Volcanoes

Here we will compare extrusive and intrusive volcanic features and actionq distinguish magma from lavaq identify intrusive and extrusive igneous features (e.g., sill, dike, batholith)q describe types of eruptions (e.g., cinder cone, composite, shield, basalt plateau, rift), the volcanoes or extrusive events that produce them, and their effects

Magma vs. Lava

Magma = molten rock below the surface of the earthLava=magma that has reached the earth’s surface

Intrusive and Extrusive Igneous Features

1. Intrusive features = parts of volcanic activity found inside the earth

Plutons = Intrusive igneous rocks of any size. Formed inside the earth.

Types of Plutons•Batholiths (Big: >100 km2) = a body of igneous rock formed beneath the Earth’s surface by the intrusion and solidification of magma.

•Stocks (Smaller than batholith: <10 km2)

Castle Crags stock, Klamath Mtns, Dunsmuir, CA

•Dikes = a sheet of rock that formed in a crack in a pre-existing rock body that cuts across layers in a planar wall rock structures.

A diabase dike crosscutting horizontal limestone beds in Arizona.

Magmatic dikes radiating from West Spanish Peak, Colorado, USA

However, when the crack is between the layers in a layered rock, it is called a sill, not a dike.

•Sill = a sheet of rock that formed in between layers in a pre-existing rock body.

2. Extrusive features = parts found outside the volcano

Main types of magma

i) Mafic (basaltic) Magmas:

ohot (900-1200oC)overy low viscosity (very fluid)- fast flowingoNo presence of SiO2 (ie., not quartz)oContain mafic minerals (olivine, pyroxene)

Low SiO2 magmas, with little gas and low viscosity, flows readily through their vents and across the land surface when the lava escapes the vents.

ii) Silicic/Felsic Magmas

•cool (~650-900oC)•highly viscous- slow flowing•~65-77% SiO2

•contain felsic minerals (quartz, feldspar)

High SiO2 magmas, gaseous and with high viscosity, tend to plug their vents until the force of escaping magma blows the vent clear; such magmas cause explosive volcanoes.

Viscosity = Resistance to flow•Viscosity depends on temperature and composition (if SiO2 is present, lava flows slower)

Lava Deposits Lava tube: A tube formed by cooling and solidifying of the lava walls while fluid lava continued to flow inside.

Pillows: A form of closed lava tube (with a bulbous end) that forms when lava flows into water (e.g., a lake or ocean) and cools very rapidly.

b) Pyroclastic material = Debris formed by a volcanic explosion.

Results when magma is very viscous.

Tephra= The general term for all pyroclastic material that is ejected from a volcano.

Different terms apply according to the size of the tephra. (syn. ejecta)

Types of Material

•Ash: tephra that is finer than 2 mm in diameter.

•Lapilli: from 2 mm to 64 mm in diameter.

Blocks: hard fragments greater than 64 mm in diameter.

Bomb: formed when a volcano ejects viscous fragments of lava during an eruption.

They cool into solid fragments before they reach the ground. http://en.wikipedia.org/wiki/Volcanic_bomb

Tuff: A deposit made up of ash.

Mt St. Helen’s eruption, May 18, 1980

Lahar: A water saturated slurry of ash and other volcanic debris that flows down slope.

Mt. St. Helen lahars

Three types of volcano:Shield volcano

Flood BasaltComposite volcano

Dominated by fluid, high temperature, low viscosity basaltic magma.

Shield volcano

The lava flows easily down the gentle slopes….reaching the ocean during some eruptions.

Low, dome-shaped profile, like an inverted shield.

Calderas form after an eruption when the surface collapses.

Each caldera is located at the site of a former eruption.

Many shield volcanoes have a central caldera:

USGS

Composite volcanoes: Volcanoes that alternate between periods of lava flows (constructive phase) and periods of explosive eruptions (destructive phase).

They are made up of both lava and pyroclastic deposits.

Pyroclastics Ejected During Eruptions

KRAKATAU: World’s largest explosion?

Over a century ago, on August 26,1883, the island volcano of Krakatau ("Krakatoa") in Indonesia, a virtually unknown volcanic island with a history of violent volcanic activity, exploded with devastating fury. The eruption was one of the most catastrophic natural disasters in recorded history. The effects were experienced on a global scale. Fine ashes from the eruption were carried by upper level winds as far away as New York City.

The explosion was heard more than 3000 miles away. Volcanic dust blew into the upper atmosphere affecting incoming solar radiation and the earth's weather for several years. A series of large tsunami waves generated by the main explosion, some reaching a height of nearly 40 meters (more than 120 feet) above sea level, killed more than 36,000 people in the coastal towns and villages along the Sunda Strait on Java and Sumatra islands.

Tsunami waves were recorded or observed throughout the Indian Ocean, the Pacific Ocean, the American West Coast, South America, and even as far away as the English Channel.

Extensive ash falls and ash flows are commonly produced during explosive phases.

Mt. St. Helens Before

Mt. St. Helens After

After an eruption a large caldera remains.

Crater Lake is a caldera that remains following an explosive eruption 7,700 years ago.

The eruption was 42 times more powerful than Mt. St. Helens.

Mt. Fuji, Japan

A stratovolcano that has erupted 16 times since 781 AD.

The most recent eruption was in 1707-1708

0.8 cubic km of ash, blocks, and bombs were ejected during that eruption.

(Greater than Mt. St. Helens and there were no fatalities).

• Generally at hots spots, spreading centers• Mantle comes directly to surface• Hot lava; low viscosity, very mafic, flows

easily, gases escape easily• Forms shields, flood basalts

EFFUSIVE ERUPTIONS

FLOOD BASALTS

Columbia River basalt flow

Composite Volcano

EXPLOSIVE ERUPTIONS1. Found at subduction zones2. Magma

a) low temp (800 degrees C), b) high viscosity, does not flow easily, c) more felsic mineralogy, d) gases trapped,

3. hard to predict explosions4. Forms composite volcanoes, cinder cones, calderas, aerial bombs,

nuee ardente gas flows, 5. very destructive