volcanoes are the common product of hotspot volcanism but they can also be found along volcanic arcs like in the Galápagos islands. Moreover, basaltic volcanoes are found near constructive plate margins and the magma is runny with and it is not sticky. There are almost no lava bombs but just lava. So, there are pyroclastic flows which are fast moving.
Lastly, andesitic eruptions have some viscosity in the lava flow Andesitic lava can be found in the Andes and other North American mountains. It is high in the rock andesite, which is the most common volcanic rock after basalt. It has course crystals. Andesitic lava flows more readily than rhyolitic lava, but not as easily as basaltic. Eruptions tend to form composite volcanoes. Composite volcanoes are what most andesitic volcanoes look like, they are the classic shape volcano, looking like a cone.
Rhyolite volcanoes are the most complex and are the most explosive of Earth’s volcanoes but often don’t even look like volcanoes. When they erupt the gases become extreme and a huge explosion removes the summit of the cone, leaving an opening several kilometer in diameter. The caldera may become flooded by the se, or a lake may form with in. They are usually so explosive
when they erupt that they end up collapsing in on themselves rather than building any tall structure. The collapsed depressions are large calderas, and they indicate that the magma chambers associated with the eruptions are huge. In fact, layers of ash (either ash falls or ash flows) often extend over thousands of square kilometers in all directions from these calderas. The last one was in 83 AD when Taupo erupted in New Zealand. The vents for these smaller eruptions sometimes follow the ring faults of the main caldera but most often they don’t. The origin of these rhyolite complexes is still not well understood. Examples of rhyolite caldera complexes include Yellowstone, La Primavera, Rabaul, Taupo, Toba, and others. Moreover, rhyolitic volcanoes are found near destructive plate margins and the magma is sticky and it is not runny. There are almost always lava bombs but no lava; these don’t happen very often.
In the last twenty-two years there have been two major volcanic explosions in Sicily (Mt Etna) and Montserrat (Soufriere Hills). Mount Etna is 3,323 metres high and it towers above Catania, the second largest city in Sicily. The most prominent feature is the Valle del Bove a 5 to 10 km horseshoe shaped Caldera all depression open to the east. This was created when the volcano experienced a catastrophic collapse during an eruption, generating an enormous landslide. Although it can be destructive, the volcano is not regarded as particularly dangerous. Thousands of people live on its slopes and in the surrounding areas, working the volcanic soils. Towards the end of 1991, lather began to pour from Bents hi on the eastern flank of the volcano. A series of protective measures were introduced to halt the lava flow. They constructed a large earth barrier across the end of the Val Calanna at the southern end of the Valle del Bove. This was several tens of metres high and water 400 m long and it held back the last several months. The aim was to slow the lava advance temporarily while other protective measures were put in place. During the spring of 1992, the accumulated lather began to spill over this barrier and down into the valley leading to Zafferna. Smaller barriers erected across the valley were rapidly overwhelmed by the advancing lava, which destroyed orchards and a few small buildings. It was decided to cut off the flow by blocking the primary feeder channel. This was first attempted by dropping concrete blocks from
helicopters through the roof of the upper lava tube. Finally, in May 1992, engineers blasted openings in the lava tube. This was to encourage new direction of flow, partly on top of the existing globe, rather than feeding it. It is not always possible to stop lava flows on Mount Etna. In 2002, there was a more serious eruption and lava flows completely destroyed the ski station of many ski resorts. It also damaged part of buildings and ash rain down on the areas. Mount Etna is a shield volcano so it is most likely to have been andesitic volcanic eruption.
Lastly, in July 1995, after lying dormant for many centuries, the volcano located in the Soufriere Hills on the island of Montserrat in the Caribbean began a rat. Early activity included ash emissions, Steve explosions and numerous earthquakes, this steam and ash reached heights of more than 2500 m. in the early 1997 there was continued don’t growth with small explosions and ballistic projectiles. The climax occurred on 25 June when large explosions within the volcano resulted in extensive pyroclastic flows from a eruption 4 to 5 million meters cubed of material. Most of this material flows down the northern flank of the volcano damaging houses and killing some inhabitants. Only 40 km of the islands 100 km was considered safe to live on. Islands capital, Plymouth was eventually buried in over 10 m of ash and mud. During this time the British government helped actuates 7000 islands in 11,000 inhabitants to neighboring islands such as Antiga, or resettlement in the UK, and financial help with all resettlement. This irruption was from a rhyolitic volcano. We can tell this tutor that Plymouth that used to have 3000 people living in it now has no one making it a ghost town due to all the ash and mind that fell on the city after the explosion.
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Differences Between Basaltic, Rhyolitic and Andesitic Volcanic Eruptions. (2016, Dec 12). Retrieved from https://graduateway.com/differences-between-basaltic-rhyolitic-and-andesitic-volcanic-eruptions/