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VOLCANOES AND VOLCANISM
The Nature and Status of Volcanoes
ACTIVE - currently erupting volcano or one thathas erupted very recently.
Kilauea, HI
Mt. Lassen, CA
VOLCANOES AND VOLCANISM
The Nature and Status of Volcanoes
DORMANT - has not erupted in the recent past, upto 2000-3000 years, but has potential to erupt.
Mt. Baker, WA Mt. Shasta, California
VOLCANOES AND VOLCANISM
The Nature and Status of Volcanoes
EXTINCT - has not erupted in very long time,generally > 10,000 years.
Shiprock, NM
Heimaey, Iceland, prior to 1973,hadn’t erupted in 5000 years.
VOLCANOES AND VOLCANISM
CAUSES OF VOLCANISM
Generally caused by magma reaching the Earth’ssurface through fractures in the lithosphere.
Occurs at plate boundaries and at hot spots.Influenced by:
Gas in volcanic magma
Magma viscosity
VOLCANOES AND VOLCANISM
CAUSES OF VOLCANISM
GAS IN VOLCANIC MAGMA1-9% of most magma is gas.Most common gases are H2O and CO2.N, SO2 and Cl are also common.With pressure, these gases remain dissolved.As magma rises, pressure decreases.Gases separate.Rise to top of magma body.Removal of pressure or overlying rock causes
dramatic release of pressure.Leads to violent eruption.
VOLCANOES AND VOLCANISM
So what?
VOLCANOES AND VOLCANISM
CAUSES OF VOLCANISM
MAGMA VISCOSITYGreater viscosity of magma, greater potential for
explosive eruption.Greater silica content, greater viscosity.
Mafic Magmas: lower viscosityrise to Earth’s surfacegases escape relatively easilyerupt quietly with gentle
outpouring of lava
Intermediate Magmas: higher viscositytend not to rise as readilygas movement is impedederupt explosively when theyreach Earth’s surfacemost common in terrestrial
volcanoes
Felsic Magmas: highest viscositybehave similarly to
intermediate magmas(often never erupt – form
granite instead!)
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
TYPES OF LAVA
BASALTIC LAVAForms at high temperatures > 1000°CHas low viscosity, high fluidity
Produces two types of lava:
VOLCANOES AND VOLCANISM
1. PAHOEHOERopy lavaVery low viscosity, flows very fastThin skin gets pushed up into folds by
the still moving lava below it.
PAHOEHOE
PAHOEHOE
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
TYPES OF LAVA
BASALTIC LAVA
2. A’AAs lava cools and degasses, it becomes more
viscous.Gets a thick, brittle crust, becomes more blocky.Surface breaks up as lava flows underneath.
A’A LAVA
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
1. LAVA TUBESTunnels along which lava once flowed.
OTHER FEATURES OF BASALTIC LAVA
Craters of the Moon NPWill Junkin
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
2. VESICLESChambers produced as gas escapes from lava.
OTHER FEATURES OF BASALTIC LAVA
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
3. COLUMNAR JOINTINGAs lava cools, it contracts.Contracts to form six-sided
polygonal columns.
OTHER FEATURES OF BASALTIC LAVA
COLUMNAR JOINTING1. Devil’s Tower, WY2. Giant’s Causeway, Ireland3. Devil’s Post Pile, CA
1.
3.
2.
Giant’s CausewayLea Korsmeyer
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
4. PILLOW BASALTSForm when basaltic lavas flow and cool
underwater.
OTHER FEATURES OF BASALTIC LAVA
PILLOW BASALTS
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
ANDESITIC LAVA
RHYOLITIC LAVA
More viscous than mafic lavas.Doesn’t flow far from vent.Contains vesicles and ‘a’a lava.Produces columnar jointing and pillow structures.
Flows extremely slowly.Erupts violently.Eruptions produce airborne solid fragments.May produce large amounts of pumice.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
PYROCLASTIC MATERIAL
Lava is expelled forcefully into the atmosphere.Termed PYROCLASTIC.
TEPHRA - pyroclastic material that cools and solidifiesfrom lava as it flows through the atmosphere.
Produces distinct layers that can be mapped andrelated to eruption events.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
TEPHRA
VOLCANIC DUST
Extremely fine tephra.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
TEPHRA
VOLCANIC ASH
Tephra with a grain-size <2mm.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
TEPHRA
VOLCANIC CINDERS OR LAPILLITephra with grain-size between
2 and 64 mm.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
TEPHRA
VOLCANIC BOMBS
Tephra with grain-size >64mm.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
PYROCLASTIC FLOWS
Can have large amounts expelled by volcanoes.Particles are generally large.Gravity tends to pull the pyroclastic material downhill.PYROCLASTIC FLOW or NUÉE ARDENTE.Trapped air and magmatic gases keep material
suspended.Very low friction, velocities >150 km/hr (90 mi/hr).
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
PYROCLASTIC FLOWSNUÉE ARDENTE
Mt. Pelee,Martinique, 1908
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
PYROCLASTIC FLOWSMaterial in nuée ardente may not cool completely
before deposition.Soft material fuses with other particles.Forms solidified tephra.Called WELDED TUFF.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
VOLCANIC MUDFLOWS
Pyroclastic material mixes with water.Flows downhill as a LAHAR.Generally happens where abundant, loose, moist soil
occurs.Tropical regions provide this setting.Helps if volcano is snow-capped or covered with
glaciers.Ash and dust may actually enhance the potential for
rain.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
LAHAR
Mt. Pinatubo, Philippines, 1991
Pillow Basalts
ActiveDormant
