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VOLCANIC ACTIVITY Chapter 18 18.1 MAGMA Magma is a mixture of molten rock, suspended minerals, and gases. All volcanoes are fueled by magma. Formation of Magma Magma forms from three factors: 1. Composition of Rocks (those that need to melt to become magma) • Forms when temperatures are high enough to melt rocks • Most rocks begin melting between 8000 and 12000 C • These temperatures exist between the lithosphere and the asthenosphere Pressure • Pressure increases with depth due to the weight of overlying rocks • As pressure increases, temperature at which substance melts also increases • The effect of pressure explains why most of the rocks in the Earth’s lower crust and upper mantle do not melt to form magma, although temperatures are high enough Water • The presence of water influences whether or not a rock will melt • A wet mineral or rock will melt at a higher temperature than the same mineral or rock under dry conditions Viscosity of Magma Viscosity is the internal resistance to flow; meaning, the ability to flow rapidly (like smooth liquid) or flow slowly (like a milkshake) • Hotter magma has lower viscosity • Cool magma flows less quickly than hot magma • If magma is low in silica, the viscosity is low Types of Magma Magmas are named after extrusive rocks. There are three types of magma, each forming from different areas of the Earth. The three types are: • Basaltic • Andesitic • Rhyolitic Magma Chart Basaltic Form from rocks in the upper mantle Low silica amounts Low Viscosity Quiet eruptions (flowing) Andesitic Form from oceanic crust and sediments About 60% silica amount (intermediate) Intermediate Viscosity Intermediate eruptions Rhyolitic Form deep beneath continental crust Highest amount of silica within this type of magma High Viscosity Explosive eruptions (most dangerous) 18.2 INTRUSIVE ACTIVITY Plutons are intrusive igneous rock bodies that represent most of the igneous activity on Earth Mountain building is responsible for the formation of many plutons. There are five types of plutons: • Batholith • Laccolith • Sill • Stock • Dike Batholiths • • • • • • Largest plutons Irregularly shaped masses Coarse-grained igneous rocks Cover at least 100km Take millions of years to form Common in interiors of major mountain chains Pluton Classification Stocks • Similar to batholiths but smaller in size • Cuts across older rocks • Forms 10-30km beneath Earth’s surface El Capitan – large granite batholith in America… Yosemite National Park Pluton Classification Laccoliths • Mushroom shaped pluton with a round top and flat bottom • Forms when magma intrudes parallel rock layers close to Earth’s surface and rocks “bow” upward due to heat and pressure of magma body • Relatively small compared to batholiths and stocks • At most, up to 16km wide • Common in Black Hills of South Dakota Sills • Forms when magma intrudes parallel to layers of rock • Can range from a few centimeters to hundreds of meters in thickness Palisades Sill, overlooking Hudson River, near NYC Pluton Classification Dikes • Pluton that cuts across pre-existing rocks • Form when magma invades cracks in surrounding rock bodies • Few cm to several m wide and up to tens of km long • Most are coarse-grained 18.3 VOLCANOES Volcanism produces various features that alter the Earth’s landscape. Common Parts of a Volcano Within a volcano, there is a portion that fuels the eruptions. This is referred to as the magma chamber. Once the magma chamber is fueled, it erupts through an opening in the crust. This opening is called the vent. Over time, the lava will solidify and accumulate to form a mountain known as a volcano. At the top of the volcano, sits the vent. The vent is surrounded by a bowl-shaped depression that is referred to as a crater. The crater connects to the magma chamber by way of the vent. In some instances, the pressure of the volcano becomes too great to support its walls and the side wall (summit) will collapse into the magma chamber, resulting in a caldera. Anatomy of a Volcano Crater Lakes Sometimes, a crater lake will form as result of too many eruptions. The top of the magma chamber will become partially empty and collapse, forming a caldera. The caldera will eventually fill up with water and become a lake, such as the one shown in the picture of Crater Lake National Park (Oregon) Types of Volcanoes The appearance of a volcano depends on two factors: 1. The type of material that forms the volcano 2. The type of eruptions that occur Based on this, three major types of volcanoes have been identified: 1. Shield 2. Cinder Cone 3. Composite Shield Volcano • Broad, gently sloping sides • Nearly circular base • Layered basaltic lava • Non-explosive eruptions • Low viscosity • Low amounts of gases and silica • Make up the Hawaiian Islands • Mauna Loa – famous example Cinder Cone Volcano • Steep sided volcano • Material is ejected high into the air and falls back to Earth, around the vent • Generally small (less than 500 m high) • More water and silica than shield volcanoes • Large volume of gases within the magma • More explosive than shield volcanoes • Tephra thrown into the air (rock fragments) • Famous example includes the Izalco Volcano in El Salvador Composite Volcano • Form from alternating layers of volcanic fragments / lava • Large amounts of silica, water and gases • Largest (in height) of all volcanoes • Highly explosive • Most dangerous to humans • Famous examples include Mount St. Helens and Mount Rainier Tephra As noted, tephra refers to rock fragments thrown into the air during volcanic eruptions. Tephra can be one of the following: – Newly cooled / hardened lava – Mineral grains that have crystallized prior to eruption – Pieces of the volcanic cone Tephra Classification by Size • DUST – less than .25mm in diameter • ASH – less than 2mm in diameter (but larger than dust) • LAPILLI (little stones) – less than 64mm in diameter (but larger than ash) • VOLCANIC ROCKS / BOMBS – can be very large. Documented sizes are comparable to a small car or house. When angular, they are called rocks and when rounded, they are called bombs. Pyroclastic Flow Pyroclastic Flow refers to rapidly moving volcanic material. This flow can travel up to speeds of 200 km/h and may contain hot, poisonous gases. Temperatures of flow can exceed 700 degrees Celsius To explain damage, one of the most dangerous flows occurred on the island of Martinique, where more than 29,000 people suffocated or were burned to death in 1902 due to the eruption of Mt. Pelee Where do Volcanoes Occur? Volcano distribution is not random. Most form at plate boundaries. Convergent Plate Boundaries – make up about 80% of all volcano locations Divergent Plate Boundaries – make up about 15% of all volcano locations Away from any plate boundaries – only approximately 5% of all activity Convergent Volcanism There are two major belts along convergent plate boundaries: 1. Circum-Pacific Belt (Pacific Ring of Fire) This stretches along the western coasts of North and South America, across the Aleutian Islands, and down the eastern coast of Asia. 2. Mediterranean Belt (Italy) Includes Mount Etna and Mount Vesuvius Divergent Volcanism As plates move apart, fractures and faults are created. This results in major separations called rift zones. Most of the world’s rift volcanism occurs under water along deep ocean ridges. This results in a process referred to as seafloor spreading. Rift Volcanism can be observed above water in Iceland, which is part of the Mid-Atlantic Ridge. Several volcanoes are present within this area. Hot Spots Some volcanoes are far from plate boundaries and form from hot spots (unusually hot regions of Earth’s mantle). Chains of volcanoes that form over hot spots provide important information about plate motions Some of Earth’s best known volcanoes form from hot spots under the Pacific Ocean Mt. Kilauea The worlds most active volcano is currently located over a hot spot on the big island of Hawaii. Sometimes, hot spots can result in flood basalts. Flood basalts erupt from fissures rather than a central vent, forming flat plains or plateaus The volume of basalt in these eruptions can be tremendous. Kilauea has what is called a “fire hose” dumping lava straight off the side of the volcanic cliff into the Pacific Ocean.