MAGMA is molten rock, but is not just liquid - also contains bits of rock crystals and gas. It ranges in consistency from wet concrete to thick oatmeal.

Rock that crystallizes from a magma is IGNEOUS ROCK.

Phaneritic Aphanitic Glassy

large crystals tiny crystals no crystals

slow cooling fast cooling very fast cooling

Ex/ Gabbro Basalt Obsidian (all the same composition – of ocean seafloor)

Rock melts when the temperature within the earth (geotherm) exceeds the melting point (solidus) of rock.

This happens for different reasons at (1) subduction zone volcanoes, (2) mid-ocean ridge volcanoes, and (3) hotspot volcanoes.

Normally the geotherm does not cross the solidus, so there is no melting.

BUT, it is very close at about 100-250 km in depth ( Asthenosphere).

What are the 3 main causes of volcanoes?

1. Mid-Ocean Seafloor Spreading

Ridges: “pressure release” melting

What are the 3 main causes of volcanoes?

2. Seafloor Subduction

Subduction Zones: “wet” melting

Mt. Fujiyama

Pyroclastic flow sweeps down the side of Mayon Volcano, Philippines, 1984.

A small lahar triggered by rainfall in Guatemala, 1989.

Mt. Pinatubo, Philippines, 1991.

Mt. Pinatubo: So much ash into the atmosphere that Earth’s temperature dropped, and sunsets were redder.

What are the 3 main causes of volcanoes?

3. Hotspot Mantle Plumes

Hawaii rises more than 5 miles above the seafloor.

Hawaii

Mauna Loa, Hawaii

Kilauea, Hawaii

Pahoehoe lava, Hawaii

“Aa” lava flow, Kilauea, Hawaii

Kilauea, Hawaii

Mt. St. Helens:

Giant Eruption

May 18, 1980

Mt. Adams

Mt. St. Helens

Crater Lake, Oregon

Mt. St. Helens:

Before

May, 1980

After

Phase 1: Small earthquakes and puffs of steam indicate that magma is rising. Bulge develops in North face.

Phase 2: A magnitude 5.1 earthquake shakes mountain, dislodging bulge which slides down mountain. Decreased pressure on magma starts lateral blast.

Phase 3: Eruption causes a second block to break free, exposing more magma and initiating an eruption column. Lateral blast goes at 300 mph, covers 230 square miles.

Phase 4: The Eruption Column reaches 80,000 feet in less than 15 minutes.

Mt. St. Helens Earthquakes: 1995-2005

The Dome is Growing Again

Mechanical Weathering (1):

Frost Wedging

Mechanical Weathering (2):

Plant growth

Mechanical Weathering (3):

Pressure release

Mechanical Weathering (4):

Large Temperature changes (also: forest fires)

Chemical Weathering (1): Dissolution

Water and carbon dioxide combine to make carbonic acid, which dissolves minerals.

Chemical Weathering (2): Hydrolysis

Addition of water to minerals makes clay minerals

Hydrolysis: Greatly accelerated in the presence of acidic rain

Chemical Weathering (3): Oxidation

Addition of oxygen to a metal-rich mineral forms a new mineral>> RUST

An end product of weathering is SOIL.