Transcript Chapter 5 - Volcanoes and Other Igneous Activity

Chapter 5
Volcanoes and
Other Igneous
Activity
The nature of
volcanic eruptions
• Characteristics of a magma determine
the “violence” or explosiveness of a
volcanic eruption
• Composition
• Temperature
• Dissolved gases
• The above three factors actually
control the viscosity of a given magma
The nature of
volcanic eruptions
• Viscosity is a measure of a material’s
resistance to flow
• Factors affecting viscosity
• Temperature - Hotter magmas are less
viscous
• Composition - Silica (SiO2) content
– Higher silica content = higher viscosity
(e.g., felsic lava such as rhyolite)
– Lower silica content = lower viscosity
(e.g., mafic lava such as basalt)
The nature of
volcanic eruptions
• Dissolved gases
– Gas content affects magma mobility
– Gases expand within a magma as it nears
the Earth’s surface due to decreasing
pressure
– The violence of an eruption is related to how
easily gases escape from magma
• In summary
– Basaltic lavas = mild eruptions
– Rhyolitic or andesitic lavas = explosive
eruptions
Materials extruded
from a volcano
• Lava flows
• Basaltic lavas exhibit fluid behavior
• Types of basaltic flows
– Pahoehoe lava (resembles a twisted or
ropey texture)
– Aa lava (rough, jagged blocky texture)
• Dissolved gases
• 1% - 6% by weight
• Mainly H2O and CO2
A pahoehoe lava flow
Aa lava flow
Materials extruded
from a volcano
• Pyroclastic materials – “fire fragments”
• Types of pyroclastic debris
– Ash and dust - fine, glassy fragments
– Pumice - porous rock from “frothy” lava
– Cinders - pea-sized material
– Lapilli - walnut-sized material
– Particles larger than lapilli
» Blocks - hardened or cooled lava
» Bombs - ejected as hot lava
• Eruption of Kilauea Volcano in Hawaii
A volcanic bomb
Bomb is approximately 10 cm long
Volcanoes
• General features
• Opening at the summit of a volcano
– Crater - summit depression < 1 km diameter
– Caldera - summit depression > 1 km
diameter produced by collapse following a
massive eruption
• Vent – surface opening connected to the
magma chamber
• Fumarole – emit only gases and smoke
Volcanoes
• Types of volcanoes
• Shield volcano
– Broad, slightly domed-shaped
– Generally cover large areas
– Produced by mild eruptions of large
volumes of basaltic lava
– Example = Mauna Loa on Hawaii
Anatomy of a shield volcano
• JDR Life Goal #38. Climb Mauna Kea (13,796 ft) shield
volcano in Hawaii. Highest mountain on the planet
from base to crest (almost 30,000 ft). Completed with
my son Jonathan in May 2000. This shows profile of
Mauna Loa shield volcano, as seen from summit of
Mauna Kea (benchmark at lower right).
Profiles of volcanic
landforms
Volcanoes
• Cinder cones
– Built from ejected
lava (mainly cindersized) fragments
– Steep slope angle
– Small size
– Frequently occur in
groups
Cinder cone volcano
The Paricutin volcano in Mexico erupted in a corn field in 1943, burying
the entire town.
Volcanoes
• Composite cone (stratovolcano)
– Most are located adjacent to the Pacific
Ocean (e.g., Fujiyama, Mt. St. Helens)
– Large, classic-shaped volcano (1000’s of ft.
high and several miles wide at base)
– Composed of interbedded lava flows and
pyroclastic debris
– Most violent type of activity (e.g., Mt.
Vesuvius)
Anatomy of a
composite volcano
Seamounts over hot spots
Volcanoes over hot spots have spawned several prominent
chains of islands in the Pacific Ocean, as shown here.
These volcanoes become increasing younger preceding
southerly, then southeasterly. Most of their mass lies
unseen, below the current sea level.
The
Hawaiian
Islands
As the Pacific Plate moves
WNW at a rate of about 6
mm/yr, magma rises through
the thin oceanic crust, causing
volcanoes. The unrelenting
erosion of waves tends to
plane off the islands, unless
protected by coral reefs
The most active area is currently the Kilauea Rift along the
southeastern coast of the island of Hawaii. Here large
slump blocks produce tensile scarps, which allows molten
lava to flow up to the surface more easily, loading the head
of the slumps. Loihi is the next island forming.
The Pali
Escarpment
The Pali Escarpment across Molokai and Oahu is a gigantic
landslide headscarp, formed when the northern side of
those islands detached itself and slid into the ocean, likely
on a layer of altered volcanic ash
Bathymetry of the detachments
Bathymetry surveys off the north
coasts of Oahu and Molokai,
showing enormous debris fields
Massive subaqueous landslide debris fields extending around the
Hawaiian Islands, initially identified by J. G. Moore of the U.S. Geological
Survey in the n1980s, which working on the 200 miles economic
exclusions zone around the islands.
Cascade
Volcanoes
• There are
13
potentially
active
volcanoes
in the
Cascade
Range of
the
northwestern United
States
Mt. St. Helens – prior
to the 1980 eruption
Mt. St. Helens after
the 1980 eruption
• Precursory stages of the Mt St Helens
eruption
• A seismically-induced landslide reduced lateral and vertical
confinement, triggering the May 18, 1980 eruption of Mt. St
Helens
• Cross
sections
thru Mt St
Helens
showing
deep
rotational
slide
blocks
which slid
off the
peak
• Atmospheric impacts of the May 18, 1980 Mt
St Helens eruption
• The Mt St Helens eruption shot volcanic ash to an
altitude of 60,000 feet, into the stratosphere
• Relative
distribution of
blown down
trees and
lahar debris
flows in the
Toutle River
Valley on the
north side of
Mt St Helens
The St Helens blast flattened Douglas fir trees over an area of 400
square kilometers
• Lahars, debris flows, and debris chocking of rivers
caused by the Mt. St. Helens eruption
Volcanoes
• Nuée ardente – A deadly pyroclastic
flow
» Fiery pyroclastic flow made of hot gases
infused with ash and other debris
» Also known as glowing avalanches
» Move down the slopes of a volcano at
speeds up to 200 km per hour
• Lahar – volcanic mudflow
» Mixture of volcanic debris and water
» Move down stream valleys and volcanic
slopes, often with destructive results
A nueé
ardente
on
Mt. St.
Helens
after the
May
1980
eruption
• Viscous lava flow passing through the village
of Goma in the Congo during the eruption of
Mt. Nyiragongo in January 2002
Other volcanic landforms
• Caldera
• Steep-walled depressions at the summit
• Generally > 1 km in diameter
• Produced by collapse
• Pyroclastic flow
•
•
•
•
Felsic and intermediate magmas
Consists of ash, pumice, and other debris
Material ejected at high velocities
Example = Yellowstone plateau
Formation of
Crater Lake, Oregon
Other volcanic landforms
• Fissure eruptions and lava plateaus
• Fluid basaltic lava extruded from crustal
fractures called fissures
• Example = Columbia River Plateau
• Lava domes
• Bulbous mass of congealed lava
• Associated with explosive eruptions of
gas-rich magma
A lava dome
Other volcanic landforms
• Volcanic pipes and necks
• Pipes - short conduits that connect a
magma chamber to the surface
• Volcanic necks (e.g., Ship Rock, New
Mexico) - resistant vents left standing
after erosion has removed the volcanic
cone
Formation of a
volcanic neck
The most famous volcanic neck
in the United States is
Shiprock, New Mexico
JDR Life Goal #54: Climb Shiprock, New Mexico, 1700’
climb, to elevation 7,178 ft, in Four Corners area .
Completed in June 1973 (after climb, discovered that
climbing had been outlawed in 1970).
Intrusive igneous activity
• Most magma is emplaced at depth in
the Earth
• Once cooled and solidified, is called a
pluton
• Nature of plutons
• Shape - tabular (sheetlike) vs. massive
• Orientation with respect to the host
(surrounding) rock
– Concordant vs. discordant
Intrusive igneous activity
• Types of intrusive igneous features
• Dike – a tabular, discordant pluton
• Sill – a tabular, concordant pluton (e.g.,
Palisades Sill in New York)
• Laccolith
– Similar to a sill
– Lens or mushroom-shaped mass
– Arches overlying strata upward
Igneous
Structures
• Relationships between volcanism and
intrusive igneous activity. Cross cutting
intrusions are called dikes, while those
emplaced parallel to structure are called sills.
• Diabase dike cutting thru preCambrian age Hakatai Shale at Hance
Rapid in the Grand Canyon. JDR Life Goal# 40. Row a rubber raft
down the Colorado River through the rapids of Grand Canyon from
Glen Canyon Dam to Lake Mead . Completed in June-July 1978.
Rowed it again in 1983, twice in 1984, and 1985.
A sill in the Salt River
Canyon, Arizona
Intrusive igneous activity
• Intrusive igneous features continued
• Batholith
– Largest intrusive body
– Surface exposure > 100+ km2 (smaller
bodies are termed stocks)
– Frequently form the cores of mountains
Classic Igneous Batholith
• When batholiths are uplifted and exposed,
they are usually resistant strata that form the
roots of mountain ranges or eroded highlands.
Batholiths
of western
North
America
Plate tectonics and
igneous activity
• Global distribution of igneous activity
is not random
• Most volcanoes are located within or
near ocean basins
• Basaltic rocks = oceanic and continental
settings
• Granitic rocks = continental settings
Distribution of some of the
world’s major volcanoes
Plate tectonics and
igneous activity
• Igneous activity at plate margins
• Spreading centers
– Greatest volume of volcanic rock is
produced along the oceanic ridge system
– Mechanism of spreading
» Decompression melting of the mantle
occurs as the lithosphere is pulled apart
» Large quantities of basaltic magma are
produced
Volatiles driven from
subducting crust
lower the melting
temperature of these
rocks, and they rise
by density contrast.
Plate tectonics and
igneous activity
• Subduction zones
– Occur in conjunction with deep oceanic trenches
– Partially melting of descending plate and upper mantle
– Rising magma can form either
» An island arc if in the ocean
» A volcanic arc if on a continental margin
– Associated with the Pacific Ocean Basin
» Region around the margin is known as the “Ring of
Fire”
» Majority of world’s explosive volcanoes
• A. Rising mantle plume; B. Rapid
decompression melting producing flood
basalts; and C. Rising plume tail produced by
linear seafloor volcanic chain
Plate tectonics and
igneous activity
• Intraplate volcanism
• Occurs within a tectonic plate
• Associated with mantle plumes
• Localized volcanic regions in the
overriding plate are called a hot spot
– Produces basaltic magma sources in
oceanic crust (e.g., Hawaii and Iceland)
– Produces granitic magma sources in
continental crust (e.g., Yellowstone Park)
• Global distribution of flood basalt provinces and
associated hot spots in the Earth’s crust. Some of
these formed in failed continental rifts, like Siberia
and the Keweenawan Rift in the USA
Volcanoes and climate
• The basic premise
• Explosive eruptions emit huge
quantities of gases (SO2) and finegrained debris
• A portion of the incoming solar
radiation is reflected and filtered out
• Past examples of volcanism affecting
climate
• Mount Tambora, Indonesia – 1815
• Krakatau, Indonesia – 1883
Volcanoes and climate
• Modern examples
• Mount St. Helens, Washington - 1980
• El Chichón, Mexico - 1815
• Mount Pinatubo, Phillippines - 1991
Pinatubo
Eruption June
1991
• Map showing
areal distribution
of pyroclastic
flows of June
1991 and
destructive lahars
that ensued in
September 1991,
killing many more
people than the
eruption
• Sulfer dioxide emissions of large volcanic
eruptions from 1979-91, in thousands and
millions of tons
• Impact of SO2 emissions on global climate, caused by
formation of H2SO4 aerosol, deflecting radiant energy
from the Sun into the Stratosphere