VolcanismVolcanism

Plate tectonics and igneous activity

Global distribution of magmatism is not random

• Most volcanoes are located within or near ocean basins

• Basalt common in both oceanic and continental settings

• Granite is rare in oceans, mostly found in continents

Distribution of major volcanoes

Plate tectonics and igneous activity

Igneous activity along plate margins• Spreading centers

– Great volumes of volcanic rock produced along oceanic ridges

– Mechanism of spreading– Lithosphere pulls apart and thins– Less pressure results in partial melting in mantle

A fissure eruption is an eruption from a long crack in the lithosphere. The Columbia River flood basalts are a continental example

Other volcanic landforms

Fissure eruptions and lava plateaus• Fluid basaltic lava extruded from crustal

fractures called fissures• e.g., Columbia River Plateau• Flood basalts cover huge areas

Flood Basalt erupted from fissures - Snake River Plain, southernIdaho

The Nature of Volcanic Eruptions

Factors determining the “violence” or explosiveness of a volcanic eruption

• Composition of the magma• Temperature of the magma• Dissolved gases in the magma

These three factors control the viscosity of magma (which controls the nature of an eruption)

The Nature of Volcanic Eruptions

Viscosity is a measure of a material’s resistance to flow (higher viscosity materials flow with great difficulty; water has very low viscosity)Factors affecting viscosity

• Higher temperature - magma is less viscous (or more fluid)

• Composition - higher silica (SiO2) content - magma is more viscous (rhyolite)

• Lower silica content = lower viscosity or more fluid-like behavior (e.g., mafic lava such as basalt)

The Nature of Volcanic Eruptions

Factors affecting viscosity continued• Dissolved Gases

– Gas content affects magma mobility– Gases expand in a magma as it nears the

Earth’s surface due to decreasing pressure– The violence of an eruption is related to how

easily gases escape from magma

The Nature of Volcanic Eruptions

Summary• Fluid basaltic lavas generally produce

quiet eruptions (Hawaiian volcanos gurgle)• Highly viscous lavas (rhyolite or andesite)

produce more explosive eruptions (Yellowstone or St Helens)

Materials extruded from a volcano

Lava Flows• Basaltic lavas are much more fluid

• Types of basaltic flows

– Pahoehoe lava (- twisted or ropey texture)

– Aa lava (rough, jagged blocky texture)

Dissolved Gases• 1-6% of a magma by weight

• Mainly water vapor and carbon dioxide

A Pahoehoe lava flow

A typical aa flow

Materials extruded from a volcano

Pyroclastic materials – “Fire fragments”Types of pyroclastic debris

• Ash and dust - fine, glassy fragments

• Pumice - porous rock from “frothy” lava

• Lapilli - walnut-sized material

• Cinders - pea-sized material

• Particles larger than lapilli– Blocks - hardened or cooled lava

– Bombs - ejected as hot lava

A volcanic bomb

Bomb is approximately 10 cm long

Volcanoes

General Features• Opening at the summit of a volcano

– Crater - steep-walled depression at the summit, generally less than 1 km diameter

– Caldera - a summit depression typically greater than 1 km diameter, produced by collapse following a massive eruption

• Vent – opening connected to the magma chamber via a pipe

Volcanoes

Types of Volcanoes• Shield volcano

– Broad, slightly domed-shaped

– Composed primarily of basaltic lava

– Generally cover large areas

– Produced by mild eruptions of large volumes of lava

– Mauna Loa on Hawaii is a good example

A size comparison of the three types of volcanoes

Olympus MonsOlympus MonsCalderaCaldera

Volcanic Domes

Volcanoes

Types of Volcanoes continued• Cinder cone

– Built from ejected lava (mainly cinder-sized) fragments

– Steep slope angle

– Rather small size

– Frequently occur in groups

Cinder cones are built from ejected lava fragments

Sunset Crater – a cinder cone near Flagstaff, Arizona

Volcanoes

Types of volcanoes continued• Composite cone (Stratovolcano)

– Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)

– Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base)

– Composed of interbedded lava flows and layers of pyroclastic debris

A composite volcano

Mt. St. Helens – a typical composite volcano (prior to eruption)

Mt. St. Helens following the 1980 eruption

Volcanoes

• Composite cones continued– Most violent type of activity (e.g., Mt.

Vesuvius)– Often produce a nueé ardente

– Fiery pyroclastic flow made of hot gases infused with ash and other debris

– Move down the slopes of a volcano at speeds up to 200 km per hour

– May produce a lahar, which is a volcanic mudflow

A nueé ardente on Mt. St. Helens

Town of St Pierre on Montserrat after Nuee Ardente

Other volcanic landforms

Calderas (form by collapse of evacuated magma chamber)

• Steep-walled depressions at the summit• Size generally exceeds 1 km in diameter

Pyroclastic flows (explosive mix of rock, gas and heat)

• Only with felsic & intermediate magma• Consists of ash, pumice, and other fragmental

debris• Material propelled from vent at high speed

Hazards related to volcanoes

LavaPyroclasticsLahararPyroclastics flows-Nuees ArdentesToxic gasesSteam explosionsClimate and atmospheric chemistry

The colder lava becomes, the thicker, more viscous, and slower-flowing it gets. When cooled enough, it may solidify altogether.

Lava

Protecting Inhabitants

One strategy would be to slow the flow—either by spraying it with water so that it congeals and dams itself, or by constructing earthen or other dams in front of it to slow it down long enough to cause it to crystallize, stiffen, and stop.

Lava

Pyroclastics

Pyroclastics

Lahars

Lahars

The mudflow has taken out the main bridges

Pyroclastic flows-Nuees Ardentes

Pyroclastic flows-Nuees Ardentes

Toxic gases

Water vapor and carbon dioxide are nontoxic, yet can nevertheless be dangerous at high concentrationsCarbon monoxides, sulfur gases, and hydrochloric acid are poisonous

How may volcanic eruptions influence global climate?

 Explosive eruptions, particularly, may put large volumes of dust and sulfuric acid aerosols into the atmosphere. These block sunlight and thus contribute to global coolingThe dust can take several months to years to settle

Climate and atmospheric chemistry

Climate and atmospheric chemistry

Climate and atmospheric chemistry

Active volcanoes have a recent history of eruption.

Dormant volcanoes: no historic erupts but not badly erodedThose volcanoes classified as extinct have neither erupted within historic record, nor do they have a very fresh appearance; they are not expected to erupt again.

Classification of volcanoes by activity

Volcanic precursors

What do scientists look for when monitoring a volcano?

seismic activitytilting or upliftmonitoring gas emissions around volcanoes

Present and future volcanic hazards in USA

Present and future volcanic hazards in USA

Present and future volcanic hazards in USA

Present and future volcanic hazards in USA

Present and future volcanic hazards in USA