Geology 100: Planet Earth

Lecture 21: Topic 14 (VOLCANOES)

11/7 and 11/8
TOPIC 13: Earthquakes (concl.)

I) Damage Due to Earthquakes

Vibration: shaking can topple and collapse buildings. This depends on the quality of
construction. Concrete slab buildings can collaps like pancakes. Flexible steel-frame
buildings are stronger.
Fire: gas and water lines broken; cities may burn after earthquake
Landslides: Unstable ground collapses; buildings on slopes are carried away, buildings at the bottom become buried.
Tsunami: Giant sea waves. Can travel across an ocean. May build to heights of 40 m and inundate the coastline.

* So -- think about where you build, if you live in earthquake country!

END OF MATERIAL FOR EXAM II ______

TOPIC 14: VOLCANOES (Reading: Chapter 9)

I) Definition:

- A vent at which lava, pyroclastic debris (ash and fragments of previously solidified rock), and gas erupt.
- Eruption may build a mountain around the vent. (Mountain is also called a volcano).
- Anatomy of a volcano: see a magma chamber at depth, and a vent to the surface.

II) Where do Volcanoes Occur?

- Mid-ocean ridge (but we don't see them)
- Convergent Margins (the majority of major volcanoes; e.g., the Andes, Japan, Aleutians)
- Rifts (e.g., East African rift)
- Hot spots (e.g., Hawaii; Yellowstone Park)

III) Nature of lava:

Characteristics of lava depend on its temperature and viscosity. (viscosity is a measure of the ability of a fluid to flow; high viscosity means sticky and slow, low viscosity means watery and fast).

* The higher the temperature of a lava, the lower its viscosity (i.e., the easier it flows)
* The lower the silica content, the lower the viscosity, because silica polymerizes and the chains get tangled up. So mafic lavas are less viscous than silicic lava.
* The greater the gas content, the lower the viscosity. Frothy lava flows more easily.

- Typically mafic lava (rich in Mg and Fe, low in silica) is also at high temperature (1100°C). Thus mafic lavas tend to flow very easily, so basalt flows in widespread thin sheets.

- But silicic lava is also at low temperature (750°C). So silicic lavas are very sticky and plug up volcanoes, unless they are very gassy.

IV) Types of Eruptions

- The nature of an eruption depends largely on the type of magma (high viscosity vs. low viscosity) that the volcano erupts.

1) Lava dominated eruption:
* Characteristic of low viscosity eruptions. Basaltic composition.
*If lava has low viscosity, it fountains out easily. You will see lava fountains, lakes and rivers, that flow long distances.

2) Explosive eruptions:
* Characteristic of silicic volcanoes. Sticky, very viscous lava clogs up the vent. Also, these lavas contain gas (CO2 and H2O) that come out of solution and make bubbles, like the bubbles in a can of soda. Gas pressure builds up and the volcano explodes.
* Result is a cloud of ash that blankets countryside (air fall ash); or an ash flow (nuée ardent) that rushes down the volcano side at 100 km/h, incinerating everything in its path.
* Ash fall creates a tuff, ash flow creates an ignimbrite (welded tuff).
- St. Pierre in Caribbean was incinerated by an ash flow from Mt. Pelée in 1902. The event killed 40,000 people; only survivor was a prisoner in the dungeon.
- Pompeii was covered by ash from Vesuvius in 79 A.D.; The town was completely preserved by the ash, for archeologists to find in the 19th century. In fact, we can still find the casts of victims.

* Can also get explosions when water gets into magma chamber and turns to steam
- this happened at Krakatoa (1883); the explosion was heard 3000 km away, and the waves it generated killed 40,000 people; great sunsets for a year.

* Can get explosion when part of the mountain slips away and pressure is released.
- this happened at Mt. Ste. Helens in 1980 (as much energy as an A-bomb). Flattened the forest like it was made of toothpicks. Ash mixed with meltwater to create a slurry (lahar) that washed away bridges and houses far away.

V) Types of Volcanoes (classified by shape)

- Shield volcano: characteristic of eruptions emitting low viscosity lava. Hawaii is an example.

- Cinder cone: cone shaped mountain built up of "cinders" (small fragments of lava that cool from drops splattered up and cooled instantly).

- Composite volcano: consists of alternating layers of lava and cinders or ash. Big "classic looking" cone-shaped volcanoes are composite volcanoes.

- Caldera: when a volcano explodes and the magma chamber beneath collapses, may get a large circular depression called a caldera.
* Yellowstone Park is an example of a large caldera (50 km across). Exploded 700,000 years ago, and covered the entire US with a layer of ash. The explosion was 100X larger than Mt. Ste. Helens.

VI) Types of Volcanic Hazards:

Lava Flows: Basaltic (lava-dominated) eruptions may emit flows that can bury highways and towns.
Ash falls and ash flows: Ash falls can bury the countryside. Ash flows can incinerate towns.
Explosive blast: The blast of an exploding volcano can flatten everything within range.
Lahars: These are fast-moving slurries made from a mixture of water and ash (the water may come from melting snow on the volcano during the eruption). Lahars rush down river valleys and wash away or bury everything in their path.

VII) Predicting Eruptions and Decreasing Consequences:

Geologists are able to determine if a volcano is about to erupt, based on several clues:

(1) Change in shape of the volcano (due to the ballooning of the magma chamber as it fills)

(2) Change in heat flow (the ground gets hot as the underlying magma chamber fills).

(3) Increase in local seismicity (as the magma chamber fills, rocks crack and break, causing small earthquakes. Also, there may be underground explosions due to the release of gas from the magma.

- If there is enough warning, the region can be evacuated. People must also get out of river valleys that could fill with lahars.

NOTE: Volcano prediction is much more possible than earthquake prediction. All we can do with earthquakes is estimate recurrence interval (statistically average time between events). We cannot exactly say when or where an earthquake will occur. But we can with a volcano.

Please report any problems to Steve Hurst