GEO-113: ENVIRONMENTAL GEOLOGY

VOLCANIC HAZARDS LECTURE NOTES

Volcanoes are the symptom of volcanic activity. Many different types of volcanoes, depending on the material being erupted. Some are more dangerous and destructive than others. They often have very different and unique eruptive behaviors or personalities. Need to study each of them as individuals.

Volcanoes can be very destructive locally, regionally, nationally, and even globally. Again depends on the type and behavior of the individual volcano.

I. Types of Volcanoes: Depends on what is the dominant material being erupted, either lava, pyroclastics (solid pieces), or gas.

Lava: Can be low in viscosity (runny) if basalt or high in viscosity (thick and pasty) if rhyolite.

Pyroclastics: Can be erupted explosively and can spread over huge areas and even around the globe.

Gas: Can be toxic or can be the driving force for explosive eruptions. Can use a Volcanic Explosivity Index (VEI: 0-8) to compare the explosive power of volcanoes. Generally, the more explosive a volcano the less frequently it erupts. Therefore, we know the least about the most dangerous because we haven't experienced them yet.

1) Flood (Plateau) Basalts - don't even look like volcanoes. Lava is very low in viscosity and it flows easily over huge areas. Flow upon flow from long fissures creates a high plateau area. Made almost entirely of basalt lava; little pyroclastics and gas. Examples: Columbia River Flood Basalts, Watchung Basalts of NJ, Deccan Basalts of India.

2) Shield Volcanoes - Broad, massive volcanoes that may be tens of thousands of feet tall and hundreds of miles across. Gently sloping. Made mostly of basalt. Usually erupt fairly quietly. Examples: Hawaii, Iceland, Martian volcanoes

3) Lava (Rhyolite) Domes - Squat plugs of thick pasty rhyolite lava. Typically a few hundred feet tall and a mile or so across. Lava oozes out of the ground like toothpaste. Mono Lakes, CA.

4) Composite or Stratovolcanoes - Classic, cone-like, symmetrical volcanoes made of alternating layers of thick, pasty lava and pyroclastics. Typically thousands of feet tall and tens of miles across. Can erupt explosively because the magma is often gas charged. VERY DANGEROUS. Can explode themselves out of existence. Examples: Cascades (NW U.S.) Andes, Japan (Mt. Fuji).

5) Cinder Cones - Made up entirely of pyroclastics. Often small structures a few hundred feet tall and a mile or so across. Usually flat topped. Often found on the sides of other, larger volcanoes. Example: Craters of Moon, ID.

6) Calderas - A large depression in the ground caused by eruption of large volumes of magma. Roof of chamber collapses causing the ground to subside. Often associated with cataclysmic eruptions. May tens of miles across and thousands of feet deep. Examples" Yellowstone National Park, Crater Lake.

Different volcano types often associated with a particular plate tectonic environment.

Divergent: Shield volcanoes and flood basalts as in Iceland.

Convergent: Composite volcanoes and lava domes as in the Cascades.

Hot Spots: In the oceans get shield volcanoes and flood basalts as in Hawaii. On the continents get large calderas as in Yellowstone.

II. Volcanic Hazards: Many different types, some obvious, some not.

1) Lava - 800 - 1200 degrees C - generally destroys property only - can outwalk most lava flows. Three types:

a) Pahoehoe - smooth ropy surface

b) AA- rough jagged surface

c) Block - large boulders and rubble

Flows move downhill and contained within valleys - easy to predict where they will go. May extend for many miles. May be contained by natural and man-made barriers. Can also be controlled by spraying water. This cools and solidifies the flow as was done at Heimaey, Iceland in 1973. Explosions may also divert flows by punching holes in solidifying crust. At least somewhat predictable

2) Pyroclastics - ash falls and flows - vary in size of particles. May spread for hundreds of or even thousands of miles and be transported around the world by winds of the upper atmosphere. Examples: Pompeii and Krakatoa

Volcanoes can eject many cubic miles of materials

3) Nuee Ardentes - means glowing cloud - hot (600-1000 C). Pyroclastics/air mixture - speeds of 150km/hr. Can spread over many square miles. Examples: Mt Saint Helens - 175 sq. miles destroyed; Mount Pelee, Martinique - 1902 - about 35000 dead.

4) Lahars - mud flows caused by ash mixing with water from glacial melting - may travel many miles (100) from volcano down stream channels. Potentially they are very destructive. Examples: Mt. Rainier, Washington; Mt Pinatubo, Philippines.

5) Gases - H2O, CO2, SO2, HCl, HF - toxic and deadly. Collect in low-lying areas (denser than air). Examples: Vesuvius, 79 A.D; Nyos volcano, Cameroon.

6) Steam Explosions (Phreatic Eruptions). Water comes into contact with magma and flashes into steam - powerful explosions. Example: Krakatoa, 1883 - shot material 80 km into the air - explosion heard 3000 miles away - ash covered 750,000 sq. km, 100 ft tsunamis, 35,000 killed.

Evaluation of Risk

1) Past behavior - eruptive style, sequence.

2) Topography - which areas affected by a particular hazard? Study path of previous lava flows, lahars, and nuee ardentes.

3) Weather patterns- prevailing winds control ash dispersal - Mt. St Helen, towns 100+ miles away covered with ash.

III. Climatic Effects

Volcanic ash blocks sunlight and some of the gases (SO4) reflect sunlight, cooling the climate. Other gases (CO2) warm the atmosphere by absorbing solar energy (the greenhouse effect) and, therefore, warm climate. Sulfuric acid may cause widespread acid rain.

1815 - Tambora, Indonesia; 1816 - year without summer.

1883 - Krakatoa, Indonesia - cooling lasted 10 years.

1991 - Pinatubo, Philippines - dropped global temperatures by about 1-2 degrees.

A large enough, prolonged eruption (1,000-1,000,000 years) may cause large-scale global climate change. Perhaps even cause glacial episodes and effect extinctions or human history.

IV. Predicting Volcanic Eruptions

Many different methods used. Often much more successful and predicting volcanic eruptions than earthquakes. However, each volcano is different and can't always apply predictive techniques that work at one volcano to another.

1) Cyclicity: global average of about 220 years between eruptions. Some erupt almost continually. Others erupt about once every 10,000-1,000,000 years. Have to know the cycle for each volcano.

2) Eruption Sequence: Mauna Loa and Kilauea show a distinct sequence of events prior to and after the eruption (typically shallowing earthquakes, inflation, eruption, deflation). The eruptions themselves also show a distinct sequence of summit eruptions followed by flank eruptions.

3) Precursory Events: Can measure changes in a number of physical properties associated with the volcano.

a) Thermal activity - hot springs, steam vents, geysers, etc. Increasing activity before eruptions. Increased heat flow measured by planes or satellites.

b) Change in gas composition - increase in HCl, SO2, and other gasses before eruptions.

c) Changes in magmatic field and electrical conduction

d) Increased seismicity - depth becomes shallower as magma rises toward surface. Distinctive harmonic tremors due to magma movement in conduits.

e) Bulging, tilting, and swelling -volcano inflates as magma rises into it.

Mt. St. Helens bulged 100 meters towards the north.

4) Animal Behavior: Very uncertain. Always anecdotal.

Much uncertainty, but can now predict some eruptions with great accuracy, though not always exactly. Best response to prediction is evacuation (avoidance principal - problem of economic disruption). Eruption may or may not occur when predicted- may go on for months or even years.

VI. U.S. Hazards

1) Hawaii: Hot spot island chain gets older to northwest. Shows plate motions of 10cm/yr for last 80 m.y. All made of basalt.

Presently, the active island is the big island of Hawaii - made up of 5 separate shield volcanoes

Mauna Loa: 13, 675 feet altitude. Base starts 18,000π below sea level. World's largest active volcano.

Kilauea: Active 80% of the time. Has been almost continuously active since 1983. Sits on eastern flank of Mauna Loa. 50 miles long and 15 miles wide. Two separate rift zones. Summit caldera periodically filled with a lava lake.

Loihi: Submarine (2,000 feet deep) volcano SE of Hawaii. Probably the next island. May emerge in 10,000 -100,000 years.

Most serious problem is destruction of property - Royal Gardens and town of Kalapana - no loss of life. $30 million in damages.

Have tried controlling lava flows by building barriers and explosive bombing. In 1935, stopped flow moving towards Hilo.

2) Cascades

Line of volcanoes stretching from British Columbia to California. About 15 major volcanoes that have all been active in the past 10,000 years. Volcanoes spaced 150-200 km apart. Source is melting caused by subduction of Juan De Fuca Plate.

Very young features - chain is 2 million years old. Some volcanoes are no more than 100,000 years old. Replace older ones that have blown themselves apart. Almost all are composite (stratovolcano) type and very explosive.

Mt. Rainer - tallest - 14,500. Last 10,000 years about 100 separate eruptions. Presents a real danger to Seattle and Tacoma, largely from lahars.

Mt. St. Helens - most active. Twenty eruptions in past 4000 years. Averages one about 150-200 years. Blew itself apart in 1980.

Others recently active Mt. Lassen, Mt. Rainer, Mt. Baker, and Mt. Shasta. About 20 million people affected by the entire range.

Mt. Mazama - Crater Lake. 6000 years ago top of volcano blew and collapsed to form a caldera now filled with water. The original volcano was 12,000 ft tall and now is 6,000 ft tall. The volume of material ejected was 50-70 cubic km. 13,000 sq. km (roughly size of NJ) was covered by 6 inches or more of ash.

3) Yellowstone National Park and Long Valley Caldera

Studies suggest YNP activity will reoccur in next 500,000 years. Cataclysmic eruptions have occurred at 2.0 m.y., 1.2 m.y., and 600,000 years. We're due! There is the potential for an eruption of 1000+ cubic km. This would bury a good portion of the western U.S. Long Valley Caldera presents a similar risk. Has been seismically and thermally active. Both areas shown signs of uplift - swelling with magma.