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Magma Objectives • Describe factors that affect the formation of magma. • Compare and contrast the different types of magma. Vocabulary – viscosity Magma Magma • The ash that spews from some volcanoes can form billowy clouds that travel around the world before raining back down to Earth. • In the last 10 000 years, more than 1500 different volcanoes have erupted, providing evidence that Earth is indeed geologically active. Magma How Magma Forms • Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases that fuels all volcanoes. • Magma forms when temperatures are high enough to melt the rocks involved, usually between __________________. • Such temperatures exist at the base of the lithosphere and in the asthenosphere. Magma How Magma Forms Pressure – Pressure, which increases with depth, is one factor that determines whether rocks will melt to form magma. – Due to the effects of pressure, most of the rocks in Earth’s lower crust and upper mantle do not melt to form magma. Water – At any given pressure, a wet mineral or rock will melt at a_________ temperature than the same mineral or rock under dry conditions. Magma Types of Magma • The three major types of magma are: _________ magma, __________ magma, and ____________ magma. – ______________ is the internal resistance to flow; the higher the viscosity, the thicker the magma. Magma Types of Magma Magma Viscosity • The viscosity of magma and lava depends on both _______________ and – _____________________. The hotter the magma or lava, the ______ the viscosity. – Magmas and lavas high in silica have ____________ viscosities than magmas and lavas low in silica. Intrusive Activity Objectives • Explain how magma affects overlying crustal rocks. • Compare and contrast intrusive igneous rock bodies. Vocabulary pluton batholith stock laccolith sill dike Intrusive Activity Intrusive Activity • Magma, because it is molten, is less dense than surrounding rocks. • This density difference forces magma to move upward and eventually come into contact with, or intrude into, the overlying crust. Intrusive Activity Intrusive Activity • Intruding magma can affect the crust in several ways: A. B. C. . Intrusive Activity Plutons • When magma cools, minerals form and over a very long period of time; they will combine to form intrusive igneous rock bodies. • ________ are intrusive igneous rock bodies that can be exposed at Earth’s surface as a result of uplift and erosion and are classified based on their size, shape, and relationship to surrounding rocks. Intrusive Activity Plutons Batholiths and Stocks – _____________, the largest plutons, are irregularly shaped masses of coarse-grained igneous rocks covering at least 100 km2 and take millions of years to form. – ________ are irregularly shaped plutons that are similar to batholiths but smaller in size. Intrusive Activity Plutons Laccoliths – A __________ is a mushroom-shaped pluton with a round top and flat bottom resulting from a Magma intrusion into parallel rock layers close to Earth’s surface. – Compared to batholiths and stocks, laccoliths are relatively small. Intrusive Activity Plutons Sills and Dikes – A ______ forms when magma intrudes parallel to layers of rock. – A ________ cuts across preexisting rocks. Intrusive Activity Plutons Volcanoes Objectives • Describe the major parts of a volcano. • Compare and contrast shield, cinder-cone, and composite volcanoes. • Contrast the volcanism that occurs at plate boundaries. • Explain the relationship between volcanism and hot spots. Vocabulary – cinder-cone volcano – vent – composite volcano – crater – tephra – caldera – pyroclastic flow – shield volcano – hot spot Volcanoes Anatomy of a Volcano • When magma reaches Earth’s surface it is called lava. • A ______ is where lava erupts through an opening in the crust. • As lava flows out onto the surface, it cools and solidifies around the vent, eventually accumulating to form a mountain known as a volcano. • A ________ is a bowl-shaped depression at the top of a volcano that is connected to the magma chamber by a vent. Volcanoes Anatomy of a Volcano • Volcanic craters are usually less than 1 km in diameter. • __________ are large depressions up to 50 km in diameter that can form when the summit or the side of a volcano collapses into the magma chamber that once fueled the volcano. Volcanoes Types of Volcanoes • The appearance of a volcano depends on two factors: 1. 2. • Based on these two criteria, three major types of volcanoes have been identified: 1. 2. 3. Volcanoes Types of Volcanoes Shield Volcanoes – A _____________________ is a mountain with broad, gently sloping sides and a nearly circular base that forms when layer upon layer of lava accumulates during ________________________ eruptions. Volcanoes Types of Volcanoes Cinder-Cone Volcanoes – A __________________________is a generally small, steep-sided volcano that forms when material ejected high into the air, in a more explosive eruption, falls back to Earth and piles up around the vent. Volcanoes Types of Volcanoes Composite Volcanoes – ___________________are large volcanoes that form when layers of volcanic fragments alternate with lava. The eruption of these volcanoes are violently explosive. Volcanoes Volcanic Material • __________ are rock fragments thrown into the air during a volcanic eruption. • Tephra are classified by size: 1. 2. 3. 4. 5. Volcanoes Volcanic Material Pyroclastic Flows – A ______________________ is a cloud of volcanic gas, dust, and other tephra traveling at speeds of nearly 200 km/h. – The temperature at the center of a ________________ can exceed __________ . Volcanoes Where do volcanoes occur? • Most volcanoes form at plate boundaries. – About 80 percent of all volcanoes are found along convergent boundaries. – About 15 percent are found along divergent boundaries. – Only about 5 percent of extrusive igneous activity occurs far from plate boundaries. – Therefore, MOST volcanoes occur where? ______________________________________ Volcanoes Where do volcanoes occur? Convergent Volcanism – The volcanoes associated with convergent plate boundaries form two major belts: • The larger belt is called the Pacific Ring of Fire. • The smaller belt is called the Mediterranean Belt. Volcanoes Where do volcanoes occur? Volcanoes Where do volcanoes occur? Hot Spots – Some volcanoes are located far from plate boundaries and form as the result of hot spots. – ______________are unusually hot regions of Earth’s mantle where high-temperature surges of mantle material rise toward the surface. – A surge does not move laterally, which results in a trail of progressively older volcanoes that formed as a plate moved over a hot spot. Volcanoes Where do volcanoes occur? Hot Spots – The _______________ continue to rise above the ocean floor as the Pacific Plate moves slowly over a hot spot.