Hazards living near a stratovolcano: M. Owens and M. Bridgeman

From GeoClasses

Contents 1 MT ST HELENS: HAZARDS LIVING NEAR A STRATOVOLCANO 1.1 INTRODUCTION: WHAT TYPE OF VOLCANO IS MT. ST. HELENS? 1.2 HAZARD THE FIRST: TEPHRA FALL 1.3 HAZARD THE SECOND: PYROCLASTIC FLOWS AND SURGES 1.4 HAZARD THE THIRD: LAHARS 1.4.1 How does the lahar-detection system work? 1.5 CONCLUSION: WORST CASE SCENARIO GUIDE FOR YOUR SURVIVAL OF AN ERUPTION

MT ST HELENS: HAZARDS LIVING NEAR A STRATOVOLCANO

Created by: Mollie Owens and Marci Bridgeman

INTRODUCTION: WHAT TYPE OF VOLCANO IS MT. ST. HELENS?

Mt. St. Helens is a stratovolcano. Sometimes called cone volcanoes, they are usually explosive due to their silica content, and they generally form from alternating layers of lava, ash, and tephra. Stratovolcanoes tend to be conical with steep slopes. The hazards of living near one of these volcanoes are substantial. Tephra Fall, Pyroclastic Flows and Surges, and Lahars make living near this volcano particularly dangerous. 1

HAZARD THE FIRST: TEPHRA FALL

The USGS describes tephra as:

“Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Such fragments range in size from less than 2 mm (ash) to more than 1 m in diameter…Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano.” 2

Thus, tephra ash is especially dangerous if inhaled and can ruin cars, buildings, agricultural products. The impact of falling tephra can be fatal. Because of ash’s fine size and abrasive nature, it can easily burry structures; thus, halting business and impeding the local economy. 3

For important information on what to expect during an ash fall,

visit this website. Here, you will find useful information on preparation for before, during, and after an ash fall.

HAZARD THE SECOND: PYROCLASTIC FLOWS AND SURGES

“An individual volcanic particle ejected during an eruption is called a pyroclast. Examples of pyroclasts include individual fragments of ash, lapilli, and blocks.” 5

Pyroclastic flows are floods of extremely hot (570°-1470°F) volcanic rocks and gases that can move up to 200 miles per hour. In the unlikely event that you are in the way of such a flow, let it be known that you will not survive. Pyroclastic flows are lethal; they destroy everything in their path, and unless you have a very fast car (albeit, one that can withstand considerable ash intake), you will not survive a pyroclastic flow. The best way to prepare for one is to evacuate the surrounding area. The 1980 eruption of Mt. St. Helens witnessed pyroclastic flows that traveled nearly five miles from the vent; obviously, one would not want to build a home or business in the near vicinity of this mountain. 3

Pyroclastic surges, like pyroclastic flows, are not escapable; however, they are less dense and are, therefore, more mobile. Because these surges are not confined to low areas, they have the potential to travel farther than flows. To survive pyroclastic flows, one would want to evacuate prior to eruption, as they are not escapable. 3

remnant of building destroyed by pyroclastic flows from El Chichon volcano, Mexico

buried river valley after pyroclastic eruption of Mt Pinatubo

pyroclastic flows from Redoubt Volcano, Alaska rapidly melted snow and ice, forming lahars

burned vegetation in valley from fluidized pyroclastic flow from lava dome on Montserrat

HAZARD THE THIRD: LAHARS

Lahars are mixtures of rock fragments and water that move down the slope of a volcano; they are triggered by rainfall, melting snow, and landslides. Not unlike wet concrete, lahars are highly mobile and are capable of carrying very large rocks (up to 10 meters). Because of the extreme temperatures, lahars can melt snow and ice, and, therefore, trigger floods in the surrounding areas. They will destroy nearly everything in their path and can cause severe economic and environmental damage. 6

Because preparation for a lahar is impossible, it is wise not to build any houses, businesses, or crops in the near vicinity of a stratovolcano. In an attempt to minimize the fatalities of lahars, geologists are working on a lahar detection system.7 With the use of this system, warnings will be more accurate; thus, allowing time for evacuation to minimize damages.

How does the lahar-detection system work?

CONCLUSION: WORST CASE SCENARIO GUIDE FOR YOUR SURVIVAL OF AN ERUPTION

In the event that you are near a stratovolcano when it erupts, here are a few tips and tricks that just might save your life:

1. Do not try to touch the magma; it will burn your hand.
2. Regardless of how talented a photographer you are, do not attempt any close up pictures of the eruption.
3. Unless you are a volcanologist, do not attempt to ‘wait and see’ what will happen before, during, or after the eruption. You will probably die from a combination of the aforementioned hazards.
4. Do not attempt to outrun the pyroclastic flows or lahars. You will not win; however, there is a good chance that you will die a slow painful death due to extreme burning of the flesh, lungs, etc.
5. MOST IMPORTANTLY: sit back, have a drink, and remain calm. It will probably be over in the morning.