Volcanoes

Destructive Seismic Events

Introduction

One of the most fascinating and exciting topics in geology, probably because some volcanoes are so awesomely powerful in eruption and often terribly violent!

What physical and chemical aspects of magmas are important in controlling types of eruptions of volcanoes?

Volcanism

Volcanism is the process by which molten rock reaches the Earth’s surface. Molten rock below the surface is magma; it is lava when it reaches the surface.

Where do Volcanoes Occur?

• Volcanoes occur in three settings:

• Spreading centers

• Hot spots

• Subduction zones

Where do Volcanoes Occur?

• Each of these comes in two flavors:

• Volcanoes on continental crust

• Volcanoes on oceanic crust

Eruptive styles of volcanoes

… and their relation to physical and chemical properties of magmas --

We recognize two styles of eruptive behavior.

Some volcanoes exhibit very complex eruptive phenomena and show both styles of eruption so these are just end-member types of behaviors:

Types of Eruptions

•Violent and explosive•Quiet and flowing

–Depends on trapped gases and magma composition

Non-explosive (or effusive) eruption

• This is when magma flows out of a vent as molten liquid.

• Flows of liquid magma called are lava flows.

• Mafic magmas generally erupt as lava flows with little associated explosive activity.

Non-explosive (or effusive) eruption

• Intermediate magmas frequently erupt as lava flows but may also erupt explosively.

• Felsic magmas erupt as lava flows least frequently--more commonly they erupt explosively.

• Rhyolites that do erupt as lava flows form slow-moving, "pasty" flows that cool very rapidly, forming a jet black glass called obsidian.

Explosive eruption

• When magmas erupt violently they get blown apart into magma droplets.

• Droplets cool or quench quickly to form tiny bits of glass called volcanic ash.

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Explosive eruption

• Sometimes drops may be larger pieces filled with gas (usually water vapor).

• Once cooled, they form pumice.• Pumice has a very low density because of

air pockets. It floats on water and is felsic to intermediate in composition.

Explosive eruption

• Air-pocket-rich fragments of mafic and intermediate composition magma that are larger than ash are called scoria.

• Ash, pumice, and any rock fragments picked up from the volcanic vent are called pyroclasts.

• As ash/pumice/rock fragments exit, they form a column of hot, very gas-rich, pyroclastic material.

Explosive eruption

• Once pyroclasts are high enough in atmosphere, some material rains down out of column to produce pyroclastic falls.

• If the erupting column of pyroclasts is not continuously recharged with more hot magma from below, entire column of ash and pumice can collapse catastrophically to form hot, gas-rich, fast-moving flows called pyroclastic flows.

Explosive eruption

• Pyroclastic flows especially dangerous because of mobility (can climb over ridges) and very fast (100-200 km/hour).

• Pyroclastic flows can also form as volcanic domes of extruded lava break apart or collapse.

Explosive eruption

• Ash and pumice deposits that form from either the material raining down out of the cloud from the atmosphere, or from the hot flows of ash moving across the ground become lithified (turned into a rock) to produce a rock called a volcanic tuff.

What controls eruptive style?

• Controlled primarily by viscosity and the water content of the magma.

• Viscosity -- measure of how resistant a material is to flow when force is applied. If a liquid has low viscosity, it flows easily. If a liquid has high viscosity, it does not flow easily.

• Honey is more/less viscous than water.

Viscosity

• Magmas which have higher viscosities are more likely to erupt explosively because they do not flow easily.

• Viscosity is in turn dependent on:• Composition• Temperature• Dissolved water

Composition

• Composition refers to what the magma is made of.

• As the silica (Si) and aluminum (Al) content increases, the viscosity increases.

• Magmas which are silica-rich therefore tend to resist being deformed (i.e. will not flow easily).

• They have a high/low viscosity when compared to water.

Temperature

• As temperature increases, viscosity decreases.

• Magmas which are erupted at higher temperature are able to flow more easily.

• Water is also more likely to remain dissolved at high temperature.

Dissolved Water in the Magma

• Magmas have the capacity to dissolve small amounts of water.

• Water is more easily dissolved in magma at higher pressures and at high temperatures.

• As long as magma stays deep within the crust and hot then the water will stay dissolved in it.

Dissolved Water in the Magma

• As magma rises pressure decreases and magma will lose heat to surrounding cooler rocks.

• At low enough pressure and temperature water exsolves ("un-dissolves") from magma.

• This can happen in shallow magma bodies or conduits beneath volcanoes.

Dissolved Water in the Magma

• This process of exsolution causes bubbles to form.

• The formation of bubbles does two things:• causes dramatic increase in viscosity of

magma and• water vapor (bubbles) create a large

amount of pressure on walls and roof of shallow magma chamber

Exolved Water in the Magma

• This is a very unstable state for the magma.

• Eventually, the top of the magma chamber can fail, or crack and magma propagates (moves) to the surface, where it fragments and forms explosive eruptions of hot gas and pyroclasts.

• Thus magmas richest in silica and water and lowest in temperature are most likely to erupt explosively.

Types of Magmas

• felsic magmas generally have high silica, high water content and low temperatures in comparison to more mafic magmas

• rhyolite and silica-rich andesite volcanoes often erupt explosively

• whereas basalt more often than not erupts as lava flows (although it can produce small pyroclastic eruptions).