Download Volcanoes and Igneous Activity Earth

Volcanism
Plate tectonics and igneous activity
Global distribution of magmatism is not
random
• Most volcanoes are located within or near
ocean basins
• Basalt common in both oceanic and
continental settings
• Granite is rare in oceans, mostly found in
continents
Distribution of major volcanoes
Plate tectonics and igneous activity
Igneous activity along plate margins
• Spreading centers
– Great volumes of volcanic rock produced along
oceanic ridges
– Mechanism of spreading
– Lithosphere pulls apart and thins
– Less pressure results in partial melting in mantle
A fissure eruption is an eruption from a
long crack in the lithosphere.
The Columbia River flood basalts are a
continental example
Other volcanic landforms
Fissure eruptions and lava plateaus
• Fluid basaltic lava extruded from crustal
fractures called fissures
• e.g., Columbia River Plateau
• Flood basalts cover huge areas
Flood Basalt erupted
from fissures - Snake
River Plain, southern
Idaho
The Nature of Volcanic Eruptions
Factors determining the “violence” or
explosiveness of a volcanic eruption
• Composition of the magma
• Temperature of the magma
• Dissolved gases in the magma
These three factors control the viscosity
of magma (which controls the nature of
an eruption)
The Nature of Volcanic Eruptions
Viscosity is a measure of a material’s resistance
to flow (higher viscosity materials flow with great
difficulty; water has very low viscosity)
Factors affecting viscosity
• Higher temperature - magma is less viscous (or
more fluid)
• Composition - higher silica (SiO2) content magma is more viscous (rhyolite)
• Lower silica content = lower viscosity or more
fluid-like behavior (e.g., mafic lava such as basalt)
The Nature of Volcanic Eruptions
Factors affecting viscosity continued
• Dissolved Gases
– Gas content affects magma mobility
– Gases expand in 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
The Nature of Volcanic Eruptions
Summary
• Fluid basaltic lavas generally produce
quiet eruptions (Hawaiian volcanos gurgle)
• Highly viscous lavas (rhyolite or andesite)
produce more explosive eruptions
(Yellowstone or St Helens)
Materials extruded from a volcano
Lava Flows
• Basaltic lavas are much more fluid
• Types of basaltic flows
– Pahoehoe lava (- twisted or ropey texture)
– Aa lava (rough, jagged blocky texture)
Dissolved Gases
• 1-6% of a magma by weight
• Mainly water vapor and carbon dioxide
A Pahoehoe lava flow
A typical aa 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
• Lapilli - walnut-sized material
• Cinders - pea-sized material
• Particles larger than lapilli
– Blocks - hardened or cooled lava
– Bombs - ejected as hot lava
A volcanic bomb
Bomb is approximately 10 cm long
Volcanoes
General Features
• Opening at the summit of a volcano
– Crater - steep-walled depression at the
summit, generally less than 1 km diameter
– Caldera - a summit depression typically
greater than 1 km diameter, produced by
collapse following a massive eruption
• Vent – opening connected to the magma
chamber via a pipe
Volcanoes
Types of Volcanoes
• Shield volcano
– Broad, slightly domed-shaped
– Composed primarily of basaltic lava
– Generally cover large areas
– Produced by mild eruptions of large volumes
of lava
– Mauna Loa on Hawaii is a good example
A size comparison of the three
types of volcanoes
Olympus Mons
Caldera
Volcanic Domes
Volcanoes
Types of Volcanoes continued
• Cinder cone
– Built from ejected lava (mainly cinder-sized)
fragments
– Steep slope angle
– Rather small size
– Frequently occur in groups
Cinder cones are built from ejected lava fragments
Sunset Crater – a cinder cone near
Flagstaff, Arizona
Volcanoes
Types of volcanoes continued
• 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 & several miles wide at base)
– Composed of interbedded lava flows and
layers of pyroclastic debris
A composite volcano
Mt. St. Helens – a typical composite
volcano (prior to eruption)
Mt. St. Helens following the
1980 eruption
Volcanoes
• Composite cones continued
– Most violent type of activity (e.g., Mt.
Vesuvius)
– Often produce a nueé ardente
– Fiery pyroclastic flow made of hot
gases infused with ash and other
debris
– Move down the slopes of a volcano
at speeds up to 200 km per hour
– May produce a lahar, which is a
volcanic mudflow
A nueé ardente on Mt. St. Helens
Town of St Pierre on Montserrat after Nuee Ardente
Other volcanic landforms
Calderas (form by collapse of evacuated magma
chamber)
• Steep-walled depressions at the summit
• Size generally exceeds 1 km in diameter
Pyroclastic flows (explosive mix of rock, gas and
heat)
• Only with felsic & intermediate magma
• Consists of ash, pumice, and other fragmental
debris
• Material propelled from vent at high speed
Hazards related to volcanoes
Lava
Pyroclastics
Laharar
Pyroclastics flows-Nuees Ardentes
Toxic gases
Steam explosions
Climate and atmospheric chemistry
Lava
The colder lava becomes, the thicker, more viscous, and
slower-flowing it gets. When cooled enough, it may solidify
altogether.
Protecting Inhabitants
One strategy would be to slow the
flow—either by spraying it with water
so that it congeals and dams itself, or
by constructing earthen or other dams
in front of it to slow it down long
enough to cause it to crystallize, stiffen,
and stop.
Lava
Pyroclastics
Pyroclastics
Lahars
Lahars
The mudflow has taken out
the main bridges
Pyroclastic flows-Nuees Ardentes
Pyroclastic flows-Nuees Ardentes
Toxic gases
Water vapor and carbon dioxide are nontoxic, yet can nevertheless be
dangerous at high concentrations
Carbon monoxides, sulfur gases, and hydrochloric acid are poisonous
How may volcanic eruptions influence
global climate?
Explosive eruptions, particularly, may
put large volumes of dust and sulfuric
acid aerosols into the atmosphere.
These block sunlight and thus
contribute to global cooling
The dust can take several months to
years to settle
Climate and atmospheric chemistry
Climate and atmospheric chemistry
Climate and atmospheric chemistry
Classification of volcanoes by activity
Active volcanoes have a recent history of
eruption.
no historic erupts but
not badly eroded
Dormant volcanoes:
Those volcanoes classified as extinct have
neither erupted within historic record, nor do
they have a very fresh appearance; they are not
expected to erupt again.
Volcanic precursors
What do scientists look for when
monitoring a volcano?
seismic activity
tilting or uplift
monitoring gas emissions around
volcanoes
Present and future volcanic hazards in USA
Present and future volcanic hazards in USA
Present and future volcanic hazards in USA
Present and future volcanic hazards in USA
Present and future volcanic hazards in USA