Volcanoes (Chapters 6 and 7)

TYPES OF VOLCANOES

Shield volcanoes: Broad, gently sloping volcanoes built with repeated basalt lava flows. Basalt is very fluid and will flow great distances before hardening, thus forming shield volcanoes. Associated with oceanic hot spots (Hawaii), divergent plate boundaries (Iceland, East African Rift) or wherever crust is pulling apart (Colorado Plateau – Basin and Range boundary, i.e. Cedar City).

Stratovolcanoes (composite volcanoes); Large cone-shaped volcanoes associated with subduction zones. These volcanoes generally have steep sides (30° to 40° slopes) and consist of a mixture of pyroclastic material, lava flows and mudflows. Andesite to rhyolite rocks.

Calderas: Large craters formed by the collapse of stratovolcanoes (Crater Lake, Oregon), or the collapse of continental hotspot volcanoes, (Yellowstone National Park).

Calderas form by the collapse of the volcano into the space left in a magma chamber after a very large eruption.

Cinder cones: Small cone-shaped mounds of loose basaltic material. Cinder cones form from fountaining lava. As lava fountains into the air, it cools and falls as solid fragments. Cinder cones, and associated basalt flows, are common in the Cedar City area.

Lava domes: Mounds of thick, rhyolitic lava extruded onto the surface. Rhyolite is a very viscous lava type and does not flow readily. Like toothpaste, it will form a blob when “squeezed” out onto the earth's surface.

PLATE TECTONICS AND VOLCANOES

Volcanoes are associated with divergent and convergent plate boundaries, and hot spots. Each of these environments produces conditions where rocks of the mantle can melt.

How rocks melt (Study diagrams in your notes):

1. Add heat: Increase temperature of rocks, usually via a mantle plume at a hot spot. Examples include the Hawaiian Islands (oceanic hot spot) and Yellowstone continental hot spot).

2. Decrease pressure: Decrease the pressure on rock by removing overlying material. This is accomplished at spreading ridges associated with divergent plate boundaries. This occurs at all mid-ocean ridges, but is exposed only ion Iceland.

3. Add water: Adding water (or any other material) will decrease the melting temperature of rock. This process is important in subduction zones where water is released from the down going plate. Examples include the Andes in South America, the Cascade volcanoes in the U.S., the Aleutians in Alaska, and Japan and other islands in south east Asia.

Different types of volcanic activity, and associated volcanic hazards, are closely related to magma composition and water content. Composition and water content are, in turn, related closely to plate tectonic setting.

Composition magma properties:

TECTONIC ENVIRONMENTS AND VOLCANISM:

Divergent plate boundaries:

1. Most volcanically active regions on Earth. Most occurs under water and is not directly observed by humans. Part of the mid-Atlantic ridge is exposed on Iceland.

2. Characterized by basaltic volcanism. Dominated by lava flows, explosive eruptions are rare.

3. Decompression of mantle material beneath spreading ridges causes partial melting of mantle material, resulting in basalts.

Also occur in continental rifts where divergent boundaries just starting to form (East African Rift) and where continental crust is being pulled apart (i.e. Colorado Plateau – Basin and Range boundary, Cedar City area).

RUN DIVERGENT PLATE BOUNDARY ANIMATION

In Iceland, many eruption occur beneath glaciers and the Icelandic ice cap. Large quantities of water originating from melting of the overlying glacial ice often collects in large subglacial pools. Sudden release of this water causes sudden flooding in an event known as a julkahalpt.

Convergent plate boundaries

Subducting plates at convergent plate boundaries carry water into the upper parts of the mantle. At about 150 Km depths, reactions occurring in the subducting plate release water into the overlying mantle, thus lowering the melting temperature of mantle material. The mantle undergoes partial melting producing basalt. This basaltic magma rises to the base of the continental crust. This magma has a temperature (1200°C-1400°C) much higher than the melting temperature if the crust (650°C-750°C), causing the crust to melt. Melting of continental crust produces magmas with felsic compositions. Mafic basaltic magmas and felsic magmas are thought to mix, thus forming magmas of intermediate composition. Thus, subduction zone volcanoes erupt andesite, and rhyolite lavas, with minor amounts of basalts. Because water is released from the subducting slab at a constant depth (about 120 km), chains of volcanoes tend to form along a line directly above the 120 km zone of melting. These volcanoes tend to be regularly spaced at about 70 km, and all are potentially eruptive at the same time.

RUN ANIMATED VERSION

Rhyolite and andesite are often saturated with water, so explosive eruptions are common. Andesite can also erupt as lava flows, and rhyolite (if dry) can erupt to form lava domes. Add occasional basalt flows to this mix, and we end up with complex composite volcanoes. These are the classic cone-shaped volcanoes we are familiar with. Composite volcanoes consist of a mix of pyroclastic materials and lava flows, having a wide range of compositions (basalt to rhyolite).

Hazards associated with subduction zone volcanoes.

1. Pyroclastic flows and ash falls:

• Ash falls; Fine-grained volcanic ash rains out of an eruption cloud. Ash can be carried great distances by wind currents. Close to the volcano, ash deposits can be tens of feet thick, covering forests, buildings, etc.

• Pyroclastic flows: Mix of fine and course grained material the flows rapidly down the flanks of a volcano. Caused by the collapse of the eruption column during large eruptions, or the collapse of rhyolite domes (smaller eruptions). Pyrocclastic flows can reach speeds of 400+ km/hr, and can be as hot as 500°C.

2. Landslides: Some eruptions are characterized by massive landslides as a parts of a volcano collapses (i.e. collapse of the lateral bulge at Mt. St. Helens).

3. Dome collapse: During the last stages of an eruption cycle, composite volcanoes frequently erupt rhyolite domes. The hot, highly viscous rhyolite extruded from a volcanic vent and forms a steep sided dome. If the dome becomes to high, parts of it may collapse, resulting in the avalanching of hot material, and small pyroclastic flows.

4. Lahars (volcanic mud flows): One of the most dangerous hazards associated with subduction zone volcanoes. Lahars may result in two ways;

• large amounts of water incorporated in a magma can condense in the eruption column, resulting in heavy rains. This water mixes with ash and other material, resulting in mud flows.

• If a volcano is glaciated, an eruption can melt the glaciers. Melt water mixes with pyroclastic material, resulting in mud flows.

• Lahars tend to flow rapidly down drainages and stream valleys, and tend to be confined to valleys.

5. Volcanic gases: Gases such as carbon dioxide (CO₂), sulfur dioxide (SO₂) and hydrogen sulfide (HS) can cause fatalities if in large enough concentrations.

Hotspot Volcanoes

These are volcanoes that form over hot plumes of material rising from the base of the mantle (mantle plumes). Mantle plumes interact with either oceanic lithosphere or continental lithosphere. In either case, the plume heats one part of the lithosphere and a volcano forms above the plume. As plates move, different parts of the lithosphere are carried over the plume, resulting in a chain of volcanoes, of which only one is active. Volcanoes along a hot spot chain become older away from the active volcanic center.

RUN HOT SPOT ANIMATION

1. Oceanic hotspots: Where a mantle plume interacts with oceanic lithosphere

• Only basaltic magmas are produced

• Basalts have low viscosities and tend to have low water contents.

• Lava flows are the primary product of eruptions

• Low viscosity flows build large shield volcanoes.

• Cinder cones develop near vents by fountaining lava and very minor pyroclastic eruptions.

2. Continental hotspots: With continental hotspots, melting of continental crust results in the development of huge felsic magma chambers. Pressure builds up within the chamber until the crust above the chamber breaks. With a sudden release of pressure, the chamber rapidly empties itself in massive eruptions, and the overlying crust collapses into the magma chamber forming large calderas. These are “super volcanoes” and represent the largest eruptions known. No eruptions of this type have occurred during recorded history. These volcanoes produce massive eruption columns, and pyroclastic flows that can extend hundreds of miles. Measurable ash falls can cover the earth, and material suspended in the atmosphere can contribute to planetary cooling.

Assessing volcanic hazards

With volcanoes, hazard assessment includes determining what hazards are most likely for a given volcano, the possible extent of these hazards, and predicting eruptions.

1. Paleovolcanology: Study of past eruptions preserved in the geologic record. Can determine the average time interval between eruptions, the size of past eruptions, and the types of hazards associated with a particular volcano. Mapping and dating of different volcanic deposits can help determine the ricks faced by different areas.

2. Forecasting eruptions: assigning a probability that an eruption will occur in a given interval of time. Usually based on paleovolcanology.

3. Prediction: Assessing when and how large an eruption will be.

• Frequency and depths of earthquakes.

• Migration of epicenters towards surface.

• Occurrence of harmonic tremors.

• Monitoring changes in gas compositions.

• Tilting / bulging of volcano flanks.

• Monitoring surface temperature.