Chapter 7 Sections 1 & 2 volcanoes
Volcanoes & Plate Tectonics(Sect 1) • Volcano – a weak spot in the earth’s crust where magma comes to the surface • There are over 600 active volcanoes on land and many more under the sea! • Volcanoes are located at 1) plate boundaries and 2) at hot spots. • Volcanoes located at plate boundaries form volcanic belts.
Volcanoes & plate boundaries Volcanic belt that surrounds or lines the Pacific Ocean is called: THE RING OF FIRE
Volcanoes can form at divergent and convergent plate boundaries. • Big divergent boundaries – Mid Atlantic Ridge and Great Rift that travels through the middle of Africa (Great Rift Valley) Great Rift Valley Mid Atlantic Ridge
Oceanic-Oceanic Convergent Boundary • The denser plate subducts, magma rises and eventually forms islands. The islands form an ISLAND ARC. The islands arrangement is in the same shape as the plate boundary that formed them. EX: Japan, New Zealand, Phillippines, Caribbean Islands.
Oceanic-Continental Crust Boundaries • Oceanic Plate subducts under the less dense continental plate. Coastal volcanoes form. EX: Mt St Helens; Andes in S. America
Volcanoes located along boundary btw North Amer Plate and Pacific Plate.
HOT SPOT VOLCANOES • These volcanoes occur in the middle of a plate instead of at a plate boundary. • There is a weak spot in the middle of a plate and sometimes islands can form. HOT SPOTS: Hawaiian Islands, Yellowstone Nat’l Park.
PROPERTIES OF MAGMA(sect 2) • Every substance has its own chemical and physical properties. These properties can be used to identify a substance and predict how it will behave. PHYSICAL property: a characteristic that can be observed without changing the composition of the substance. (color, density, mass, hardness, melting point, etc) CHEMICAL property: characteristics that describe a substance’s ability to form new substances. (corrosive, flammable, etc)
VISCOSITY • Magma/lava can be described by its viscosity. • VISCOSITY: resistance of a liquid to flowing. • High Viscosity – flows very slowly (ketchup) • Low Viscosity – flows quickly (water)
Viscosity of magma depends on 2 things: • 1) Silica content • 2) Temperature • Silica Content: ranges from 50% - 70% Higher silica Higher viscosity High silica magma is lighter in color, doesn’t flow Very far; cools to form rhyolite. Less silica – magma is darker; flows faster; cools to form basalt
Viscosity & Temperature • Temperature of magma ranges from 750o – 1175oC Higher temperature Lower viscosity Hotter magma flows faster; cooler magma flows more slowly.
Two Types of Lava: • Pahoehoe: low viscosity, fast moving, wrinkles, billows, ropy • Aa: high viscosity, cooler, Slower, rough, and jagged Think about what you would say if you walked on this barefooted… “Aa”