Lesson 8: Mountain Hazards

What are avalanches?

The sudden release and movement of vast amounts of snow down a mountainside under the influence of gravity

What did early european records of avalanches look like?

Significant natural hazard in early european alps records

Sizeable problem between the 16th and 18th century With Increased snowfall and glacial advance with the glacial ice age

What were some of the ealiest and most fally recorded avalanches?

Snow avalanches in WW1 in the Australian alps caused the death of around 10,000 soldiers

First major problem in NA in the 1800s goldrush era;

Earliest fatalities in the east side of Canada of 22 people in the winter of 1782 in Nain, Newfoundland and Labrador ; most northern and largest community: torngat mountain

Most fatal: in 1910: 58 workers were killed as they were cleaning a section of railway in Rogers pass

What is snowpack?

When snow falls in the mountains, it accumulates in layers within the snowpack, which is the total amount of snow on the ground. 

Different weather conditions and snowfall events create different types of layers in the snow over the course of a winter season. 

What influences stability of snowpack?

Influenced by how well the different layers of snow adhere to one another and the surface on which they fell. This bond and anchorage of snow layers, called shear strength, resists the downslope force of gravity, which is called shear stress.

What occurs when shear stress outweighs shear strength?

Ab unstable mass of snow breaks loose, creating a snow avalanche.

How do avalanches vary in size and severity?

Avalanches range in size from small sluffs that wouldn’t harm a person to large powerful slides capable of destroying forests or even small villages.

Sometimes considerable quantities of mud, rock or timber are carried within avalanches, increasing their destructive potential.

What are the two ways avalanches are categorized? what is this based on?

1. Loose-snow avalanches

2. Slab avalanches

based on cohesiveness of snow

What is a loose-snow avalanche?

Also called a point release avalanche= little internal cohesion, small amount of loose snow slides down the slope and causes additional snow to go into motion. Grows wider.

More frequent in fresh snow.

Shallow, small and cause little damage.

More damaging in the spring with increased mass.

What is a slab avalanche?

When a slab of snow slides down and then breaks up.

Less frequent but more dangerous.

Requires 4 conditions

Can lie teetering on the verge of release for days or even months. The weak layers beneath the slabs can be extremely sensitive, and any rapid addition of weight or stress can initiate a Failure.

What 4 condition are required for slab avalanches?

1. Slab of snow : dense mass

2. Placed on a weak layer (less cohesive strength_

3. Steep slope: >30 degrees (36-39 on average)

4. Trigger: weak layer beneath the slab; stress can initiate stress that causes an avalanche.

What is the difference between natural and artifical triggers for avalanches?

Natural trigger = usually triggered by new snow, also rainfall, rockfall, earthquake even warming temperatures.

Artificial triggers = majority of disastrous avalanche to people are human caused

What is a distinguishing characteristic of slab avalanches?

the snow breaks away with enough internal cohesion to act as a single unit before breakup.

What marks the area of release on a slab avalanche?

a distinctive upper fracture line, or “crown,” which is perpendicular to the slope and extends down to the sliding surface, or “bed surface.”

What is a common feature of slab avalanches?

Slide paths are a common feature on the landscape, recognizable due to the lack of trees.

What are the 3 areas of the avalanche path?

1. Starting zone: uppermost area where avalanche starts

2. Track: area where the avalanche is covered = treeless

3. Run out zone: where debris from the avalanche ends up

What is the speed of a dry slide avalanche?

50-200 km/hr; if >35km/hr can create a powder cloud which moves above the dense flow of the avalanche

There can be a large air blast which can explode lungs!

This extreme violence grinds up the snow into dense particles

Small grains sinter faster; can sinter 10,000X faster than larger grains

The kinetic energy of the flow heats up the snow and creates water and seizes up the debris

What are wet-snow avalanches?

Much slower speeds without a dust cloud

Still cause severe damage

Climax avalanches in the spring

What are professional avalanche safety committees?

Require special procedures and strategies

Based on precise observations and meticulous research; locating the danger, understanding the hazard and assessing the risk are critical

Avoidance is safest; But we can also defend and attack

What is the rogers pass?

A very important transportation corridor that connects the east and west of canada

134 avalanche paths across 40 km

Up to 14 meters of snow

→ Big hazard

What are 3 ways we mitigate the hazards of avalanches?

1. Avalanche forecasting; weather; snowfall, precipitation ,temperature wind and wind direction; snowpack observations; layering in snow pack, test weak layers, avalanche observation; are they running, how large

2. Static defences; avalanche snowsheds through the pass, key to keeping the highway opening in the winter; with avalanche berms to allow the avalanche debris to run over the snowshed

3. Active control program: they use a gun to fire the peaks; the national defence will shoot and fire the guns to start avalanches in the right area

What is a landslide?

Downslope movement of rock and debris.

They occur when shear stress > shear strength, causing the slope to fail.

How does vegetation act as a barrier for landslides?

Shear strength increases with vegetation; can act as a barrier to prevent downslope movement and as an anchor for soil

Smooth surfaces like rocks = low frictional strength + water = less friction

What are a few ways to decrease slope stability?

Streams that can undercut the slope and make them steeper and prone to failure

Human activities influence the likelihood of landslides; in clearcutting there is less trees to act as a barrier to prevent downslope movement

Mining, road construction and homebuilding can undercut or overload slope

What are landslides classified based on?

Composition

Water content

Movement

What are the 6 kinds of landslides?

Rockfalls

Topples

Translational slides

Rotational slides

Earth flows

Debris flow

What are rock falls and topples?

They are the most basic kinds of landslides

Rocks suddenly detaches from slope

Extreme steepness causes the rock to fail; can be weathering or by faulting of the rock

Triggered by rain or freeze and thaw cycles; expansion and cracking of rocks

Resulting masses may travel for some distance

What is the difference between a rockfall and topple?

Rockfalls = a single rock falls off

Topples = bedrock which falls end to end

What is an example of a very large rockfall?

Turtle Mountain near the Crowsnest Pass

• One of the largest rockfalls in Canadian history; in the spring of 1903, the mtn partially collapsed, with approx. 82 million tons of limestone falling into the valley below, burying most of the town of Frank and killing around 80 people.

• Multiple factors led to the rock slide, but Turtle Mountain's unstable geology was a primary cause)

What are translational and rotational slides?

Landslides that occur when a plane of weakness in the rock or sediment causes an overlying layer of rock to slide

Usually occur in unconsolidated sediment; clay, sand or silt

Triggered by increased moisture, sediment of or earthquake

Steep head scare: steep layer where the failure surface ruptures the ground surface

What is the difference between a rotational and translational slide?

Rotational = concave or convex movement (slump), doesn’t remain intact and rotates backward

Translational = runs parallel to the surface, often remains in tact with original orientation

What are earthflows?

Fluid like movement of fine sediment downslope

Present when unconsolidated sediments overlays a layer which is impermeable to water

When water saturates the sediment it forces the grains apart and allows it to flow down the plain

Not as well defined

Can happen very quickly ; in hours, or even years or can be gradual = earth creeps

What are debris flows?

Similar to earthflows

Fluid like movement down a slope but larger sediments than earth flows

This is very dangerous

Triggered by a large influx of water into the system

Quickly and fast moving; Flow down stream

What are some landslide mitigations?

To protect roads and railways, rockfall shelters or tunnels are often constructed in the areas of highest risk; another option is to place drape nets across vertical rocky cliffs, and to build catchment fences.

Levees are built along streams that are prone to debris flows to prevent them from overflowing onto land.

Diversion structures may also be constructed to deflect landslides and redirect them away from communities and infrastructure.

What are some preventative measures for landslides?

Metal anchors can be inserted into mountain sides to reinforce and stabilize rock masses.

Ditches, culverts and drains are built to facilitate drainage and prevent water from accumulating in high risk areas.

Tree planting is also used to help stabilize slopes

How do volcanoes form?

Can form at convergent plate boundaries at subduction zones , at divergent boundaries and at hot spots

The heat of the mantle melts the subducting plate and turns it into magma which can erupt and produce a volcano

How are volcanoes classified?

Formed by different kind of magma; silica content defines magma types which then determines viscosity

What is basaltic magma?

Upwelling melting magma which flows easily; Low silica, high magnesium and ammonia and dark in colour that cools to form basalt. .

Doesn’t pass through continental crust. It is formed at divergent plate boundaries as it directly fills gaps between plates. Also at hotspots.

What are two examples of volcanoes with basaltic magma?

1. Skaftafell national park in iceland

-> Shield volcanoes; larva is fluid and flows down slope before cooling, which makes broad volcanoes. Mild eruptions. Found worldwide.

2. Tamu Massif in the Pacific ocean; largest known volcano

What is rhyolitic magma?

High silica content, low iron = lighter colour. It cools to form rhyolite.

Formed when ballistic magma flows through the continental plate as it picks up rocks.

Formed in subduction zones and hotspots.

What are examples of rhyolitic volcanoes?

→ Stratovolcanoes = magma doesn’t flow much and piles up = traditional magma. Traps gas and builds pressure = explosive eruptions

EG Krakatoa, Vesuvius (pompeii and herculean), Mt St Helens and Mt Pinatubo

What does viscosity influence in volcanoes?

Determines how well lava flows once its erupted which influences the shape of the volcano

Influences gas, influences explosiveness.

More gas = more explosive

What is an example of an intermediate type of magma in a volcano?

Cinder cone volcanoes: Paricutin ; grew out of a corn field in Mexico in 1943 from a vent and erupted for years until it reached 423 meters

What are the 4 direct hazards of volcanoes?

1. Lava flows

2. Volcanic ash

3. Plumes

4. Pyroclastic flows

T/F Lava flows are fast?

False

Lava flows are very slow; The least hazardous volcanic process – because they’re usually not life-threatening (usually, lava flows at a rate that is slow enough that people can be evacuated before it reaches communities)

Even though the intense heat of volcanoes burns anything in its paths

what is an example of a lava flow?

Kailua in Hawaii

What is volcanic ash?

produced by explosive eruptions; magma is excluded into the air as small shards of glass and rock which is very abrasive

What is a plume?

Volcanic ash that if projected high enough it enters the stratosphere and travels thousands of Km

Produces ash clouds which block solar radiation

Can coat anything and destroy a lot of things

What is the danger of volcanic sludge? an example?

Sludge can collapse roofs and can clog airplane engines

Can also cause health effects and breathing problems

EG Eyhafallenkull: grounded airplanes in Europe for a week

What are pyroclastic flows? an example?

Very dangerous and very fast lava flows that can reach speeds of up to 700 km an hour and reach 100s of degrees

This is very deadly

EG: Mount Pelee in the caribbean buried the town of st pierre killing 30k peopleW

What is a lahar? An example?

When large amounts of water mix with loose volcanic ash and rock which pours into creeks and causes them to overflow; like wet, hot cement that bury and destroy everything in their path. It doesn’t require a large eruption

EG Nevado Del Ruiz in 1985: tiny eruption but caused several tiny lahars destroying people along the rivers. Killed 23K people. Far reaching impacts

What is a tsunami? and an example?

When volcanoes erupt near water is the displacement of large volumes of sea water, which can generate large waves

The 1883 eruption of Krakatau in Indonesia created massive tsunamis that inundated the surrounding coastal areas, killing more than 36,000 people.

How do CO2 and SO2 impact the environment?

Carbon dioxide = greenhouse gas which can heat up the atmosphere as it absorbs heat and locks it into the environment; carbon is sequestered in rock, and is recycled by volcanic activity

Sulfur Dioxide = shorter time scale of impact, creates sulphuric acid which falls to earth as acid rain which is harmful to human and plant health. Can deteriorate structures. Can remain in the atmosphere for several years and reflect heat into space and cool down the atmosphere = volcanic winter

What is an example of a volcano that caused a volcanic winter?

Mt Pinatubo eruption lowered global temperature by 1 degree celsius (creates beautiful sunsets)