There are three types of volcanoes - geowordsgeowords.com/p_/05b-ge101-volcanism.pdf · There are three types of volcanoes or Stratovolcano Composite Volcanoes have alternating layers

There are three types of volcanoes

or Stratovolcano

Composite Volcanoes have alternating layers of lava and rock fragment recording multiple eruptions. A Cinder Cone Volcano is a steep conical, relatively small, hill built by liquid lava blobs and pyroclastics that rain back to earth around a volcanic vent to form a cone .

Shield Volcanoes built mainly of low viscosity basaltic lava flows have broad summit areas and low slopping sides.

Plate Tectonics and Volcanism Magma is formed at three main plate-tectonic settings: mantle plumes (decompression melting) divergent boundaries (decompression melting) convergent boundaries (flux melting)

A cutaway view along the Hawaiian island chain showing the inferred mantle plume that has fed the Hawaiian hot spot on the overriding Pacific Plate.

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The Yellowstone region has produced three exceedingly large volcanic eruptions in the past 2.1 million years. The pyroclastic flows from these eruptions resulted in rock formations called "tuffs."

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Illustration: University of California - Berkeley

A fissure eruption in or along rifts and rift zones is through a linear volcanic vent, often a few meters wide and often many kilometers long. The lava, mostly basaltic, erupts usually without any explosive activity. Large flood-basalt plateaus are the result of repeated outpourings from mantle plume heads.

Illustration of a hot mantle plume "head" pancaked beneath the Indian Plate. The theory by Richards and his colleagues suggests that existing magma within this plume head was mobilized by strong seismic shaking from the Chicxulub asteroid impact, resulting in the largest of the Deccan Traps flood-basalt eruptions.

Mantle Plume under India

The Deccan Traps (flood basalts), India, straddling the Cretaceous-Tertiary boundary, and covering more than 510,000 km2, constitute an important Large Igneous Province (LIP).

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Pinatubo June 1991

Pyroclastic flow

Calderas are large volcanic craters that are the result of an explosive volcanic eruption or the collapse of a volcano’s top into an empty magma chamber or both as was so for Crater Lake, Oregon.

Eruptions of ash and pumice: The cataclysmic eruption started from a vent on the northeast side of the volcano as a towering column of ash, with pyroclastic flows spreading to the northeast. Caldera collapse: As more magma was erupted, cracks opened up around the summit, which began to collapse. Fountains of pumice and ash surrounded the collapsing summit, and pyroclastic flows raced down all sides of the volcano. Steam explosions: When the dust had settled, the new caldera was 5 miles (8 km) in diameter and 1 mile (1.6 km) deep. Ground water interacted with hot deposits causing explosions of steam and ash. Today: In the first few hundred years after the cataclysmic eruption, renewed eruptions built Wizard Island, Merriam Cone, and the central platform. Water filled the new caldera to form the deepest lake in the United States.

Figure modified from diagrams on back of 1988 USGS map “Crater Lake”

A caldera forming eruption can produce massive amounts of:

far reaching (spread by winds aloft) pumice ash fall deposits called tuffs.

pumice-rich pyroclastic flows that move at very high speed down volcanic slopes, typically following valleys. Most consist of two parts: a lower (basal) flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. They accumulate as deposits called welded tuffs or ignimbrites.

Most of the explosive volcanism in the world occurs along “Ring of Fire” defined by a nearly continuous series of oceanic trenches, island arcs, and volcanic mountain ranges and/or boundaries of several plates, which encircle the periphery of the Pacific Ocean Basin.

May 22, 1915 eruption of Lassen Peak as seen from Red Bluff, California

Lava domes are formed by viscous magma, too thick and sticky to flow very far, that piles up high around a vent.

1984 USGS picture of the growing Mount St. Helens Lava dome. This lava dome started developing shortly after the iconic May, 18th 1980 eruption and dome growth continued until 1986.

Lava domes (dark gray mounds in this photograph) on the flanks of a volcano are dangerous as they are subject to landslide collapse and so the sudden release of pressure on underlying lava that could then explode out pyroclastic flows (nuée ardente).

Devil’s tower, Wyoming

Shiprock. New Mexico