Big Bend National ParkСтраница 36 из 50
Lahars: volcanic mudflows, and the fossil forests they buried
Lahars, or volcanic mudflows, are among the most destructive volcanic phenomena. They are mixtures of water, volcanic ash, tephra, rock fragments and chunks of ice that can flow like wet concrete. The word comes from Indonesian. Lahars can strike with little or no warning, mostly run down river valleys, and can reach 45–50 miles per hour (75–80 km per hour) or more on steep slopes. Some are relatively dilute and others a thick slurry that can carry large boulders. Because they can travel far down valleys, they can reach population centers away from the volcano.
Lahars fully filled the Drift River Valley during the April 2009 eruption of Redoubt Volcano in Lake Clark National Park. USGS photo.
Types
| Type | Origin |
|---|---|
| Hot (primary) | Formed during eruptions, with elevated temperature: ash, pyroclastic surges and flows, or lava melting snow and ice; wet volcanic landslides liquefying; eruptions into crater lakes; heavy rain during eruptions. Snow- and ice-covered volcanoes are especially prone |
| Cold (secondary) | Not tied to eruptions, no elevated temperature; usually started by heavy rain or snowmelt that mobilizes loose volcanic deposits, or by landslides, flooding or glacial outburst floods. Often smaller than hot lahars and may occur long after eruptions |
| Debris flows | Not from eruptions; in the Cascades the term is reserved for small events from slope failure, rain, snow or ice melt or glacial outburst floods. They happen more often than large lahars |
Examples in national parks
Mount Rainier. Lahars are the greatest geohazard from Mount Rainier and the most significant from any Cascade volcano. Several large prehistoric lahars ran all the way to the Tahoma area near Puget Sound. Ice, abundant surface water and hydrothermally altered rock, which weakens slopes by producing clay minerals, make the mountain especially susceptible. The Osceola Mudflow, the largest in the last 10,000 years, began when a weakened part of the northeast summit collapsed during an eruption 5,600 years ago; it ran down the White River to Puget Sound, and its deposits cover about 212 square miles (550 square kilometers).
The Osceola Mudflow about 31 miles (50 km) downstream of Mount Rainier. USGS photo.
Lassen Peak. The May 19, 1915 eruption shattered the growing lava dome, sending hot blocks onto snow-covered slopes. The resulting avalanche of rock and snow turned into a lahar as the snow melted and flowed 7 miles (11 km) down Lost Creek. A larger one on May 22, triggered by snowmelt from a pyroclastic flow, followed the same path and went farther. Both flooded downstream.
Hazard map for Lassen Volcanic National Park showing lahar hazard zones. USGS image.
Redoubt Volcano. Eruptions in 1989–1990 and in 2009 both produced lahars. In 1990, dome collapse in the crater produced meltwater, and pyroclastic flows melted glacial ice; the largest hot lahar flooded the Drift River valley to the coast. The 2009 hot lahars left deposits wall to wall on the valley floor up to 16 ft (5 m) deep, carrying blocks of snow and ice as well as rock.
Petrified wood in lahar deposits
Lahars can bury trees and logs. Abundant ash and warm groundwater help silica replace the wood. Yellowstone and Florissant Fossil Beds contain fossil wood in lahar deposits.
Fossil forest on Specimen Ridge in Yellowstone National Park. NPS photo by Neal Herbert.
Specimen Ridge, east of Tower Junction in Yellowstone, holds the world’s largest concentration of petrified wood, in lahar deposits from the 50-million-year-old (Eocene) Absaroka Volcanic Field. The trees include spruce, ash and oak, with sequoias among the most common; some stand where they grew, others were carried and deposited upright.
A large redwood stump in Florissant Fossil Beds National Monument.
The fossil stumps at Florissant Fossil Beds are about 34 million years old (Eocene) and were also preserved by lahar deposits. Thirty-one have been documented, most vertical. Big Stump is 12 feet (3.7 m) tall with a diameter of nearly 11 feet (3.3 m); the redwood is estimated to have been about 750 years old and 230 feet (70 m) tall when buried.
Sources
- National Park Service, “Volcanic Processes—Lahars” (https://www.nps.gov/articles/000/volcanic-processes-lahars.htm); rewritten in hubnx's own words.