chapter 7 sections 1 & 2. volcanoes & plate tectonics (sect 1) volcano – a weak spot in...
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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 boundariesVolcanic 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)
Mid Atlantic Ridge
Great Rift Valley
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
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”