Volcanology The study of volcanoes

Magma

forms wherever temperature and pressure are

high enough to melt rock.

Some magma forms at the aesthenosphere

Magma also forms at plate boundaries, where

intense heat and pressure develops from friction

between the plates.

Volcanoes

Melted rock expands as it heats up,

becoming less dense than

surrounding rock.

Magma moves upwards through

fractures;

If magma reaches the surface, it

erupts through an opening called a

volcano.

Types of Magmas

Felsic magmas: high silica,

thick, light-coloured, slow-

moving

Mafic magmas: low silica,

thinner, dark-coloured, flow

more easily

Volcanic Eruptions

Magmas contain dissolved gases (i.e., water vapour, carbon dioxide)

As magma reaches the surface, pressure is reduced and dissolved gas comes out of solution as bubbles of gas.

Bubbles expand and explode…therefore…

LOTS OF GAS = MORE EXPLOSIVE ERUPTIONS!

Mafic magmas are more fluid and let gases

escape more easily than felsic magmas

MAFIC MAGMAS = LESS EXPLOSIVE

FELSIC MAGMAS = MORE EXPLOSIVE

Lava and Lava Fragments

If magma reaches the surface of

the earth, it is called lava

Explosive eruptions produce lava

fragments called TEPHRA

TEPHRA CLASSIFICATION:

Ash < 2mm

Lapilli 2 mm – 64 mm

Bombs >64 mm

Pyroclastic Flow

in explosive eruptions, tephra

combines with gases to form a

dense, superheated cloud

traveling at very high speeds

(> 100 km/h)

Volcano Hazards

1. Ash

Gritty sand-sized particles blasted

from erupting volcano

Can reach very high altitudes.

Large amounts can block sunlight,

causing world temperatures to drop.

Can destroy crops, buildings, clog

rivers, damage machines.

Volcano Hazards

2. Lava

Molten rock may flow over large

areas, destroying everything in its

path.

Volcano Hazards

3. Pyroclastic Flow

Destructive mix of

superheated gas, ash and

debris which can move faster

than 100 km/h

Volcano Hazards

4. Lahar

Flow of mud, water, ash and debris

that can result when snow-covered

volcanoes erupt

Types of Volcanoes

There are three main ways volcanoes are formed:

Convergent boundaries

Divergent boundaries

Hot Spots

1. Convergent Boundaries

(Composite Volcanoes)

As ocean plate sinks beneath the

continental plate, increasing heat

melts the rock, forming magma.

Magma is lighter than the

surrounding solid rock, so it rises up

through the edge of the continental

plate to form a volcano.

1. Convergent Boundaries

(Composite Volcanoes)

Form classic, cone-shaped volcanoes that

erupt ash and lava

Cone shape results from layers of ash and

lava building up over time

Magma is usually felsic (very thick), and

traps gas producing explosive eruptions

i.e, Mt. Baker

2. Divergent Boundaries

(rift eruptions)

Magma flows out of volcanoes at

mid-ocean ridges;

occasionally, volcanoes grow high

enough to rise above the surface of

the ocean and produce islands

i.e, Iceland

3. Hot Spots

(Shield Volcanoes)

Sometimes extremely high temperatures are found beneath

the middle of oceanic plates, in areas called hot spots.

At these locations, the melting of crustal rock produces

magma that rises up to form a volcano.

3. Hot Spots

(Shield Volcanoes)

As the plate moves, it carries the

hot spot with it, so that chains of

volcanic islands are formed

Magma is usually mafic, so

eruptions are not explosive and

volano’s sides have a shallow slope.

i.e., Hawaii

Plutons and Volcanism

Magma squeezes through fractures and

rock layers as it rises.

Igneous rock masses that form inside

other rocks are called PLUTONS.

Structures are named according to

size and orientation.

Dikes

sheets of igneous rock that cut across the rock

layers they intrude (usually mafic)

Sills

sheets of igneous rock that are parallel to the

rock layers they intrude (also usually mafic)

Laccoliths

Dome-like masses of igneous rock formed by stiff

magma which, instead of spreading, bulges

upwards (usually felsic)

Volcanic Neck

Plug of magma that hardens inside the vent of a

volcano

Batholiths and Stocks

Batholith: Largest of intrusions; forms the core of

many mountain ranges (usually felsic)

Stock: Small batholith (less than 100 square

kilometres exposed at surface)