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Igneous Activity
Origin of magma
| Where do volcanoes form? | Volcanic
Eruptions | Extrusive Igneous Activity
| Types of Volcanic Eruptions | Benefits
of Volcanoes | Methods Of Forecasting Volcanic
Eruptions | Intrusive
Igneous Activity | Links
- Origin of magma
- Magma originates when essentially solid rock,
located in the crust and upper mantle, melts
- Factors:
- Role of heat
- Earth’s natural temperature
increases with depth (geothermal gradient) is not
sufficient to melt rock at the lower crust and upper
mantle
- Additional heat is generated by
- Friction in subduction zones
- Crustal rocks heated during subduction
- Rising, hot mantle rocks
- Role of pressure
- Increase in confining pressure causes
an increase in melting temperature
- Drop in confining pressure can cause
decompression melting
- Lowers the melting temperature
- Occurs when rock ascends
- Role of volatiles
- Primarily water
- Cause rock to melt at a lower temperature
- Play an important role in subducting
ocean plates
- Partial melting
- Igneous rocks are mixtures of minerals
- Melting occurs over a range of temperatures
- Produces a magma with a higher silica
content than the original rock
- Emplacement of magma
- Magma at depth is much less dense
than the surrounding rock
- Increased temperature and pressure
causes solid rock to deform plastically
- The more buoyant magma pushes
aside the host rock and forcibly rises in the
Earth as it deforms the “plastic” host
rock
- At shallower depths, the host rock
is cooler and exhibits brittle deformation
- Movement of magma here is accomplished
by fractures in the host rock and stoping
- Overall, the emplacement of magma
is very similar to the emplacement and intrusion
of salt domes
- Where do volcanoes form?
- Volcanoes form at:
- Hot Spots (10% of all volcanic activity)
- Spreading Centers (80% of all volcanic activity)
- Convergent Plate Boundaries (10% of all volcanic
activity)
- Ocean–Continental
- Ocean – Ocean
- Volcanic Eruptions
- Factors that determine the violence of an
eruption
- Composition of the magma
- Temperature of the magma
- Dissolved gases in the magma
- The above three factors actually control
the viscosity of a given magma which in turn controls the
nature of an eruption
- Viscosity is a measure of a material's resistance
to flow
- Factors affecting viscosity
- Temperature
- hotter magmas are less viscous
- Composition (silica content)
- High silica – high viscosity
(e.g., rhyolitic lava)
- Low silica – more fluid (e.g.,
basaltic lava
- Dissolved gases (volatiles)
- Mainly water vapor and carbon dioxide
- Gases expand near the surface
- Provide the force to extrude lava
- Violence of an eruption = how easily
gases escape
- Plate-Tectonic Setting of Volcanoes Revisited
- Why more volcanic activity at spreading
centers?
- Low SiO2 content
- High temperature
- Low pressure as plates pull apart
- Fluid basaltic lavas generally produce
quiet eruptions
- Why less volcanic activity at subduction
zones?
- High SiO2 content
- Lower temperatures
- Higher pressures
- Highly viscous lavas produce more
explosive eruptions
- Extrusive Igneous Activity
- Materials associated with volcanic eruptions
- Lava flows
- Molten rock that has flowed out onto
the Earth’s surface
- Types of basaltic lava
- Pahoehoe lava (resembles braids
in ropes)
- Aa lava (rough, jagged blocks)
- Gases
- One to 5 percent of magma by weight
- Mainly water vapor and carbon dioxide
Materials associated with volcanic eruptions
- Pyroclastic materials
- Ash and dust – fine, glassy
fragments
- Pumice – from "frothy"
lava
- Cinders – "pea-sized"
- Lapilli – "walnut"
size
- Particles larger than lapilli
- Nuée Ardente (or pyroclastic flow)
- Fiery pyroclastic flow made of hot
gases infused with ash
- Flows down sides of a volcano at
speeds up to 200 km (125 miles) per hour
- Lahar
- Volcanoes
- General features
- Conduit, caries gas-rich magma to
the surface
- Vent, the surface opening (connected
to the magma chamber via a pipe)
- Crater, steep-walled depression at
the summit
- Caldera (a summit depression greater
than 1 km diameter)
- Parasitic cones
- Fumaroles
- Types of volcanoes
- Shield volcano
- Low Viscosity, Low Volatiles, Large
Volume
- Broad, slightly domed
- Primarily made of basaltic (fluid)
lava
- Generally large size
- Hawaiian Islands
- Cinder cone
- Low Viscosity, Medium Volatiles,
Small Volume
- Built from ejected lava fragments
- Steep slope angle
- Rather small size
- Frequently occur in groups
- Sunset Crater, Flagstaff, Arizona
- Composite cone (or stratovolcano)
- High Viscosity, High Volatiles, Large
Volume
- Most are adjacent to the Pacific
Ocean
- Large in size
- Interbedded lavas and pyroclastics
- A size comparison of the three types
of volcanoes
- Other volcanic landforms
- Calderas
- High Viscosity, High Volatiles, Very
Large Volume
- Steep walled depression at the summit
formed by collapse
- Size exceeds one kilometer in diameter
- Famous (or infamous) collapsed calderas:
- Long Valley, California (Mammoth)
- Crater Lake (Mount Mazama), Oregon
- Yellowstone, Wyoming
- Krakatau, Indonesia, 1883
- Santorini and the Lost Continent
of Atlantis
- Formation of Crater Lake
- Fissure eruptions and lava plateaus
- Low Viscosity, Low Volatiles, Very
Large Volume
- Basaltic lava extruded from crustal
fractures
- Incredibly large volumes of lava
pour out of fissures over 2-3 million years
- Can affect global climate
- Formation of Flood Basalts
- The Columbia River basalts
- Lava Domes
- Bulbous mass of congealed lava
- Most are associated with explosive
eruptions of gas-rich magma
- One is currently developing in Mt.
St. Helens
- Volcanic pipes and necks
- Pipes are short conduits that connect
a magma chamber to the surface
- Volcanic necks (e.g., Ship Rock,
New Mexico) are resistant vents left standing after
erosion has removed the volcanic cone
- Types
of Volcanic Eruptions
- Hawaiian
- Fissure, caldera, and pit crater eruptions;
- mobile lavas, with some gas;
- quiet to moderately active eruptions;
- occasional rapid emission of gas-charged
lava produces fire fountains;
- only minor amounts of ash;
- builds up lava domes.
- Strombolian
- Stratocones (summit craters):
- moderate, rhythmic to nearly continuous
explosions, (spasmodic gas escape);
- clots of lava ejected, producing bombs
and scoria;
- periodic more intense activity with outpouring
of lava;
- light-colored clouds (mostly steam) reach
upward only to moderate heights.
- Vulcanian
- Stratocones, (central vents);
- associated with viscous lavas;
- lavas crust over in vent between eruptions,
allowing gas buildup below surface;
- eruptions increase in violence over longer
periods of quiet until lava crust is broken up, clearing
vent, ejecting bombs, pumice
- and ash;
- Vulcanian
- lava flows from top of flank after main
explosive eruption;
- dark ash-laden clouds, convoluted, cauliflower-shaped,
rise to moderate heights more of less vertically, depositing
tephra
- along flanks of volcano.
- Vesuvian
- More Paroxysmal then Strombolian or Vulcanian
types;
- extremely violent expulsion of gas-charged
magma from stratocone vent;
- eruption occurs after long interval of
quiescence of mild activity;
- vent tends to be emptied to considerable
depth;
- lava ejects in explosive spray (glow
above vent), repeated clouds (cauliflower) that reach
great heights and deposit tephra.
- Plinian
- More violent form of Vesuvian eruption;
- last major phase is uprush of gas that
carries cloud rapidly upward in vertical column for miles;
- narrow at base but expands outward at
upper elevations; cloud generally low in tephra.
- Peléan
- Results from high-viscosity lavas;
- delayed explosiveness; conduit of stratovolcano
usually blocked by dome or plug;
- gas (some lava) escapes from lateral
(flank) opening or by destruction or uplift of plug;
- gas, ash, and blocks move downslope in
one or more blasts as nuées ardentes or glowing
avalanches, producing directed deposits.
- Katmaian
- Variant of a Peléan eruption characterized
by massive outpouring of fluidized ashflows;
- accompanied by widespread explosive tephra;
- ignimbrites are common end products,
also hot springs and fumaroles.
- Benefits of Volcanoes
- Produce great amounts of new land
- Frequently produce very fertile soils
- Provide Geothermal Power
- Recreation
- Methods Of Forecasting
Volcanic Eruptions
- Geophysical methods
- Seismology
- Structure of the volcano can be determined
- Movement of the magma can be traced
- Gravity Surveys
- Can recognize inflation and deflation
- Precise surveying - either conventional
or by laser
- Tiltmeter measurements reveal inflation
and deflation
- Heat flow studies
- Geochemical methods
- The Cl, F, and S composition of hot springs
may change before eruptions
- Geochronology
- Precise dating may reveal regular repeat
intervals
- Intrusive Igneous Activity
- Plutons are classified according to
- Orientation with respect to the host
(surrounding) rock
- Discordant – cuts across existing
structures
- Concordant – parallel to features
such as sedimentary strata
- Types of igneous intrusive features
- Dike, a tabular, discordant pluton
- Sill, a tabular, concordant pluton
- e.g., Palisades Sill, NY
- Resemble buried lava flows
- May exhibit columnar joints
- Laccolith
- Similar to a sill
- A sill in the Salt River Canyon,
Arizona
- Lens shaped mass
- Arches overlying strata upward
- Batholith
- Largest intrusive body
- Often occur in groups Surface exposure
100+ square kilometers (smaller bodies are termed
stocks)
- Frequently form the cores of mountains
- A batholith exposed by erosion
Origin
of magma | Where do
volcanoes form? | Volcanic Eruptions
| Extrusive Igneous Activity | Types
of Volcanic Eruptions | Benefits of Volcanoes
| Methods Of Forecasting Volcanic Eruptions
| Intrusive Igneous
Activity | Links
- California
- Cameroon
- Columbia
- Greece
- Hawai'i
- Kratatoa
- Oregon
- Washington
- Yellowstone
- General
Origin of magma
| Where do volcanoes form? | Volcanic
Eruptions | Extrusive Igneous Activity
| Types of Volcanic Eruptions | Benefits
of Volcanoes | Methods Of Forecasting Volcanic
Eruptions | Intrusive
Igneous Activity | Links
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