VOLCANOESChapter 7

Sections 1 & 2

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

Video – Click to begin

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

Africa’s 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

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.

Hot Spots Video Clip – click

Hot Spot Islands

Hawaiian Islands

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)

Which has higher viscosity?

Viscosity of magma depends on 2 things: 1) Silica content 2) Temperature

Silica Content: ranges from 50% - 70%Higher silica Higher viscosityHigh 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 –

1175oCHigher temperature Lower

viscosityHotter 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”

Lava video – Click to watch