Volcanic Activity chapter 18

18.1 Magma: mixture of molten rock, suspended mineral grains and dissolved gases beneath the Earth’s surface 3 factors that control the formation of magma: 1) Temperature: Depending on composition, rocks melt around 800-1200 degrees C. 2) Pressure: As pressure increases, so does the melting point. 3) Water: the more water a rock has in it the lower the melting point.

Section 19.1 page 497 Viscosity: A fluids resistance to flow. High viscosity = thick (traps gas and therefore is explosive) Three types of magma 1) Rhyolitic: forms when molten material rises and mixes with overlying silica and water rich continental crust and has a high viscosity and is explosive 2) Andesitic: Found at ocean crust subduction zones with an intermediate viscosity 3) Basaltic: Forms when rocks in the upper mantle melt, has a low viscosity and erupts quietly They are named after the igneous rocks which can be found in the same row in your ESRT’s.

Factors that effect viscosity 1. Composition: The more silica, the more viscosity 2. Temperature: The hotter it is the lower the viscosity

18.2 Intrusive activity Plutons: large intrusive igneous rock bodies, most commonly made of granite. Batholiths: The largest of plutons, can cover at least 100 square Km Stocks: Irregularly shaped pluton that is similar yet smaller than a batholith Laccoliths: A mushroom shaped pluton with a round top and flat bottom Sill: A pluton that forms when magma intrudes into parallel layers of rock

Section 19.1 page 497 Dike: A pluton that cuts across preexisting rocks and is perpendicular to the rock layers.

Plutons and Tectonics Most Plutons are formed during a mountain building process such as the continental/continental convergent boundaries. They can also form as a result of ocean to ocean convergence zones.

Section 19.1 page 498 18.3 Volcanoes Anatomy of a volcano. Vent: The opening through which the lava erupts. Crater: The bowl shaped depression around the vent

Calderas: A larger depression than a crater that can form as a result of the summit or side of a volcano collapsing into the magma chamber. Section 19.1 page 498 This is a picture of a calderas called Crater Lake

Section 19.1 page 498 Volcanoes are classified by two factors. 1. The type of material that forms the volcano. 2. The type of eruption that takes place. There are three types of volcanoes. 1. Shield volcano: Is a mountain with broad, gently sloping sides and a nearly circular base. These form when basaltic lava flows during non explosive eruptions. These are the largest of the volcanoes.

Section 19.1 page 498 2. Cinder-Cone Volcanoes Forms when material ejected high into the air falls back to Earth and piles around a vent. These are the smallest of the volcanoes often no higher than 500 m.

Section 19.1 page 498 3. Composite Volcanoes Form when layers of volcanic fragments alternate with lava. As with cinder cone volcanoes, there is also a lot of water, silica and gases. These are much larger than cinder cone volcanoes and are a threat to people and the environment.

Section 19.1 page 498 Volcanic material Tephra: Rock fragments thrown into the air during a volcanic eruption

Section 19.1 page 498 Volcanic material Tephra is classified by size. Dust < 0.25 mm Ash < 2 mm Lapilli < 64 mm Blocks any thing larger than 64 mm and can be as large as houses and can shoot 10 km into the air.

Pyroclastic Flow: The clouds of ash, gas, and tephra that can travel down slope at speeds at nearly 200 km/h. The temperature can exceed 700 degrees c and kill on contact.

Volcanoes occur in one of three places. 1. Convergent boundaries 2. Divergent boundaries 3. Hot spots - hot regions of the Earth’s mantle where mantle material reaches the Earth’s surface. Chains of volcanoes form from hot spots as the plates continue to move.