Stratovolcano the Composite Cones Essay Example | Topics and Well Written Essays - 500 words

Stratovolcano the Composite Cones - Essay Example Stratovolcanoes are common at subduction zones forming a chain of volcanic features along plate tectonic boundaries where the oceanic crust is drawn under continental crust. The magma that is responsible for stratovolcanoes formation rises when water that is trapped both in the porous basalt rock and in hydrated minerals of the upper oceanic crust layer. It is then released into the mantle rock of the asthenosphere of earth’s crust that is above the sinking oceanic slab. After the occurrence of dewatering at high pressures and temperatures for each mineral, the plate descends to greater depths. The released water from this rock then lowers the melting point by cooling the temperatures of the overlying mantle rock. This in turn makes the rock to undergo partial melting and then rises due to its lighter density relative as compared to the surrounding mantle rock and the pools that form temporarily at the base of the lithosphere. The magma then rises thought the earth’s cr ust, carrying silica-rich crustal rock leading to a final intermediate composition. On reaching near the top surface, it then forms a pool in the magma chamber within the volcano, due to low pressure, water and other compounds such as chlorine sulphur and carbon dioxide dissolve in the magma to escape from the solution. Once a critical large volume of magma and gases accumulates the rock that acts as an obstacle is then overcame leading to a sudden explosive eruption (Biley ). Stratovolcanoes are rated as the most hazards to civilians as they cause large numbers of death when they occur. Many deaths are caused by mud and the pyroclastic flow that fast-moving mixture of hot volcanic debris ash and gases that can travel at high speed of about 160km/h. The eruption of Mt Pel’ee on Martinique Island in the Caribbean, El Chi chon volcano in Mexico which killed thousands of lives, the Pinatubo volcano that erupted in 1991 in Manila and the Unzen volcano on the island of Kyushu. Stratovolcanoes mainly form at the convergent plate margins usually where one plate descends beneath an adjacent denser plate at the site of a subduction zone.  

Physiology Essay Example | Topics and Well Written Essays - 3500 words

Physiology - Essay Example This pathway consists of A & A fibers characterised with high velocity (30 to 110 m/sec) of impulse conduction. The dorsal column-lemniscal pathway has also high degree of spatial orientation of the nerve fibres; particularly the medial fibers convey the information from the lower parts of the body while the lateral fibers related to higher segments. The spasticity in the muscles is related to the pyramidal insufficiency. This condition can occur if pyramidal pathways controlling voluntary movements are damaged by the mechanic or other agents. Intoxications, chronic neurodegenerative diseases, traumas, and metabolic disorders can play role in the origin of muscular spasticity. The attenuation reflex is important protective mechanism providing the best sound perception. The contraction of the stapes muscle protects the inner ear from damaging. The role of outer ear structures is important also - they can reduce resonance in the middle ear. Summation is a process of gradual release of the transmitter and the transmission of the impulse from several presynaptic neurones to one postsynaptic neurone (spatial summation) or from only one presynaptic and one postsynaptic neurone (temporal summation). The summation leads to the release of sufficient the impulse is transmitted across the synaptic cleft. The pain control system of CNS is represented by three major components: the periaqueductal grey and the nucleus raphe magnus (in the brain) and the pain inhibitory neurons of the dorsal horns in medulla spinalis. The opioid receptors of these structures are activated by endorphins or exogenous opiates and block the spreading of pain impulses. 3.3 Why is it difficult to localize slow, chronic pain (2) Chronic pain usually irradiate to other areas thus its localisation could be difficult. Furthermore, the efferent pain stimuli are conducted to CNS