Chapter 7

Question 1

What are the consequences of a volcanic eruption

Answer 1

The ash generated from the eruption will affect crops between 100 and 2000km away, depending on the wind.

Question 2

What volcano erupts every day?

Answer 2

Stromboli, a volcano island off the coast of Sicily, erupts every 20 minutes to 1 hour every day.

Question 3

What is magma?

Answer 3

molten rock, and may be a mix of actual hot liquid, gases and scattered mineral crystals

Question 4

Define lava

Answer 4

When magma spills out on surface, it is called lava

Question 5

Define pyroclastic material

Answer 5

When magma is blown out the top of a volcano into the air, it is called pyroclastic material in the form of large blobs (bombs), solid blocks, or pulverized material (ash)

All pyroclastic material is hot when ejected and forms volcanic material when cooled

Question 6

What is plutonic rock

Answer 6

Magma that cools in a volcano

Question 7

Define volcanic rock

Answer 7

Erupted magma that cools

Question 8

What is igneous rock

Answer 8

Rock that cools from magma, whether volcanic or plutonic.

Question 9

What is viscosity

Answer 9

The property of materials that provides resistance to flow. Materials that do not resist flow are fluid, rather than viscous

Question 10

What determines viscosity

Answer 10

1st importance: temperature (higher tempreature = higher fluidity)

2nd most important: chemical composition

Question 11

What strucure was discussed in the text that increases viscosity, and what is its chemical composition? What influences the abundance of this structure in a substance?

Answer 11

Silicon and 4 oxygens come together to from a “silica tetrahedra” structure

Temperature affects the affinity these ions have for each other, and hence determines its abundance.

Question 12

Explain the process of crystal formation in basalt. What is a general order in which these substances form?

Answer 12

As magma cools, crystal structures that form at high temperatures will form first, followed by crystals that form at lower temperatures.

1. Olivine
2. Pyroxene
3. Amphibole
4. Biotite Mica
5. Potassium Rich Feldspar Muscovite Mica
6. Quartz (SiO₂)

Question 13

Give an example of a volatile compound in magma and tell why it is so.

Answer 13

Water is a volatile compound because of its assymetrical structure with the positive hydrogen ends and the negative oxygen side.

Question 14

Why is the volatile compound content in magma significant?

Answer 14

This water breaks apart silica tetreahedral structures, called “dissolved water” when this happens. This makes the magma more fluid.

Question 15

What are the types of volcanic rock?

Answer 15

1. Basalt
2. Andesite
3. Rhyolite

Question 16

What happens when the dissolved water content limit is reached. What is the resulting outcome and product? What does this have to do with temperature?

Answer 16

When there are no ions for the water to attach to, the water will be “exsolved water” and released in the form of bubbles.

Also as temperature decreases, water becomes less able to attach to such structures, and we get more exsolved water.

Question 17

From where does volcanic rock originate? What is the original form called?

Answer 17

The melting of the earth’s asthenosphere.

If this is retrieved with no modifications after melting, we have basalt.

Question 18

What are the textural types of basalt. How are they different

Answer 18

aa - bubbly looking

paloehoe - smooth-surfaced, ropey looking

aa contains less water.

Question 19

What is Andesite?

Answer 19

Andesite is formed by the modification of Basalt after melting. It is more silicon- and volatile-rich than the original.

Question 20

What is Rhyolite?

Answer 20

even more modified than Andesite.

High Si

Even higher volatile content

Question 21

Recite the stats table for the three rocks

Answer 21

Question 22

What causes volanoes? What are the proportions of these causes?

Answer 22

10% hot spots from mantle plumes

90% spreading centres

Question 23

Describe an oceanic spreading centre

Answer 23

At oceanic spreading centres, the lithosphere is a mere 3km thin, making it very close to the asthenosphere. As the spreading centre opens, the pressure decreases, meaning that the temperature increases and the asthenosphere hot plastic melts as a result.

Question 24

What is peroditite, its densities at solid and liquid form, and relative melting points?

Answer 24

Peroditite is an igneous rock that only partially melts from the spreading centre meachanism. Its solid density is 3.3 g/cm3 and its liquid density is 2.9 g/cm3

Question 25

Where is the ring of fire

Answer 25

Around the Pacific Ocean, up the coast of South America, all along the West Coast of North America, across the Aleutian Islands of Asia and all the way down the South Pacific again.

Question 26

How do volcanoes form at subduction zones?

Answer 26

The volcanoes from at first by a partial melt of asthenosphere, where water is added to a relatively dry rock to make it melt easier.

Question 27

What is the water source that promotes partial melts in the asthenosphere?

Answer 27

A the water surrounded slab drops deeper within the crust (and gets hotter), the partial melting occurs further, and magma moves upwards.

Question 28

What happens when the slab melts around granite.

Answer 28

The magma becomes chemically changed. This is significant because the energy change in the reaction can drastically affect the severity of the reaction.

Question 29

Explain assimilation

Answer 29

Magma seeps through microfractures and breaks apart weak fragments. These fragments fall into the magma and melt, changing the chemical composition of the magma

Question 30

Explain fractional crystallization

Answer 30

As the magma moves up, it cools, and crystals that solidify at the current temperature begin to form. If these are denser than the magma, they fall to the bottom, and form a rock consisting entirely of these minerals

Question 31

Explain magma mixing

Answer 31

A mafic magma pool from deeper in the crust may move up towards a more silicic magma higher up. If these meet, they will mix to form an intermediate mix

Question 32

How does magma near the surface differ from magma deeper in the lithosphere/asthenosphere?

Answer 32

Basaltic magma deeper within the earth is in its purest form with few volatile compounds and silica structures The magma that makes it to the surface will be cooler, more silica-rich, more volatile-rich and thus considerably more viscous and explosive than a basaltic magma. The rocks produced from such magma are commonly andesite or rhyolite (see table 1)

Question 33

What type of volcano is more more likely to be hazardous? What are the hazardous volcanic products?

Answer 33

Subduction zone volcanoes are much more hazardous than spreading centre volcanoes, which aren't hazardous at all The volanic products are lava, pyroclastic products and gas.

Question 34

What is lava

Answer 34

Usually, basalt, it is magma that runs down the side of a volcano. usually you don't see andesite or rhyolite.

Question 35

Explain the formation of pyroclastic material

Answer 35

pyroclastic products are fragmented rock and magma ejected from a volcano

Question 36

What is ash

Answer 36

A fine form of pyroclastic material, which comes in the form of hot pulverized rock, is carried long distances by the wind. It buries villages and crops, killing them nuée ardente (glowing cloud) is a term for the cloud of ash that engulfs areas, destroying life.

Question 37

How are magma volatiles released, and give an event where they have been deadly.

Answer 37

They are released in the form of volcanic gas (mainly H₂O, CO₂, and SO₂) It was deadly only at Lake Nyos, Cameroon, West Africa, killing 1700 people from a huge carbon dioxide emission on August 21, 1986 in a 50 m thick cloud. This occurred because there was a magma flow beneath a lake, and because of its shape the lake the bottom layers of the water did not mix with the top, and eventually the CO2 buildup became unstable.

Question 38

What is a Lahar and the Washington and Armero disasters?

Answer 38

Lahars are mudflows consisting of pyroclastic material, water and any other product picked up en-route. They are a viscous sludge when moving but it quickly turns to solid when stopped. 5600 years ago, a Lahar appears as a wall of solidified mud and geological garbage 180 m high at the point where flow ceased. In a town called Armero in Columbia in 1985, 25,000-30,000 people died from one lahar flow. A volcano named Nevado del Ruiz produced lahars that buried: * Spanish colonists in 1595 * The same town rebuilt in 1845 * The same town rebuilt in 1985 This was caused by pyroclstic material melting hot ice and causing a mudslide.

Question 39

What are the four states of a volcano

Answer 39

"active", "repose", "dormant", and "extinct" If a volcano is active it doesn’t mean that it is erupting (just as someone with a cough does not cough all the time). Periods in between eruptions are called repose, which eventually last for so long that the volcano is considered dormant and eventually extinct.

Question 40

Recite volcanic eruption stats

Answer 40

Only ~1 % of volcanic eruptions over the last 100 years have resulted in any fatalities, even though there are about 50 eruptions on land throughout the world every year. About 70,000 people have died as a result of volcanic eruptions since 1900, and nearly 80% of those in two particular days: 8 May 1902 at St. Pierre when 30,000 were overcome by a nuée ardente, and 13 November 1985 at Armero (the lahar event we mentioned above).

Question 41

How do we predict volcanic eruptions?

Answer 41

There are two kinds of volcanic eruption predictions: long-term and short-term. The former are based on historical and geological records and the latter on surveillance. Survellance can be expensive

Question 42

How might surveillance be used to predict eruptions?

Answer 42

The point of surveillance is that the ascent of magma causes a range of measurable repercussions which become more frequent and more intense as the eruption becomes imminent. We will consider seismic activity, temperature changes and ground deformation.

Question 43

What are the criteria for a short term prediction to be effective

Answer 43

must be clear and very precise, time should preferably be expressed in terms of days rather than weeks, danger zones must be identified exactly, together with type of danger.

Question 44

What Mountain was studied to determine precursors of a volcanic eruption

Answer 44

Mt. Helens

Question 45

Which precursors did or didn't change as the time of the eruption drew closer

Answer 45

More earthquakes occurred before the eruption, as well as expansion of the mountain dome. Gas emissions, however do not change closer to eruptions

Question 46

How does the ascent of magma cause seismic activity

Answer 46

As the magma shoots up, there is too much pressure to be contained by the rocks, and they begin to fracture. These initial fractures have small relaeses of energy, which cause short-period earthquakes. As the magma moves up, the hypocentres go from being deep within the crust to shallower depths. As the magma passes the top of the magma, long-period earthquakes occur, indicating an imminent eruption.

Question 47

How does ground tilt indicate an imminent eruption and how do we monitor this

Answer 47

Tiltmeters are used to monitor the tilt difference in the ground to a degree of one parts in a million. GPS satellites are used as well. This is the same system to monitor earthquake activity. Light signals are sent to earth and the time taken for them to return is recorded, and distance is calculated

Question 48

How does gas evolution indicate an eruption

Answer 48

Although Mt. Helens did not produce gas, the composition of the gasses emitted may indicate an eruption. CO2, a lighter gas, tends to lead the magma up and moves faster than SO2 A higher CO2:SO2 ratio indicates an eruption. If the magma gets stuck, the SO2 will have a chance to come up, so this may indicate a lower likelihood of eruption

Question 49

How can we predict eruptions in volcanoes under summit lakes?

Answer 49

Many volcanoes have summit lakes. If magma degassing connects with one of these lakes (as would be likely), the temperature of the water would increase, the pH of the water would decrease (due to the addition of chlorine and sulfur), and the colour of the water will likely change due to the elements dissolved in the rising volatiles. The major problem in using these as signs of an impending eruption is that they sometimes precede the eruption by months if not years.

Question 50

How do we detect thermal anomolies around the world?

Answer 50

There are other satellites available that use infrared for routine weather studies. These satellites are able to pick up thermal anomalies from remote-sensing imagery when magma rises close to the surface.

Question 51

What did satellites in space determine for the central Andes region? What year was this?

Answer 51

These satellites emonstrated in 1989 that in the central Andes there were 60 active volcanoes in the region, rather than the 16 recorded in the Catalogue of Volcanoes.

Question 52

How did scientists predict small eruptions at Mt. Helens after the big 1980 one?

Answer 52

Through earthquakes and earth tides

Question 53

What are earth tides and why do they correlate to volcanic eruptions?

Answer 53

When the magma is close enough to the opening, an earth tide may be the factor that causes it to spill out. Between 1980 and 1987, 14 of the 17 eruptions were predicted in part by earth tides.

Question 54

How many of the high risk volcanoes are monitored

Answer 54

50 of 89

Question 55

What is the most dangerous / difficult to predict type of volcanic eruption

Answer 55

Subduction zone volcano

Question 56

Explain the Pinatubo evacuation and how the predictions were made. What factors were associated?

Answer 56

Over 100,000 people were evacuated in June 1991 * mud flows were restricted to the prediction areas

Question 57

What is the most valuable resource to predicting eruptions in the long-term. What are some limitations of this methoology

Answer 57

The geological record Historical records not as useful: * North america made up for lack of historical records by using geological record * Japan has over 2000 years of historical record, not always the best The limitiations of the geological record are that geological evidence left by eruptions is often minimal, and what little shows up by blasts is often eroded away quickly.

Question 58

What is the VEI and some VEI's of certain eruptions?

Answer 58

The VEI is an open-ended logarithmic scale to represent the eruption explosivity of the eruption Mt. Helen's of 1980 eruption got 4 The Yellowstone mega eruption 600,000 years ago got 8