Volcanoes

Anatomy of a Volcano Magma Chamber –

the source of the volcano’s magma.

Conduit – A tubelike structure connecting the magma chamber to the vent.

Vent – the opening at the top

Crater vs. Caldera

Small Explosion outward

Large Collapse of the chamber

inward

Types of Volcanoes Composite Cones

Steep Slopes due to vertical eruptions

Composed of alternating layers of ash and lava

Explosive eruptions Generally found at

subduction zones where oceanic crust is pulled under continental crust

Andesitic magma

Types (continued) Shield Cones

Flat, broad slopes due to passive oozing lava

Composed of layered lava flows

Passive eruptions Found at rift valleys

and hot spots in the ocean

Basaltic magma Hawaiian Dangers

Pa’hoe’hoe flows A’a’ flows

Pa’hoe’hoe Fluid

Oozing

A’a’ Rocky

Jagged

Types (continued) Cinder Cones

Small cones Steep slopes Small, explosive

eruptions Found on the flanks of

composite and shield cones

Composed of tephra (ash)

Warning Signs of a Composite Cone Eruption

Strange Animal Behavior Presence of sulfur gas Increased earthquake activity Increased avalanches and landslides Swelling of the mountain Minor small eruptions

Dangers of a Composite Cone Eruption Pyroclastic flow –

dense, fast moving currents of ash and rock 450 mph 1800 degrees

farenheit

Dangers (continued) Lahar – a mudflow

mixture of ash and water Can be 400 feet deep Can cover 100 square

miles

Dangers (continued) Tephra – Fragments

of volcanic rock blasted from an eruption As small as ash As large as boulders

Intrusive Volcanism

Formation of Magma Comes from the Upper Mantle when it melts

Forms in 3 general areas Subduction Zones Spreading Centers Mantle Plumes

Subduction Zone The friction of the plate

subducting adds heat that melts the rock

The seawater lowers the melting point allowing it to melt compared to the rock around it

A decrease in pressure also helps rock melt

Spreading Center When the plates split

apart, the asthenosphere moves upwards and the loss of pressure helps melt the rock

Mantle Plumes Mantle rock rises

beneath the lithosphere and the loss of pressure helps melt the rock.

Factors that affect the magma Amount of silica present

Silicates form in long chains and cause the magma to slow

High Viscosity (resistance to flow)

Amount of water present Magma cools when it rises helping it solidify but

the decreasing pressure also helps keep it liquid. Dropping pressure tends to override the cooling

effect unless there is a lot of water present. In this case the water is lost as steam and helps

the magma solidify faster

Igneous Rocks Igneous rocks are rocks that have formed from

the direct cooling and crystallizing of magma.

They can be classified based upon two things: The type of magma The rate of cooling

There are 2 types of magma that form most igneous rock

Andesitic Magma The type of magma that is

explosive

High silica

Is sticky and doesn’t flow well.

Creates lighter colored rocks

Has more water content because it is trapped by the thicker magma

Basaltic Magma Magma that is very fluid

Low silica

Flows very well

Produces darker colored rocks

Low water content because it escapes as steam

Cooling Rates Igneous rocks that are

formed at or near the Earth’s surface cool very quickly. These are called

extrusive rocks

Igneous rocks that are formed deep below Earth’s surface cool very slowly. These are called

intrusive rocks

Extrusive Rocks These rocks cool so

quickly that the minerals do not have the time to develop Small crystal size

Intrusive Rocks These rocks cool so

slowly that the minerals take a long time to develop Large crystal size

Other Properties Some igneous rocks

have holes in the surface

This is from the escape of gases as the rock cools

This rock is considered vesicular when this occurs.