Magma and Volcanism PDF

Magma and Volcanism Earth's internal heat is essential in sustaining life on Earth as it drives most geological processes including volcanism and plate tectonics. Magma Rocks commonly found in the surface of Earth are in solid state. However, rocks also exist in liquid state at certai...

Magma and Volcanism Earth's internal heat is essential in sustaining life on Earth as it drives most geological processes including volcanism and plate tectonics. Magma Rocks commonly found in the surface of Earth are in solid state. However, rocks also exist in liquid state at certain locations deep inside Earth. The molten rocks that are found beneath Earth's surface is called magma. They are less dense than the surrounding solid rock, and therefore capable of rising the surface. When magma emerges at the surface, it is called lava.  Magma also has the ability to flow since it is a liquid. The degree of resistance to flow is called viscosity. The more viscous a substance is the less fluid it becomes. The viscosity of magma generally depends on its silica content. Magmas with high silica are more viscous than low-silica magmas. Generally, gas content is higher when the temperature is higher and this makes the magma less viscous. For instance, lava that just came out from a volcano has very high temperature and flows readily, but as the temperature decreases, it becomes more viscous and eventually stops flowing. The melting temperature of the rock, called eutectic temperature, is lower than the melting temperature of its constituent minerals. When a rock begins to melt, only certain minerals are melted. This process is called partial melting. The rock will eventually be melted completely when the temperature is high enough. Formation of Magma Magmas are formed under certain circumstances in special locations deep in the crust or in the upper mantle. They are formed when conditions are right to cause preexisting solid rocks to melt. Magma can be generated in several ways. Melting can occur when the temperature stays the same but the pressure decreases. This process is called decompression melting. Another melting trigger is when volatiles or gaseous substances are added into the hot solid rocks in a process called flux melting. Magma can also be formed via a process called heat transfer melting. It is the melting of surrounding rocks caused by very hot magma bringing in additional heat. Rocks are composed of several different minerals, which cause it to melt over a range of temperatures. The melting temperature of the rock, called eutectic temperature, is lower than the melting temperature of its constituent minerals. When a rock begins to melt, only certain minerals are melted. This process is called partial melting. The rock will eventually be melted completely when the temperature is high enough. If the magma is separated from rock that has not undergone complete melting, the magma will consists of components that melted at lower temperature and the remaining solid rock will contain components with higher melting temperature. This process is called fractionation. Volcano Volcanoes are visible manifestation of the process of rock formation. It is a vent that serves as the conduit of lava or the molten rock that reaches Earth's surface. The funnel-shaped depression where materials are ejected is called crater. The event when the lava is spews out of the volcano is called an eruption. There are two types of eruptions, effusive and explosive. Effusive eruption is dominated by the flow of lava and formation of fountains and lava flows. Explosive eruption ejects ash and larger fragments of broken up pyroclastic materials, forming ash clouds that eventually collapse and cover the slopes of the volcano. The composition of the lava controls the type of eruption. Lavas with low silica content are less viscous and flow more rapidly, generating effusive eruption. Explosive eruption is associated with lavas having high silica content and is more viscous. There are three different kinds of volcanoes according to its shape: Shield volcano forms very broad dome with gentle slope that covers a very wide area. Cinder Cone volcano are granular materials formed by lava fountains. They are built from particles and blobs of congealed lava ejected from a single vent. Stratovolcanoes or composite volcanoes have relatively steep sides and are more cone-shaped than shield volcanoes. They are formed from viscous, sticky lava that does not flow easily. Metamorphism The process when a rock changes its form into a new one without undergoing melting is called metamorphism. This implies that the change occur in a solid state. The transformation may take thousands to million of years and would involve several processes; 1. Recrystallization refers to the changes in shape and size of minerals without changing its identity. 2.Phase change is the process that transforms a grain of one mineral into a grain of another mineral having the same composition but different crystal structure. Example: Coesite was formed from Quartz under high Quartz Coesite temperature. 3. Neocrystallization is the growth of new minerals that differ from those in the protolith. 4. Pressure solution refers to the dissolution of mineral grains when a rock is squeezed dominantly at relatively low temperature and pressure and in the presence of water. 5. Plastic deformation occurs when some minerals become flattened or elongated without changing either the composition or crystal structure due to their plastic behavior when exposed to high temperature and pressure. Causes of metamorphism At high temperature, atoms vibrate rapidly, causing the chemical bonds to stretch and bend. If the bonds break, atoms detach and move to form new bonds with other atoms. This process leads to rearrangement of atoms within the mineral or to migration into other minerals. Recrystallization and neocrystallization occur because of this process. Types of Metamorphism 1. Contact metamorphism- is a type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma. Types of Metamorphism 2. Regional metamorphism- is a type of metamorphism where rock minerals and texture are changed by pressure over a wide area or region. Ex: When two plates converging into one another.