Volcanoes
Here we will compare extrusive and intrusive volcanic features and
action
q distinguish magma from lava
q 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 earth
Lava=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 flowing
oNo 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 Basalt
Composite volcano
Shield volcano
Dominated by fluid, high temperature, low
viscosity basaltic magma.
The lava flows easily down the gentle
slopes….reaching the ocean during some
eruptions.
Low, dome-shaped profile, like an inverted shield.
Many shield volcanoes have a central caldera:
Calderas form after an eruption when
the surface collapses.
Each caldera is located at the site of a
former eruption.
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).
EFFUSIVE ERUPTIONS
• 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
FLOOD BASALTS
Composite Volcano
Columbia River basalt flow
EXPLOSIVE ERUPTIONS
1.
2.
3.
4.
5.
Found at subduction zones
Magma
a) low temp (800 degrees C),
b) high viscosity, does not flow easily,
c) more felsic mineralogy,
d) gases trapped,
hard to predict explosions
Forms composite volcanoes, cinder cones, calderas, aerial bombs,
nuee ardente gas flows,
very destructive