Geology Test 2, Module 5

How do volcanic eruptions trigger lahars?

They are generated directly from a volcanic eruption as massive amounts of water are generated either by the rapid melting of ice and snow, or by the disruption of crater lakes.

How would you use satellite data and your observations to relate volcano type, tectonic setting, volcanic hazards, and magma chemistry? (lab).

Type- Using satellite data, you can use the volcano's dimensions to determine what kind of volcano it is. Steeper ones without a crater are composite. Larger, more gradually tall volcanoes are shield. Conic volcanoes are usually shorter than 300 meters in height and have a crater at the bottom.Tectonic Setting- Tectonic boundaries create volcanoes. Most volcanoes are found at or along volcanic boundaries. Magma chemistry can be understood once you learn the type of volcano.

Describe two major historical geologic events that were the result of volcanic eruptions. (lecture and video)

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How do volcanic eruptions impact other spheres of the Earth system? (video)

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How would you determine the likelihood of an eruption using geologic evidence? (lecture and video)

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What geologic evidence could lead to a volcanic eruption? (lecture)

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Pyroclastic Flows.

A dense, destructive mass of very hot ash, lava fragments, and gases ejected explosively from a volcano and typically flowing downslope at great speed. Speeds are typically greater than 50 mph. Sometimes they are responsible for eroding, melting and mixing with snow and ice, and sending a sudden torrent downstream. As well as damming or blocking streams in volcanic valleys and increasing the rate of stream runoff and erosion.

Ashfall.

A deposit of volcanic ash produced by a volcanic explosion. Has widespread distribution by wind, is a major hazard to aviation, poses significant disruption and damage to buildings, transportation, water, power supply, communications equipment, agriculture, primary production, and when ingested can cause health impacts to humans and animals.

Lahar.

A destructive mudflow on the slopes of a volcano. The initial flow may be relatively small, but it may grow in volume as it entrains and incorporates anything in its path - rocks, soil, vegetation, and even buildings and bridges.

Volcanic Landslides.

A mass movement of rock fragments, soil and debris downslope. They are commonly associated with volcanic activity because the volcanic mountain is weakened by the magma and pressure brewing inside. Common on volcanic cones because they are tall, steep, and weakened by the rise and eruption of molten rock.

Volcanic Gas.

As magma rises towards the surface and pressure decreases, gases are released from the liquid portion of the magma and are eventually released into the atmosphere. Significant amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide, and hydrogen halides can be emitted. Depending on their concentrations, these gases are all potentially hazardous to people, animals, agriculture, and property.

____ is the most widespread and frequent volcanic hazard.

Ashfall

____ are the most common type of volcano. A _____ has a cone shape, but is much smaller than a composite volcano. They rarely reach 300 meters in height but they have steep sides. They grow rapidly. They are composed of small fragments of rock piled on top of one another. The exact composition of a ____ depends on the composition of the lava ejected from the volcano. _____ usually have a crater at the summit.

Cinder Cone(s)

The viscosity of the lava means that eruptions at these volcanoes are often explosive. The viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopes of a _____ volcano. _____ Volcanoes are made of felsic/intermediate rock.

Composite

Climate Change.

During major explosive eruptions huge amounts of volcanic gas, aerosol droplets, and ash are injected into the stratosphere. The most significant climate impacts from volcanic injections into the stratosphere come from sulfate aerosols. The aerosols increase the reflection of radiation from the Sun back into space, cooling the Earth's lower atmosphere or troposphere.

Tephra.

Fragments of rock that are produced when magma or rock is explosively ejected.

____ viscosity magmas, that trap gas, produce explosive eruptions.

High

_____ silica content means higher viscosity.

Higher

What are some examples of everyday materials with high viscosity?.

Honey, Peanut Butter, Syrup, and Milkshakes have high viscosity because it takes a "higher amount of time for them to flow" where you want them to. These materials will trap air, build pressure, and eventually violently explode.

How would you describe the volcanic hazards that were associated with the volcanic eruptions similar to Mt. St. Helens? (lecture).

Lahars are the big ones here. There's also lots of ash and pyroclastic flows.

____ viscosity magmas, that allow gas to escape, produce mild eruptions.

Low

_____ Volcanoes get their name from their shape. Although ____ volcanoes are not steep, they may be very large. ____ volcanoes are common at spreading centers or intraplate hot spots.

Shield

Lava Flows.

Streams of molten rock that pour or ooze from an erupting vent. Everything in the path will be knocked over, surrounded, buried, or ignited by the extremely hot temperature of lava. Deaths caused directly by this hazard are uncommon because most move slowly enough that people can move out the way easily.

What factors influence magma viscosity? (lecture and video).

Temperature. The hotter the lava is, the easier it flows. The cooler it is, the slower it flows. Composition. If a rock is felsic in composition it has more silica and naturally cools slower, making it more viscous. If a rock is mafic, it naturally cools quicker making it less viscous. Gas Content. If magma has lower gas content that means that gas has been allowed to escape, meaning it is less viscous. If magma has higher gas content that means gas hasn't been allowed to escape, making it more viscous.

How would you describe the tectonic setting and the geologic events that preceded the eruption of Mt. St. Helens? (lecture)'.

The fierce 1980 eruption occurred because of the destructive nature of the margin between the Juan de Fuca oceanic plate and the North American continental plate. The Juan de Fuca plate and North American Plate were moving towards each other or converging together. The converging caused the subduction of the oceanic plate. As the subduction happened, eventually the old crust turned into more magma that continued to put pressure on the above plates. To make things worse, water got in through small faults, hit the magma, and immediately turned to steam, adding to the pressure. This pressure was immense and caused the 5.1 magnitude earthquake that triggered the landslide that triggered the volcanic explosion.

What is the difference between mafic, intermediate, and felsic magmas? (lecture and video)

The rate of cooling, the silica content, and the rate at which flows.

Define viscosity.

The state of being thick, sticky, and semifluid in consistency, due to internal friction. The resistance to flow.

What are some examples of everyday materials with low viscosity?

Water, Milk, Juice, and Coffee have a lower viscosity because it takes them a "lower amount of time" to flow where you want them to. These materials allow air to pass through easily.