VOLCANOES AND VOLCANISM The Nature and Status of Volcanoes ACTIVE - currently erupting volcano or one that has 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, up to years, but has potential to erupt. Mt. Baker, WAMt. 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’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 H 2 O and CO 2. N, SO 2 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

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 1. LAVA TUBES Tunnels along which lava once flowed. OTHER FEATURES OF BASALTIC LAVA

Craters of the Moon NP Will Junkin

VOLCANOES AND VOLCANISM PRODUCTS OF VOLCANISM 2. VESICLES Chambers produced as gas escapes from lava. OTHER FEATURES OF BASALTIC LAVA

VOLCANOES AND VOLCANISM PRODUCTS OF VOLCANISM 3. COLUMNAR JOINTING As lava cools, it contracts. Contracts to form six-sided polygonal columns. OTHER FEATURES OF BASALTIC LAVA

COLUMNAR JOINTING 1. Devil’s Tower, WY 2. Giant’s Causeway, Ireland 3. Devil’s Post Pile, CA

Giant’s Causeway Lea Korsmeyer

VOLCANOES AND VOLCANISM PRODUCTS OF VOLCANISM 4. PILLOW BASALTS Form 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 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

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