VolcanoesIt’s all about the magma

What qualifies as a volcano?Any vent in the Earth’s surface that magma is expelled through

So what is magma?Liquid rock that forms under Earth’s surface

Why is it liquid?

How does magma form?1. High temperatures2. Lower the pressure (what?)3. Add water (no kidding)

Why would magma come to the surface?Lighter than surrounding rock

Pressure

Can create more magma by melting surrounding rock

LavaOnce magma gets to the surface, it changes its name to lava

It’s the lava that builds the part of the volcano you can see

Where do volcanoes happen?1. subduction

zones2. mid-ocean

ridges3. hot spots

Subduction ZonesOne tectonic plate moves under another

● ocean vs. continent - Cascades● ocean vs. ocean - Aleutian Islands

Mid-ocean RidgesThe largest amount of magma coming to the surface

Underwater volcanos

Pillow lava

Iceland

Hot SpotsNot at plate boundaries - in the interior

Mantle plumes that remain stationary (stay in place)

Hawaii

Back to MagmaThe type of magma determines the type of eruption

Viscosity (resistance to flow)● low viscosity - runny or liquid● high viscosity - thick or sticky

Depends on what it is made of and trapped gas● Mafic: magnesium and iron, dark colored, runny

○ more common in ocean plates

● Felsic: silicates, light colored, sticky○ more common in continental plates

Quiet EruptionsLow viscosity

● Mafic magma● gas escapes easily● oceanic volcanoes

○ underwater○ Hawaii○ Japan

Explosive EruptionsFelsic lava

● cooler and stickier● more trapped gas● continental volcanoes

What magma becomes: Lavaassociated with quiet eruptions

flows out of volcanoes

rock that results depends on speed of flow and speed of cooling

What magma becomes: Pyroclasticassociated with explosive eruptions

materials that are thrown into the air

rock that results depends when and where it cooled and solidified

Pyroclastic materials1. volcanic dust - tiny particles from trapped gas exploding cooled lava2. volcanic ash - slightly larger particles formed the same way3. lapilli (little stones) - little lava blobs that cool in midair4. volcanic bombs - big lava blobs that cool in midair5. volcanic blocks - solid rock blasted from the vent

Back to MagmaThe type of eruption determines the type of volcano

Lava vs. pyroclastics determine the shape of the cone

Shield VolcanoBroad and sloping

Quiet eruptions - lava flows

covers a large area, builds slowly

Cinder Cone

Very steep slopes

Explosive eruptions - pyroclastic materials

Not usually very large

Composite VolcanoBoth types of eruptions

Alternating layers of lava and pyroclastics

Form large volcanic mountains

CalderaThe end of a volcano’s life cycle

Magma chamber empties

Volcano collapses

Huge crater left at the surface

Yellowstone

Questions?