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VOLCANOES AND VOLCANISM
The Nature and Status of Volcanoes
ACTIVE - currently erupting volcano or one that
has erupted very recently.
Mt. Lassen, CA
Kilauea, HI
VOLCANOES AND VOLCANISM
The Nature and Status of Volcanoes
DORMANT - has not erupted in the recent past, up
to 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.
Heimaey, Iceland, prior to 1973,
hadn’t erupted in 5000 years.
Shiprock, NM
VOLCANOES AND VOLCANISM
CAUSES OF VOLCANISM
Generally caused by magma reaching the Earth’s
surface 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 MAGMA
1-9% 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 VISCOSITY
Greater viscosity of magma, greater potential for
explosive eruption.
Greater silica content, greater viscosity.
Mafic Magmas: lower viscosity
rise to Earth’s surface
gases escape relatively easily
erupt quietly with gentle
outpouring of lava
Intermediate Magmas: higher viscosity
tend not to rise as readily
gas movement is impeded
erupt explosively when they
reach Earth’s surface
most common in terrestrial
volcanoes
Felsic Magmas:
highest viscosity
behave similarly to
intermediate magmas
(often never erupt – form
granite instead!)
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
TYPES OF LAVA
BASALTIC LAVA
Forms at high temperatures > 1000°C
Has low viscosity, high fluidity
Produces two types of lava:
VOLCANOES AND VOLCANISM
1. PAHOEHOE
Ropy lava
Very low viscosity, flows very fast
Thin 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’A
As 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
OTHER FEATURES OF BASALTIC LAVA
1. LAVA TUBES
Tunnels along which lava once flowed.
Craters of the Moon NP
Will Junkin
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
OTHER FEATURES OF BASALTIC LAVA
2. VESICLES
Chambers produced as gas escapes from lava.
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
OTHER FEATURES OF BASALTIC LAVA
3. COLUMNAR JOINTING
As lava cools, it contracts.
Contracts to form six-sided
polygonal columns.
COLUMNAR JOINTING
1. Devil’s Tower, WY
2. Giant’s Causeway, Ireland
3. Devil’s Post Pile, CA
1.
2.
3.
Giant’s Causeway
Lea Korsmeyer
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
OTHER FEATURES OF BASALTIC LAVA
4. PILLOW BASALTS
Form when basaltic lavas flow and cool
underwater.
PILLOW BASALTS
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
ANDESITIC LAVA
More viscous than mafic lavas.
Doesn’t flow far from vent.
Contains vesicles and ‘a’a lava.
Produces columnar jointing and pillow structures.
RHYOLITIC LAVA
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 solidifies
from lava as it flows through the atmosphere.
Produces distinct layers that can be mapped and
related 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 LAPILLI
Tephra 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 FLOWS
NUÉE ARDENTE
Mt. Pelee,
Martinique, 1908
VOLCANOES AND VOLCANISM
PRODUCTS OF VOLCANISM
PYROCLASTIC FLOWS
Material 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
Dormant
Active