Avalanches
Avalanches are masses of snow, ice, and rocks that fall rapidly down a mountainside.
Falling masses of snow and ice, avalanches pose a threat to anyone on snowy mountainsides. Beautiful to witness from afar, they can be deadly because of their intensity and seeming unpredictability.
Humans trigger 90 percent of avalanche disasters, with as many as 40 deaths in North America each year. Most are climbers, skiers, and snowmobilers. Learning about avalanches, and the conditions that cause them, can help people recreate more safely in the back country.
Types of avalanches
The recipe for an avalanche may seem simple: a mountain slope and a thick layer of snow. It is actually a trifecta that causes avalanches: terrain, snow-pack and weather conditions. Avalanches do occur naturally, but when you add humans into the mix, they can be deadly.
Avalanches come in many shapes and sizes. Many are small slides of powdery snow that move as a formless mass downslope. Outdoor recreationists often trigger these small “sluffs,” as well as more medium-sized avalanches.
Disastrous avalanches occur when massive slabs of snow break loose from a mountainside. The mass of snow shatters like broken glass as it races downhill. These hazards can travel as fast as cars on a freeway, up to 160 kilometres per hour.
Snow slides can start on mountain slopes with at least a 30-degree incline, but they occur most frequently on slopes of 35-50 degrees. Although avalanches need a slope to start, large avalanches accelerate downhill with sufficient momentum to cross flat terrain for short distances.
In some cases, a large chunk of ice or small glacier breaks off a mountainside, gathering snow and momentum and rushing down in a dangerous steamroll. One of the most deadly avalanches on Mount Everest, which killed 16 Sherpa's, occurred when an ice mass the weight of 657 buses plummeted down on top of hikers.
Features of avalanches
Avalanches contain three main features: the starting zone, the avalanche track, and the runout zone. Avalanches launch from the starting zone. That’s often the most unstable part of the stope, and generally higher on the mountain.
Once the avalanche starts to slide, it continues down the avalanche track, the natural path it follows downhill. After avalanches, large clearings or missing chutes of trees provide clues to an avalanche’s trajectory.
The avalanche finally comes to a stop at the bottom of a slope, in the runout zone, where the snow and debris pile up.
What conditions cause an avalanche?
Avalanches are most common during the winter, December to April in the Northern Hemisphere, but they do occur year-round.
To get an avalanche, you need a surface bed of snow, a weaker layer that can collapse, and an overlaying snow slab. The highest risk period is during and immediately after a snow storm.
Underlying snowpack, overloaded by a quick deluge of snow, can cause a weak layer beneath the slab to fracture naturally.
Human-triggered avalanches start when somebody walks or rides over a slab with an underlying weak layer. The weak layer collapses, causing the overlaying mass of snow to fracture and start to slide. Earthquakes can also trigger strong avalanches.
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Landslides
What is a landslide?
A landslide is defined as the movement of a mass of rock, debris, or earth down a slope due to gravity. The materials may move by falling, toppling, sliding, spreading, or flowing.
What causes a landslide?
Almost every landslide has multiple causes. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope. Landslides can be triggered by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors.
What are submarine landslides?
Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas.
What causes a landslide?
Landslides: Thredbo
Sinkholes explained: How are they caused and what are the warning signs?
What causes them?
The cause of a sinkhole is fairly straightforward. It's the stuff immediately below the surface shifting to somewhere else.
A sinkhole can range anywhere between a slight depression in the ground right up to an enormous hole reaching down half a kilometre. The world's deepest sinkhole is in Chongquig, China, reaching down 662 metres.
They have no natural surface drainage, so any water that gets into a sinkhole can't get out via the surface — and so usually drains downward, into the subsurface layers.
What causes the shift?
There are three main situations that cause sinkholes:
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If the rocks below the surface are made of materials that dissolve in water, such as limestone or gypsum. Over hundreds or thousands of years, the water trapped beneath the surface causes natural underground currents that dissolve the rocks and forge chasms under the ground.
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The top of the chasm gradually grows towards the surface and, eventually, the surface layer gets thinner and thinner until it becomes too weak to support what is above it and falls in.
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When the rock under the surface doesn't dissolve in water, but instead is made of grains that are small enough to be carried away by underground water currents. In Guatemala City, where a three-story building vanished into the sinkhole, the underlying rock was predominantly weak crumbly volcanic rock, fine ash and other debris that had erupted from a volcano.
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The third situation has nothing to do with underground water currents, it's all about water movements above-ground.They are triggered by intense rainstorms or floods, and burst or long-term leakage from sewer or storm-water pipes.
In all these cases, the water can't go where it used to, so it finds a new path.
Where do sinkholes form?
Sinkholes form mainly in limestone.
Limestone areas in a tropical, moist climate are much more likely to form sinkholes than those in dry climates. So because of the dry conditions in Australia, we don't often see sinkholes forming here.
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