Plate Tectonics and Volcanism:

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Plate Tectonics and Volcanism Types of Plate
Boundaries

• Divergent
• Where new crust is generated as plates move away
  from each other
• Convergent
• Where crust is destroyed as two plates meet
• Transform
• Plates slide past each other horizontally

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Distribution of Vocanoes (on land)
Plate boundaries (blue lines)Earthquakes
(yellow)Volcanoes (red
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Divergent (continental)
convergent
divergent
transform
convergent
NOTE some volcanoes are NOT at plate boundaries !
http//pubs.usgs.gov/publications/text/Vigil.html
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• Volcanic Gases
• 50 to 80 is water vapor, also carbon dioxide,
  nitrogen, sulfur dioxide, hydrogen sulfide,
  carbon monoxide
• Gases contained in rising magma expand and can
  contribute to violent explosions

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Volcanic Gases
Evidence of volcanic gases is seen in fumaroles,
on the ground
Fumarole vent from which volcanic gas escapes
into the atmosphere.
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Evidence of volcanic gases in the soil
Mammoth Mountain ski resort, on edge of Long
Valley caldera
CO2 released from the magma reservoir under Long
Valley caldera is accumulating in the soil, and
destroying the root systems of trees.
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Evidence of volcanic gases explosivity of
eruptions
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• Intermediate to Explosive Volcanic Activity
• Convergent Margins
• Intraplate locations within continents
• Less fluid to viscous, lower temperature magmas
• with higher silica content and more gas content

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SUBDUCTION ZONE VOLCANOES
Typical volcano type stratovolcano Explosivity
Can be extremely high because of stiff, viscous
high silica lava with large amounts of trapped
gas that can escape violently Composition
Generally intermediate to sialic
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The Pacific Ocean is rimmed by subduction zones,
and hence is surrounded by volcanoes.
These make up the Ring of Fire
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Convergent Plate Boundaries

• Continental Volcanic Arc
• e.g., WA

• Volcanic Island Arc
• e.g., Japan

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What causes SUBDUCTION ZONE MAGMATISM?
Release of superheated water from subducting slab
causes melting of overlying material
This water ends up in the magma that eruptions at
subduction zones, which is one of the reasons
that subduction-zone eruptions tend to be very
explosive
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Continental arc volcanoes violently erupt
silica-rich magma as ash from composite volcano.
As basalt moves through continental crust, it
melts and absorbs some crust, becoming more
silica-rich.
Added fluids produce basalt magma from melting
mantle.
Heating of ocean crust releases fluids (mostly
water) into mantle above.
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Intraplate Volcanism - Continent

• forms flood basalts, then more silica-rich
  volcanoes (calderas)
• Columbia River Basalts Yellowstone

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• Composite Volcanoes
• Also called stratovolcanoes, are composed of
  alternating layers of pyroclastics and lava flows
• Composition is intermediate, with andesite common
• Eruptions are infrequent, violent, and may
  involve lahars

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The Ideal Stratovolcano

• Gentle low slopes, rising steeply to summit
  (concave upward shape)
• Summit crater may be quite small.

Mt Mayon, Phillipines
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Composite Volcano
Mount St. Helens before and after eruption in
May, 1980. A composite volcano consists of
layers of pyroclastic material and lava flows.
The eruption was pyroclastic
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Examples of explosive eruptions at subduction
zones
Vesuvius buried Pompeii in AD79 Krakatau 1883
(Indonesia) - tsunami Mont Pelée 1902 (West
Indies) 28,000 died in St Pierre Mt Pinatubo
1991. 100,000 people affected but few deaths
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• Intermediate to Explosive
• Volcanic Eruptions
• Explosive Eruption
• Products - Pyroclastic
• Airfall Tephra
• Pyroclastic Flow
• 2. Lavas
• 3. Lahars Avalanches
• (or landslides)

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• Gas-charged, explosive eruptions
• of high-silica melt (rhyolite)

ASH - Very fine-grained fragments of ash,
crystals, and rock fragments. LAPILLI - Pea- to
walnut-size collection of pumice, crystals and
small solidified chunks of magma.. BLOCKS AND
BOMBS - Large pieces, usually composed of lava
that has solidified in the air.
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Pyroclastic eruptions and cinder cones
Pyroclastic material forms when magma is erupted
into the air. Particles range in size from ash
to large blocks.
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Cinder cones are composed entirely of pyroclastic
material, usually scoria whose origin is airfall
(tephra)
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Distribution of Tephra Fallout, May, 1980
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When these solids are mixed with hot gases, the
eruption becomes a Pyroclastic Flow most deadly
of all volcanic phenomena

• Fluidized, heavier-than-air mixture of hot gases
  solid particles
• Travel FAST up to 100 km/hour
• Travel FAR 10s of km from source
• HOT temperature can be over 1000C

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Pyroclastic Flows, Mt. St. Helens
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II. Lava flows
Recall that the ability of a lava to flow is
related to its viscosity high silica (rhyolite)
lavas are highly polymerized (rich in complex
arrangements of silica tetrahedra), and hence
dont easily flow.
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High silica lava flows tend to be steep-sided,
and dont travel far. Many are best considered
domes.
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Lava domes form when a viscous, silica-rich lava
flows onto the surface and degasses
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Mount St Helens Oct 2004
High-silica lavas quench rapidly to glass, which
is called obsidian.
Snowflake obsidian
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III. Lahars Avalanches
products of eruption that do NOT originate
directly from the vent
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(a) Lahar

• mixture of water and rock fragments up to boulder
  size
• consistency like wet concrete
• can flow rapidly.
• can be hot (up to 100C) or cold

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1985 lahar at Nevado del Ruiz volcano killed
20,000
Small eruption caused melting of ice cap mixture
of ash, soil, rocks, trees and snow melt roared
at 50-60 km/hr down canyon. Town of Armero was,
unfortunately, situated at mouth of canyon...
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Armero was here
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MT. RAINIER The Future Site of a Catastrophic
Lahar?
Largest Cascade volcano (14,410 feet high). A
mudflow caused by steam explosions about 5,700
years ago was one of the largest known in the
world.
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Frequency of eruptionEruptive stylePresence of
snow, summit lakesProximity to population
centersResult is hazard map
VolcanicRisk Assessment
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Forecasting Eruptions typical warning time is
one week to one day

• Monitor ground motion
• small earthquakes - using remote
  seismometers
• Monitor changes in shape and size of volcanoes
  using tiltmeters or GPS elevation measurements
  from space
• Monitor changes in volume and composition of
  gases
• Watch (live volcanocams)

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III. Transform Plate Boundary
VOLCANISM IS TYPICALLY NOT ASSOCIATED WITH
TRANSFORM BOUNDARIES. Volcanism in the Cascades
IS associated with a small subduction zone off
the coast of Washington and Oregon
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Typical type of eruption Plinian (or Vesuvian).
. . . powerful convecting plumes of ash ascending
up to 45 kilometers into the stratosphere.
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Mt. St. Helens Plinian Eruption Column May
18, 1980
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(b) Avalanche large mass of snow, ice, soil, or
rock (or mixture) that slides rapidly down a
steep slope.
At Mount St Helens, injection of magma steepened
the volcanos slopes, leading to an avalanche,
which led in turn to the 1980 eruption.
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Explosive Eruption, Mount St. Helens, May 18, 1980
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Mt. St. HelensMay 18, 1980 Devastation
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Post May 18, 1980 Crater and Resurgent Dome
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Caldera A large volcanic depression, more or
less circular, with a diameter that is many times
greater than that of any included vent.
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Some Calderas do not blow their top during
pyroclastic eruption. They are COLLAPSE features!
Example Crater Lake, Oregon
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Crater lake, Wizard Island, OR 8 kBP Eruption Mt.
Mazama
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Supervolcanoes produce the largest known
pyroclastic eruptions. They invariably erupt
from calderas situated on continental crust.
Only the largest calderas are supervolcanoes!
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Supervolcanoes include only the largest calderas
and are relatively RARE
There are only three young supervolcanoes in
North America

• Long Valley, CA
• Yellowstone, WY
• Valles, NM

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Calderas formed by collapse
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• Yellowstone Caldera
• eruption 640,000 years ago.
• caldera, measures 28 miles wide by 47 miles long
• subsequent lava flows dominated by rhyolite,
  most recently 70,000 years ago, filled in parts
  of the caldera
• geysers and hot springs heated by magma 0.5
  miles beneath surface

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Map of Yellowstone Caldera and Adjacent Volcanic
Features
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Map of Yellowstone Caldera
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Yellowstone Caldera Rim and Flows
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Porcelain Basin, Norris
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Norris Geyser Basin
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Map of Elevation Change at Yellowstone Hot Spot
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Map of Yellowstone Hotspot Trail
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Olympus Mons, Mars

• summit caldera 24 kilometers (15 miles) above
  the surrounding plains.
• surrounding the volcano is an outward-facing
  scarp 550 kilometers (342 miles) in diameter and
  several kilometers high.
• beyond the scarp is a moat filled with lava,
  most likely derived from Olympus Mons

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Even the Krakatoa Tambora eruptions - among the
worlds most explosive historic events - were
small in comparison!
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.they are hazardous
Tambora (Indonesia) 1815 92,000 dead Mount
Pelée (Martinique) 1902 29,000 dead Nevado del
Ruiz (Columbia) 1985 22,000 dead
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Quiet Hawaiian type Volcanic Eruptions

• Intraplate and Divergent Boundaries
• Single Vent and Fissure Eruptions
• More fluid, high temperature Magmas of
  Basaltic Composition with low gas content
• Main product is lava instead of pyroclastic
  material

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Divergent Plate Boundaries

• Most are at mid-ocean ridges
• Mantle melts as pressure is reduced
• Melt is basaltic in composition
• Cooling basalt forms new oceanic crust

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Intraplate Volcanism

• Head of hot mantle collects at core-mantle
  boundary

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• Head of hot mantle collects at core-mantle
  boundary
• When it reaches the base of the crust it makes a
  huge amount of basalt (flood basalts)

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• Head of hot mantle collects at core-mantle
  boundary
• When it reaches the base of the crust it makes a
  huge amount of basalt (flood basalts)
• As plate moves over plume, a trail of volcanism
  forms

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Intraplate Volcanism - Ocean
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• Shield Volcanoes
• Low, rounded profiles slope angles 2-10
  composed of numerous thin flows of fluid mafic
  lava with little explosive activity
• Largest of all volcanoes

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All Hawaiian volcanoes are SHIELD
volcanoes (broad, convex upward)
Why this shape? Viscosity of basalt is low that
flows move great distances from vents before
solidifying
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Mauna Loa
Largest volcano on our planet, descending 5 km to
the sea floor, which is depressed by Mauna Loa's
great mass another 8 km! This makes the
volcano's summit about 17 km (56,000 ft) above
its base! Mauna Loa was last active in 1984,
Kilauea has been continuously erupting since
1983, and Loihi, a seamount south of Kilauea,
erupted in 1996
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A Classic Shield Volcano - Mauna Loa as Seen from
Mauna Kea
Joe Dellinger
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Summit of Mauna Loa, Big Island, Hawaii
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Digital 3-D Map of the Big Island (Hawaii)
Showing the Major Shield Volcanoes
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Note young lava flows at Mauna Loa and Kilauea in
Landsat Image to left
http//mael.soest.hawaii.edu80/space/hawaii/navne
w/images/images.all.bigisle.html
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BASALT (low silica) flows travel great distances
due to their relatively low viscosity.
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Lava fountaining pahoehoe flow
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Lava Flows on Kilauea Volcano, Hawaii
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Basaltic fissure eruptions
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Two types of lava flow on Hawaii
Pahoehoe Surfaces are smooth, billowy, or
ropy. Aa Surfaces are fragmented, rough, and
spiny, with a "cindery" appearance
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Pahoehoe toes
pahoehoe
http//volcanoes.usgs.gov/Products/Pglossary/pahoe
hoe.html
Ropey pahoehoe
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Kinds of lava flows fluid, basaltic magma
Pahoehoe (ropy) lava typical of hotspot
volcanoes like Kilauea
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Pahoehoe Lava Flows, Kilauea Volcano, Hawaii
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aa The spiny surface of a solidified aa flow
makes walking difficult and slow. The clinkery
surface actually covers a massive dense core.
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Aa Lava Flow, Kilauea Volcano, Hawaii
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http//wwwhvo.wr.usgs.gov/kilauea/erz_xsec.html
Pathway of molten rock during eruption of Puu Oo,
located on the east rift zone about 20 km from
the caldera.
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Pillow lavas are common where basalt lavas flow
underwater.
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• Intraplate Lava Plateaus
• Result of Fissure Eruptions
• of very Large Volumes of
• Basalt
• Example - Columbia Plateau

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Pacific Northwest Volcanic Features, Including
Columbia Plateau Flood Basalts
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Dry Falls, Columbia Plateau, Washington
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Lava Plateaus - Columbia Plateau Flood Basalts at
Palouse Falls State Park WA
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I. Divergent Plate Boundary
Most divergent plate boundaries are found on the
ocean floor. example Mid-Atlantic
Ridge. Spreading rate 2.5 cm/yr 25
km/million yr
http//pubs.usgs.gov/publications/text/ridge.html
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Ocean-floor basalts 60 of Earths surface
Length of ridge system 60,000 km!
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Ocean Ridges...
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Pillow Lavas Common at spreading centers
Chilled, glassy margins of lava tubes insulates
interior from the cooling effect of seawater,
allowing hot magma to continue flowing inside
tubes. Because of their sack-like shape, these
tubes sometimes resemble pillows.
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Also. Hydrothermal Vents
In late 1970s, submarine hot springs - up to
350C - were discovered on the sea floor. These
hydrothermal systems, and many others since
discovered, spew hot fluids through chimney-like
vents. The fluids are blackened by sulfide
precipitates rich in iron, copper, and zinc.
The sulfide minerals precipitate out of solution
due to cooling of the hydrothermal waters
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Iceland is one of the few places where the
mid-ocean ridge surfaces

• Iceland
• straddles mid-Atlantic Ridge
• almost entirely volcanic in origin
• eruptions about once every 5 yrs

http//pubs.usgs.gov/publications/text/understandi
ng.html
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Divergent plate boundaries can also be found on
some continents
East Africa Rift
An example of a continental rift - the early
stages of seafloor spreading!
http//pubs.usgs.gov/publications/text/understandi
ng.html
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Nyiragongo
1977 - extremely fluid (non-viscous) lava escaped
from lake through a flank fissure. Lava flowed
very rapidly (up to 60 km/hr), killing 50-100
people and a small elephant herd. This was a
rare example of death by lava !
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Lava lake began filling again in 1994
Note benches showing level of former lava lake
crater is 3,600 ft deep
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January 17, 2002
Lava flows from Nyiragongo dissected the town of
Goma (400,000 people), 18 km S of crater.
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The town of Goma in the news.
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