Transcript Volcanoes and Other Igneous Activity

화성쇄설류(Pyroclastic Flows)
• Stratovolcanoes erupt
violently…
– Often produce a nueé
ardente (glowing
avalanche):
• Fiery pyroclastic flow
made of hot gases
infused with ash,
pumice, and other
debris.
• Felsic and intermediate
magmas.
• Material ejected at high
velocities.
Nueé Ardente on Mt. St. Helens
Pyroclastic Flows
• Nueé ardente (glowing
avalanche):
– Forms from the collapse
(overcome by gravity) of tall
eruption columns.
– Moves down the slopes of a
volcano at speeds up to 200 km
per hour.
– Traveling up to more than 60
miles from the vent.
– Ground-hugging portion is rich
in particular matter suspended
by jets of buoyant gases (nearly
frictionless).
Lahars
• Stratovolcanoes may produce lahars, or
volcanic mudflows:
– Mixture of volcanic debris and water.
– Moves rapidly down slope (30 kph or more)
following stream valleys.
– Triggered when large volumes of ice and
snow melt during an eruption.
– Also generated when heavy rainfall saturates
weathered volcanic deposits.
– Highly destructive.
Lahar from Mt. St. Helens -- On March 18, 1980, an explosive eruption on Mt. St.
Helens generated a 14-kilometer-high eruption plume. Melted snow from the
eruption produced the dark-colored lahar seen in this photo. Part of the lahar flowed
into Spirit Lake (lower left). However, most of the lahar flowed into the North Fork
of the Toutle River valley (right), eventually reaching the Cowlitz River, 80
kilometers downstream. Courtesy of Thomas J. Casadevall, USGS.
Eruptive Frequency of
Cascade Range
Stratovolcanoes
Stratovolcanoes and
Plate Tectonics
• Stratovolcanoes are the product of subduction
zone igneous activity along oceanic-oceanic and
oceanic-continental convergent plate
boundaries.
– Occur in conjunction with deep oceanic trenches.
– Descending plate causes partial melting (wet melting)
of the mantle.
• A volcanic island arc if in the ocean – evolves
from early-stage mantle-derived basalts to
mature andesites and rhyolites.
• A continental volcanic arc if along a continental
margin – primarily andesites and rhyolites lesser
basalts.
섭입대에서
의 화성활동
Stratovolcanoes and
Plate Tectonics
• Associated with the Pacific Ocean
Basin margin is known as the “Ring
of Fire”.
• Most of the
world’s
explosive
volcanoes are
found here.
세가지 화산유형의 크기 비교
•
•
•
•
Calderas
Large collapse depressions with approximate circular diameter.
Steep-walled depressions at the summit.
Size generally exceeds 1 km in diameter.
Formed by the eruption of large volumes of magma from a shallow
underground magma reservoir.
• Results in loss of structural support for the overlying rock.
• Leading to collapse of the ground and formation of a large depression.
Calderas
Form by one of the following processes:
1. Collapse of the summit of a large composite volcano
following the eruption of silica-rich pryroclastic
eruption (Crater Lake).
Crater Lake Caldera, Oregon (6 miles diameter)
Calderas
2. Collapse of the top of a shield volcano caused
by subterranean drainage from the central
magma chamber to rift zone – flank
eruptions (Mauna Loa).
Calderas
3.
Image courtesy of the United States Geological Survey.
Collapse of large
area, independent
of any preexisting
volcanic
structures, caused
by discharge of
colossal volumes
of silica-rich
pyroclastics along
ring fractures
(Yellowstone).
Yellowstone
Caldera
Calderas and Plate Tectonics
• Calderas are a product of intra-plate
volcanism in continental crust.
– Activity within a tectonic plate.
– Associated with plumes of heat upwelling in the
mantle – mantle plume.
– Form localized
volcanic regions in the
overriding plate called
a hot spot.
– Produces granitic
magmas creating:
• Calderas
Fissure Eruptions
(열하분출) and Lava
Plateaus (용암대지)
• Huge volumes of volcanic
material are extruded from
fractures in the crust called
fissures (열곡).
• Fluid basaltic lava extrude
via fissure eruptions (flood
basalts) building up a thick
lava plateau.
• e.g., Columbia River
Plateau (nearly 1 mile thick
of flood basalts).
Flood Basalts and Plate Tectonics
• Flood basalts are a product of
intra-plate volcanism in
continental crust.
– Activity within a tectonic plate.
– Associated with plumes of heat
upwelling in the mantle –
mantle plume.
– Form localized volcanic regions
in the overriding plate called a
hot spot.
– Produces basaltic magmas
creating:
• Flood basalts in continental
environments (Columbia
Plateau).
Global distribution of flood basalt provinces (black) and associated hotspots (red dots). Red dashed lines are hot spot tracks, which appear as lines
of volcanic structures on the ocean floor. The Keweenawan and Siberian
Traps formed in failed continental rifts where the crust had been greatly
thinned. Whether there is a connection between the Columbia River basalts
and the Yellowstone hot spot is still a matter of ongoing research.
• Spreading Centers
– The greatest volume
of volcanic rock is
produced along the
oceanic ridge system.
– Continental or oceanic
rifting.
– Results in partial
melting of mantle
(decompression
melting).
– Large quantities of
basaltic magma are
produced.
Lava Domes (용암돔)
Lava Dome on Mt. St. Helens
• Bulbous mass
of congealed
lava.
• Form in the
summit crater.
• Most are associated with composite cones that
produce explosive eruptions of silica- and gas-rich
lavas.
• Typify late stages if mature, chiefly andesitic
cones.
화도(Volcanic Pipes)
• Most volcanic pipes are conduits that connect a magma chamber
to the surface.
• Pipes may extend in a tube-like manner to depth exceeding 200 km
(rare).
• Enables ultramafic rocks from the mantle to reach the surface that
have undergone very little alteration.
• Diamond-bearing kimberlite dikes.
암경(Volcanic Necks)
• Erosion of the outer flanks of the
volcanic cone leave behind the moreresistant neck.
Shiprock, NM
– A Volcanic
Neck
주상절리(Columnar Joints)
• Form as igneous rocks cool at or near the surface.
• Shrinkage fractures formed by tensional forces that
cracks the rock as it contracts during cooling.
• Produces elongated, pillar-like columns.
• Common is lava flows and sills.
Plutonic Igneous Activity
Plutonic Igneous Bodies
• Most magma is emplaced at depth in the
Earth.
• An underground igneous body, once
cooled and solidified, is called a pluton
(심성암).
Classification of Plutons
•
Classification of Plutons:
1. Shape
•
•
Tabular (sheetlike)
Massive
3. Size
2. Orientation with
respect to the host
(surrounding) rock:
– Discordant – cuts
across existing
structures or rock
units.
– Concordant –
parallel to existing
structures of rock
units.
심성암체의 구분
• 암맥(Dike) – a tabular, discordant pluton
where magma has injected into fractures.
– Pathways that fed ancient lava flows.
– Occur in clusters – radiating around neck.
Vertical
Dike Near
Granby,
Colorado
Types of Intrusive Igneous Features
• 암상(Sill) – a tabular, concordant pluton where magma
has injected along sedimentary bedding surfaces.
– Occur in shallow environments less force required to move
overlying rock layers.
– Frequently cut across layers and resume concordant nature at a
higher level.
A Sill in the Salt River Canyon,
Arizona
Types of Intrusive Igneous Features
• 병반(Laccolith): 돔형태
– Similar to a sill (near-surface environment).
– Lens or mushroom-shaped mass.
– Generated by accumulation of more viscous
magma.
– Arches
overlying
strata upward.
Types of Intrusive Igneous Features
• Two Types of Plutons (based on size)
– 저반(Batholiths)
• Largest intrusive body.
• Surface exposure of 100+ square
kilometers.
• Frequently
form the
cores of
mountains.
Types of Intrusive Igneous Features
• Two Types of Plutons (based on size)
– 암주(Stocks)
• Smaller
• Surface exposure less than 100 square
kilometers.
• Batholiths consist of
large numbers of
distinct plutons
(including stocks) that
were intruded over
millions of years.
Batholiths of Western
North America
The Sierra Nevada Batholith exposed in
Yosemite Natl. Park