Volcanic eruptions are among the most spectacular natural hazards on Earth.
A volcanic hazard is any volcanic process that threatens life or destroys land or infrastructure.
In this lesson we will characterize the principle types of volcanic hazards using examples from a variety of composite cone volcanoes and a few shield volcanoes.
As we will learn, people who perish from volcanic eruptions are rarely consumed by glowing floods of lava
Instead they are more likely to be buried by mudflows or engulfed in hot clouds of toxic gas.
The residents of Portland, Seattle and Vancouver, Canada, might be interested in this discussion.
They sit to the west of a chain of active volcanoes known as the cascade range.
More than 2000 flights travel over the Cascade Range each day.
Cascade volcanoes have erupted on average of 1 to 2 times per century during the past 4000 years.
And are guaranteed to erupt again in the not too distant future.
Most active volcanoes around the world are composite volcanoes that are located along convergent plate boundaries.
The Cascades represent examples of these types of volcanoes.
They form on the North American plate as it overrides the diminutive Juan de Fuca plate adjacent to Washington and Oregon.
Most hazards associated with composite volcanoes are characterized by four eruption products.
Tephra that is blasted into the air.
Lahars or mudflows that flow outward from the mountain.
Pyroclastic flows or hot clouds of toxic gases that tumble down the volcanic slopes.
And a limited volume of lava that tends to form localized flows or volcanic domes.
Lava is a more abundant product of other types of volcanic eruptions.
And we'll discuss in the context of eruptions on the Hawaiian Islands.
The term tephra is used to describe the rock fragments and other particles ejected from a volcano.
Tephra is the most far flung product of a volcanic eruption.
This material is blasted into the atmosphere and represents ash or lava bombs
and even larger blocks of rocks rip from the cone of the volcano.
The finest material can be blasted more than 20 kilometers into the atmosphere and carried much farther downwind
than the larger blocks which fall to Earth on the volcano itself.
These images illustrate tephra clouds from recent eruptions from volcanoes in the Aleutian Islands of Alaska.
Tephra can also result in significant economic losses in areas downwind from the volcanic eruption.
These tiny shards of glass destroyed car and jet engines
A recent eruption in Iceland shut down many European flights for weeks.
Tephra is slippery when wet and can block transportation routes or make travel hazardous.
Ancient tephra deposits often form thick layers that can be tied back to their volcanos to analyze the size of past eruptions.
Tephra that falls back to the ground or even tephra deposits from previous eruptions
Can mix with water in streams from heavy rainfall or from melting glacial ice create mudflows.
The volcanic mudflows are termed lahars and can be a dangerous hazard for communities many miles down slope from the volcanoes.
When Mount Saint Helens erupted in 1980 the lahars filled river valleys leaving a mudline
several meters up trees and carrying away bridges and buildings. Remember those dead trees later.
A lahar resembles a river of wet cement and can vary in speed depending on the amount of water and the size of the debris carried in the flow.
The most fluid lahars travel fast making them impossible to outrun.
More than 20,000 people were killed in Columbia when the town of Armero was overrun by a lahar
when an eruption melted snow and ice triggering a massive mudflow.
Many communities west of Mount Rainier are located on top of a giant 5000 year old lahar deposit.
You can see the chaotic mix of debris including large boulders in this section of the lahar that is 50 kilometers downstream from Mount Rainier.
Remember those dead trees from earlier? We can see whole tree trunks and branches preserved in this lahar deposit generated from an eruption at Mount Baker.
A pyroclastic flow may be the nastiest product of volcanic eruptions.
These hot dense clouds of toxic gases mixed with tephra roll down the flanks of a volcano at a great speed.
Anything in their way is incinerated.
The greatest volcanic disaster in the last century occurred on the Caribbean island of Martinique
when a pyroclastic flow overwhelmed the city of Saint-Pierre killing all its residents with the exception of one lone survivor deep in the cells of the local jail.
Pyroclastic flow deposits can be identified on the north side of Mount Saint Helens where the flow lost energy as the slope decreased.
The blocks of light colored pumice were deposited at the toe of a pyroclastic flow.
Elsewhere we can identify pyroclastic flow deposits over 100 meters thick
that were formed after a single massive eruption of a volcano 7000 years ago in what is now Oregon.
The one hazard with the relatively modest impact on most active composite cone volcanoes is lava.
Composite volcanoes have relatively sticky, viscous lava that doesn't flow very far and doesn't get much distance from the crater.
At Mount Saint Helens lava has built up to form a small lava dome inside the crater left behind by the 1980 eruption.
In contrast fluid lava is a primary product of shield volcanoes such as those in Hawaii.
The more fluid and magic lavas flow greater distances from the crater. Consequently they build up wide volcanic shapes.
Lava flows are one of the least deadly of all the volcanic hazards. This is partly because lava flows generally don't move very fast.
Even the more runny lava flows typically only travel a few miles per hour.
Having said that we wouldn't recommend walking up to a lava flow because temperatures can reach as much as 1400 degree Celsius.
That is definitely hot enough to singe your eyebrows off.
Lava flows can kill vegetation and destroy property by burning homes and destroying infrastructure such as bridges and roads.
While lava rarely kills anyone these flows are unstoppable and can flow through populated areas burying everything below several meters of hardened basalt.
So for today we only had one learning objective for this lesson
How confident are you that you could complete this task?