Volcano and Metamorphism PDF

VOLCANO and METAMORPHISM VOLCANO: A volcano is an opening in the Earth’s crust that allows molten rock, gases, and debris to escape to the surface. Alaska, Hawaii, California, and Oregon have the most active volcanoes, but other states and territories have active volcanoes, too. A volcanic eru...

VOLCANO and METAMORPHISM VOLCANO: A volcano is an opening in the Earth’s crust that allows molten rock, gases, and debris to escape to the surface. Alaska, Hawaii, California, and Oregon have the most active volcanoes, but other states and territories have active volcanoes, too. A volcanic eruption may involve lava and other debris that can flow up to 100 mph, destroying everything in their path. Volcanic ash can travel 100s of miles and cause severe health problems. Types of Volcano: A. STRATOVOLCANOES These volcanoes are majestic giants with steep sides and a symmetrical cone shape. They form from very thick, viscous, or sticky, lava that won't flow easily. The lava therefore builds up around the vent forming a volcano with steep sides – we call this a stratovolcano and it has a familiar triangular shape. Because the magma is so viscous, gas can't leave the magma, therefore, when the magma rises to the surface the gas pressure builds up inside the volcano, resulting in an explosive eruption. STRATOVOLCANOES B. SHIELD VOLCANOES Where a volcano produces low viscosity, runny, lava it spreads far from the source forming a volcano with gentle slopes. This type is called a shield volcano. Mauna Kea and Mauna Loa are shield volcanoes. They are the world's largest active volcanoes, rising nearly 9 km above the sea floor around the island of Hawaii SHIELD VOLCANOES C. CINDER CONE VOLCANOES Cinder cone volcanoes (also called scoria cones) are the most common type of volcano, and are the symmetrical cone-shaped volcanoes we typically think of. They may occur as single volcanoes or as secondary volcanoes known as "parasitic cones" on the sides of stratovolcanoes or shield volcanoes. Airborne fragments of lava, called tephra are ejected from a single vent. The lava cools rapidly and fall as cinders that build up around the vent, forming a crater at the summit. Cinder cone volcanoes are fairly small, generally only about 300 feet (91 meters) tall and not rising more than 1,200 feet (366 meters). They can build up over short periods of a few months or years. CINDER CONE VOLCANOES Two predominant types of volcanic eruptions: A. Effusive eruptions Magma rises through the surface and flows out of the volcano as a viscous liquid called lava. B. Explosive eruptions Magma is torn apart as it rises and reaches the surface in pieces known as pyroclasts. EFFUSIVE ERUPTION EXTRUSIVE ERUPTION METAMORPHISM - Mineralogical and structural adjustments of solid rocks to physical and chemical conditions differing from those under which the rocks originally formed. - The most important agents of metamorphism include temperature, pressure, and fluids. TWO METAMORPHIC PROCESSES: A. Mechanical dislocation- Rock is deformed as a consequence of differential stress. B. Chemical recrystallization- A mineral assemblage becomes out of equilibrium due to temperature and pressure change but the basic composition remains unchanged. TYPES OF METAMORPHISM: A. Dynamic-or cataclasis, results mainly from mechanical deformation with little long-term temperature change. Textures produced by such adjustments range from breccias composed of angular, shattered rock fragments to very fine-grained, granulated or powdered rocks with obvious foliation and lineation. Large, pre-existing mineral grains may be deformed as a result of stress. B. Contact- occurs primarily as a consequence of increases in temperature when differential stress is minor. A common phenomenon is the effect produced adjacent to igneous intrusions where several metamorphic zones represented by changing mineral assemblages reflect the temperature gradient from the high-temperature intrusion to the low-temperature host rocks; these zones are concentric to the intrusion. Because the volume affected is small, the pressure is near constant. Resulting rocks have equidimensional grains because of a lack of stress and are usually fine-grained due to the short duration of metamorphism. C. Regional-results from the general increase, usually correlated, of temperature and pressure over a large area. Grades or intensities of metamorphism are represented by different mineral assemblages that either give relative values of temperature or absolute values when calibrated against laboratory experiments. Regional metamorphism can be subdivided into different pressure-temperature conditions based on observed sequences of mineral assemblages. It may include an extreme condition, where partial melting occurs, called anatexis. D. Retrograde- the response of mineral assemblages to decreasing temperature and pressure E. Metasomatism- the metamorphism that includes the addition or subtraction of components from the original assemblage; poly- metamorphism, the effect of more than one metamorphic event F. Hydrothermal- the changes that occur in the presence of water at high temperature and pressure which affect the resulting mineralogy and rate of reaction.