chapter 5 volcanoes and volcanism. introduction about 550 volcanoes have been active in historical...

Chapter 5

Volcanoes and Volcanism

Introduction

About 550 volcanoes have been active in historical times.

About 12 or so erupt each year; most eruptions are small and go unnoticed

Introduction

Volcanism is the eruption of magma and associated gases at the surface.

Some magma erupts explosively as pyroclastic ("fire-broken") rock and others erupt as lava flows.

Introduction

How can volcanism be both constructive and destructive? Volcanism may cause

injuries and fatalities, and destroy property.

Volcanism is also responsible for the origin of many oceanic islands, fertile farmland, and critical atmospheric gases.

Fig. 5.1, p. 111Pompeii and Mount Vesuvius


Types of Volcanoes Not all volcanoes erupt from giant cones, some erupt

from fissures (fractures) in the Earth's crust.

Volcanoes were once active on all the inner planets of our Solar System and our Moon. Today, volcanoes are still active on Earth and possibly on Venus.

Jupiter's moon, Io, has active sulfur volcanoes.

Triton, a moon of Neptune, probably has volcanoes that erupt liquid methane (natural gas or CH4).


Volcanic Gases

Gases, primarily water vapor, constitute only a small portion of magmas

Lesser amounts of carbon dioxide, nitrogen, sulfur dioxide, and hydrogen sulfide

Very small amounts of carbon monoxide, hydrogen, and chlorine

Fig. 5.2, p. 112


Volcanic Gases Most volcanic gases dissipate

without harm to humans

Chlorine gas may contribute to the deterioration of the ozone layer, which protects organisms from ultraviolet radiation (Geo-Focus, pp. 122-123)

Sulfur gases from large eruptions may cool global climates for 1-2 years

Fig. 5.2, p. 112


Low Viscosity Mafic Lava Flows

Mafic lavas may flow through near surface lava tubes

Mafic lava flows may reach speeds of 50 km/hr

Fig. 5.4, p. 113


Mafic Lava Flows on Land Aa lava flows consist of angular blocks and

fragments. Pahoehoe lava flows have a smooth

surface, much like taffy.

Fig. 5.5, p. 114


Underwater Mafic Lava Flows

Pillow lavas are bulbous, interconnecting masses that result from underwater eruption of basaltic lavas.

Fig. 5.6a, p. 114


Lava Flows

Columnar Joints Columnar joints form in response to the cooling

and contraction of lava. They most commonly occur in basalt and andesite lava flows.

Fig. 5.7, p. 115

Fig. 5-7, p. 115

Stepped Art

As lava cools and contracts, three-pronged cracks form that grow and intersect to form four- to seven-sided columns, most of which are six-sided.

Surface view of the columns from (b). The straight lines and polish resultedfrom abrasion by a glacier that moved over this surface.

Columnar joints in a basalt lava flow at Devil’s Postpile NationalMonument in California. The rubble in the foreground is collapsed columns.

Pyroclastic Materials


Pyroclastic materials are magma fragments that are explosively ejected by volcanoes, most are solid

Ash < 2mm in diameter (smaller than sand grains)

Lapilli, 2-64 mm Bombs, partially molten, >64 mm Blocks, solid, >64 mm


Pyroclastic Materials Volcanic ash, unlike larger pyroclastic materials, may be

transported by winds over long distances.

Ash is unhealthy for lungs and dangerous to aircraft because it fouls jet engines.

Ash falls settle out of the atmosphere.

Ash flows are turbulent clouds of ash and gas that travel close to the surface during pyroclastic eruptions.

Types of Volcanoes

A volcano is a hill or mountain that forms around a vent, where lava, pyroclastic materials and gases erupt.

Fig. 5.12c, p. 119

Types of Volcanoes

Calderas

A caldera is a large oval to circular volcanic depression that forms when the summit of a volcano collapses into its magma chamber following voluminous eruptions.

Fig. 5.8, p. 116

Types of Volcanoes

Shield Volcanoes

Shield volcanoes have gentle slopes and are largely composed of basalt with possibly some mafic pyroclasts. The Hawaiian volcanoes are shield volcanoes.

They are usually non-explosive and pose little danger to humans.

Fig. 5.9, p. 117

Types of Volcanoes

Shield Volcanoes

The largest volcano in the Solar System, Olympus Mons on Mars, is a shield volcano.

Fig. 5.10b, p. 117

Types of Volcanoes

Cinder Cones

Cinder cones consist of cinders and other pyroclastic materials. They accumulate as steep-sloped cones that rarely exceed 400 meters high.

Fig. 5.11, p. 118

Types of Volcanoes

Composite Volcanoes (Stratovolcanoes)

Composed of layers of lava flows, pyroclastic debris and volcanic mud flows (lahars). They are explosive and are the most dangerous to humans. The volcanoes rimming the Pacific ocean are mostly composite volcanoes. Most composite volcanoes have intermediate compositions.

Fig. 5.12a,b, p. 119

Types of Volcanoes

Some Notable Volcanic Eruptions, Many Involving Deadly Composite Volcanoes

Table 5.1, p. 120

Table 5.1, p. 120

Types of Volcanoes


Lahars are mudflows consisting of mixtures of liquid water and pyroclastic materials. They may be hot or cold, and often form from rain during eruptions.

Fig. 5.13, p. 119

Types of Volcanoes


Lahars may also form without an eruption if a submit of a volcano heats up, catastrophically melts snow and ice, and the resulting water mixes with pyroclastic materials from previous eruptions.

Fig. 5.13, p. 119

Types of Volcanoes

Lava Domes

Bulbous lava domes form when viscous, usually felsic or intermediate, lavas are forced up through the conduits of some volcanoes. These volcanoes can erupt explosively and commonly eject nuée ardentes (hot clouds of pyroclastic materials and gas).

Fig. 5.14, p. 120

Types of Volcanoes

Lava Domes

Nuée ardentes are hot clouds of pyroclastic materials and gas that are released by explosive eruptions.

In 1902, a nuée ardente engulfed the city of St. Pierre on Martinique in the Caribbean. 28,000 people were killed.

Fig. 5.15, p. 121

Types of Volcanoes

Supervolcano Eruptions

No supervolcano eruptions have occurred within recorded history

Erupt hundreds of square kilometers of material and produce huge calderas

Three supervolcano eruptions have occurred in Yellowstone within the past 2 million years

Figure 5.16, p. 124

Other Volcanic Landforms

Fissure Eruptions and Basalt Plateaus Basalt plateaus form when fluid mafic lava erupts

from long fissures (not vents), known as a fissure eruption. Basalt plateaus are made up of numerous overlapping basalt lava flows.

Fig. 5.17a,b, p. 124

Other Volcanic Landforms

Pyroclastic Sheet Deposits Huge sheet-like eruptions of pyroclastic

materials, especially ash, can cover large areas when erupted from fissures during caldera formation.

Volcano Belts

Volcanoes are usually not randomly distributed, but occur in well-

defined zones or belts along plate boundaries.

Fig. 5.18, p. 125

Volcano Belts

About 60% circum-Pacific belt, 20% Mediterranean, belt, 20% are at or near mid-oceanic ridges

Fig. 5.18, p. 125

North America’s Active Volcanoes

Alaska’s Volcanoes Alaska’s volcanoes stretch from the mainland of

Alaska through the Aleutian Islands.

Fig. 5.18, p. 125


Alaska’s Volcanoes Most are composite volcanoes, some with huge calderas. This

volcanic arc is extremely active with many explosive eruptions.

Fig. 5.18, p. 125


The Cascade Range: Lassen Peak in California to British Columbia, Canada

Mostly composite volcanoes, but there are also two huge shield volcanoes and numerous cinder cones

Fig. 5.19 a-c, p. 127


The Cascade Range: Lassen Peak in California to British Columbia, Canada

Results from the subduction of the oceanic Juan de Fuca Plate underneath the continental North American Plate

Fig. 5.19 a-c, p. 127

Plate Tectonics, Volcanoes, and Plutons

Igneous Activity at Divergent Plate Boundaries

Volcanic activity at or near mid-oceanic ridges is mainly submarine, but in a few places such as Iceland, it occurs above sea level. The volcanoes that form are mostly mafic shield volcanoes.

Fig. 5.6a, p. 114


Igneous Activity at Convergent Plate Boundaries

The volcanism in the circum-Pacific and Mediterranean belts results from subduction.

Most of the magmas are produced by the partial melting of the subducted plates.

Fig. 5.18, p. 125


Intraplate Volcanism

Mafic hotspot volcanism

The Hawaiian Islands formed as a series of volcanoes originating from a stationary mantle plume as the Pacific Plate moved over it.

Fig. 5.4, p. 113

Volcanic Hazards, Volcano Monitoring, and Forecasting Eruptions

Lava dome eruptions with huge amounts of pyroclastic materials and gases are the most dangerous volcanic eruptions.

Lahars are also dangerous and they may occur without an eruption.

Fig. 5.20, p. 129

What are the most dangerous manifestations of volcanoes?


How Large Is an Eruption and How Long Do Eruptions Last?

VEI - the most widely used indication of the size of a volcanic eruption is the volcanic explosivity index

The VEI measures the explosive intensity of an eruption using: volume of material erupted eruption cloud height

Fig. 5.21, p. 130

Fig. 5.21, p. 130

Plinian

VEI 4-7

CATACLYSMIC

Fig. 5-21, p. 130

Stepped Art

EXPLOSIVE

Strombolian

VEI 1-3Vulcanian

VEI 3-5

GENTLE


Is It Possible to Forecast Eruptions?

Monitoring volcanoes helps geologists to forecast imminent eruptions

Fig. 5.22, p. 130

Fig. 5.22, p. 130



Monitoring involves recording and analyzing both physical and chemical changes at volcanoes

Tiltmeters and geodimeters to detect changes in slope, elevation, and shape of the volcano

Fig. 5.22, p. 130



Monitoring involves recording and analyzing both physical and chemical changes at volcanoes

Seismometers to detect harmonic tremors

Gas emissions are also measured

Fig. 5.22, p. 130

Volcanic Activity and Extinction

chapter 5 volcanoes and volcanism. introduction about 550 volcanoes have been active in historical...

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