What Are Avalanches?
Types, Causes, and Risks
A complete 7-part backcountry safety course. Part 1 covers the science of avalanches — what they are, why they happen, and the five types every backcountry traveller must understand.
View Full Course →Backcountry travel offers some of the most rewarding mountain experiences — from carving pristine powder to summiting breathtaking peaks. But with these adventures come real risks, and avalanches are among the deadliest hazards you can face. Whether you're skiing, snowboarding, hiking, or mountaineering, understanding avalanche safety is not optional — it's essential.
This 7-part course gives you a comprehensive foundation in avalanche danger. By the end, you'll understand what avalanches are, how to read terrain and snowpack, how to use your safety equipment, and how to make informed, conservative decisions in the field.
"Avalanches are not random — they follow patterns that you can learn to identify, understand, and avoid."
How Fast and Powerful Are Avalanches?
Avalanches occur when the snowpack — a layered accumulation of snow — loses stability, causing one or more layers to slide downhill. They can accelerate to speeds of 60–80 mph (100–130 kph) within seconds, making escape nearly impossible once caught. They carry hundreds of tons of snow, ice, and debris, crushing everything in their path. Even a small avalanche can bury a person under compacted snow so dense that digging out by hand is nearly impossible.
The Five Types of Avalanche
Understanding avalanche types helps you recognise warning signs in the field and make smarter terrain choices.
Most DeadlySlab Avalanches
Slab avalanches occur when a cohesive layer of snow fractures and slides downhill on a weaker layer below. These are by far the most dangerous avalanche type, accounting for nearly 90% of all fatalities.
Causes
- Human triggers — a skier or snowboarder disturbing the snowpack's balance. This is the most common cause.
- Natural triggers — wind-loading or additional snowfall exceeding the snowpack's stability threshold.
Key Characteristics
- Fracture line: A distinct break at the top of the slide, often spanning wide areas of the slope.
- Propagating fractures: The initial fracture can spread across an entire slope, causing massive slides.
- Speed: Can reach over 80 mph in seconds — escape is almost impossible once triggered.
Also Called Point-Release AvalanchesLoose Snow Avalanches
Loose snow avalanches start from a single point — often triggered by a skier, snowboarder, or a natural event like falling ice. They gather unconsolidated snow as they descend, creating a fan-shaped slide.
Causes
- Triggered in light, dry, unconsolidated snow — typically on very steep terrain (40° or more).
Key Characteristics
- Smaller in scale compared to slab avalanches.
- Typically less destructive, but still dangerous — especially if they carry a person into hazardous terrain such as cliffs or gullies.
Spring & Warm-Weather HazardWet Avalanches
Wet avalanches occur when water saturates the snowpack, significantly weakening it. More common during warm periods — springtime, after heavy rain, or on sun-exposed slopes in the afternoon.
Causes
- Warm temperatures melting surface snow.
- Rain-on-snow events adding weight and reducing friction between layers.
Key Characteristics
- Slower-moving than dry avalanches — typically 10–20 mph.
- Extremely heavy due to high water content, causing significant destruction despite slower speed.
Ridge & Summit HazardCornice Collapses
Cornices are large overhanging masses of snow that build along ridgelines due to wind. When they collapse — often suddenly — they can trigger significant avalanches on the slopes below.
Causes
- Strong winds depositing snow unevenly on the leeward side of ridges.
- Sudden warming or additional snow load causing the cornice to break.
Key Characteristics
- Difficult to see from above — particularly dangerous for climbers or skiers travelling along ridgelines.
- Can trigger both slab and loose snow avalanches on slopes below.
Unpredictable & Difficult to ForecastGlide Avalanches
Glide avalanches occur when an entire snowpack slides slowly downhill due to a lubricating layer of water at the ground level. Unique in that they can move gradually for hours or days before releasing suddenly and without obvious warning.
Causes
- Warm weather or rain increasing water content at the snow/ground interface.
- Smooth ground surfaces — grass or rock slabs — reducing friction.
Key Characteristics
- Often occur on smooth, grassy, or rocky slopes.
- Extremely difficult to predict — the snowpack can release suddenly after days of slow movement. If you see glide cracks, do not camp or linger below them.
Why Do Avalanches Happen?
The key to understanding avalanches lies in the delicate balance between snowpack, terrain, and external forces. When that balance is disrupted, a slide becomes possible.
How Dangerous Are Avalanches?
Avalanches are one of the leading causes of fatalities in the backcountry. Understanding what kills is the starting point for protection.
- Burial — Victims can be buried under heavy, compacted snow within seconds. The snow sets almost like concrete, making self-rescue nearly impossible. Survival depends almost entirely on your group rescuing you quickly.
- Trauma — Many fatalities result from collisions with rocks, trees, or other terrain features before burial. Trauma injuries can be severe even in smaller avalanches.
- Hypothermia — Even if a victim survives burial, the cold environment inside the snowpack quickly leads to life-threatening conditions. Time is critical.
The 15-Minute Survival Window
- Within 15 minutes: Survival rates for buried victims are relatively high if companions can locate and uncover the airway quickly — this is why your group is almost always the rescue team.
- After 30–45 minutes: Survival rates drop sharply, primarily due to asphyxiation and hypothermia. External rescue services rarely arrive in time.
- Key implication: Carrying and knowing how to use an avalanche transceiver, probe, and shovel is not optional. You are each other's rescue service.
Key Takeaway — Chapter 1
Understanding avalanche types and their causes is the foundation of backcountry safety. Slab avalanches are responsible for nearly 90% of all fatalities and are most often triggered by human activity. Slopes between 30° and 45° carry the highest risk. By recognising the warning signs and understanding what drives avalanche formation, you can begin making far better terrain choices. The next chapter covers the gear that keeps you and your group alive when things go wrong.
- Part 1: What Are Avalanches? Types, Causes, and Risks YOU ARE HERE
- Part 2: Essential Avalanche Equipment: Your Lifeline in the Backcountry
- Part 3: Recognising Avalanche Terrain: A Critical Skill for Backcountry Safety
- Part 4: Field Observations and Snowpack Testing
- Part 5: How Weather Affects Avalanche Risk
- Part 6: Understanding Avalanche Danger Ratings
- Part 7: Best Practices for Avalanche Safety
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