Volcanoes pose eight types of immediate hazard, most of which are shown schematically below:

1. Lava flows
2. Pyroclastic flows (and surges)
3. Ash (tephra)
4. Lahars
5. Gas emissions
6. Landslides and debris avalanches
7. Volcanic earthquakes
8. Tsunamis

The style of eruption and type of hazard produced depend largely on the size of the eruption, the composition of the erupting magma, and the environment in which the eruption occurs. For example, a small, subaerial, basaltic eruption in a remote part of British Columbia (e.g. close to Mount Edziza in northwestern British Columbia) would constitute a minimal hazard compared with a large, explosive, dacitic eruption from Mount Meager, in southwestern British Columbia.

Lava flows commonly accompany volcanic eruptions, especially basaltic eruptions. They are among the least hazardous processes associated with a volcanic eruption, although they can destroy any immobile object in their path, including houses, roads, and volcano observatories! Flows travel slowly, a few kilometres an hour to a fraction of a kilometre an hour, and people and animals can normally move easily out of the way. Fire-fountaining (ejection of lava into the air) is commonly associated with lava flows, but does not usually exceed a few hundred meters in height, so poses a threat only to the immediate vicinity of a volcano. Lava flows, however, can cause many secondary hazards. They can start forest fires and will usually destroy any structures in their path. The expulsion of poisonous gases can also accompany the eruption of a lava flow, as happened in 1783 at the Laki volcano in Iceland. A lava flow can dam rivers, change their courses, kill resident fish, and even act as a barrier to migrating fish. This happened along the Nass River in northern British Columbia during the eruption of the Tseax cone.

Of special concern in western Canada is the possibility of lava erupting onto or beneath glacial ice. Such an event might cause large-scale melting of the ice and generate a lahar or catastrophic flood; such floods are referred to by their Icelandic name, 'jökulhlaups'. In addition, water entering a basaltic vent can produce explosions much larger than those normally associated with a basaltic eruption. Consequently, the presence of water, snow, or ice will increase the risk of significant impact from an eruption.

Pyroclastic flows are dense avalanches of hot gas, hot ash, and blocks (tephra) that cascade down the slopes of a volcano during an eruption (shown above). They are most commonly associated with explosive eruptions, and form when the towering column of ash rising above the volcano collapses. They also form when a less energetic eruption 'boils' over the edge of a crater or caldera or when a lava flow or dome on a steep slope disintegrates. Pyroclastic flows originating from column collapse can have considerable momentum and travel great distances. Speeds between 50 and 150 km/h have been measured and distances of 30 km are not unusual. Although extremely rare, the largest eruptions have produced pyroclastic flows that extend 100 km from the volcano. All pyroclastic flows are extremely destructive, destroying buildings, trees, or any objects that are in their path by impact, burial or fire. People caught in a pyroclastic flow have little chance of survival. Pyroclastic flows may also interact with snow and ice to generate lahars. In Canada, deposits from pyroclastic flows have been found at Mount Meager, Hoodoo Mountain, and Mount Edziza.

Pyroclastic surges are masses of hot turbulent gas and rock fragments similar to pyroclastic flows but less dense, with a much higher ratio of gas to rock. They are more violent and travel much faster than pyroclastic flows; surges have been clocked at over 360 km/h. They are extremely destructive because of their high speeds. People and structures in their path have little hope for survival. White Horse Bluffs, in British Columbia, was built from successive surge deposits, but in general, these deposits are difficult to preserve and what little evidence may exist is easily eroded away.

Tephra is finely comminuted (broken) volcanic rock and accompanies nearly all explosive volcanic eruptions. In very energetic, explosive eruptions, tephra is carried upward into the upper atmosphere and the finest tephra (ash) can be carried by the jet stream for hundreds and thousands of kilometres. Ballistic projectiles (e.g. volcanic blocks and bombs) are the larger blocks of desegregated and comminuted magma that are most commonly thrown short distances from the vent, but may occasionally be thrown several kilometres from the vent. In less energetic eruptions, most of the tephra falls within a few kilometres of the vent and the accompanying ballistic projectiles fall a correspondingly shorter distance.

Because it is often widely dispersed, volcanic ash may be a significant health hazard and create economic problems over a wide area. For example, the town of Yakima, 130 km northeast of Mount St. Helens, received 5 mm of ash after the May 18, 1980 eruption. Ash can pollute water supplies and disrupt transportation; thick accumulations of heavy ash can cause buildings or other structures to collapse. Inhaled ash can aggravate respiratory conditions such as asthma and bronchitis. Coarser particles can lodge in the nose (causing irritation) and the eyes (causing corneal abrasions). Silicosis has been attributed to long-term exposure to volcanic ash. Ash, however, is rarely a direct cause of death unless the fallout is so severe that suffocation results.

Ash can damage mechanical and electrical equipment. It is abrasive and, at great distances from the volcano, fine enough to work its way into bearings or other moving parts of equipment and machinery, causing damage or mechanical failure. Computer equipment is particularly sensitive to this type of damage. Electrical power can be disrupted because transformers conduct heat poorly and will overheat and explode when covered by only a few millimetres of ash.

Ash can affect aircraft flying at high altitudes. The adverse impact of ash on high-performance jet engines was well-publicized for the first time in 1982 when a Boeing 747 jet on a flight over Indonesia encountered ash from Galunggung volcano. Although the plane was severely damaged and all four engines failed, it was able to make a successful emergency landing in Jakarta. Since then, the adverse effects of ash have been studied more carefully. Ash sucked into high-performance jet engines abrades the outer parts of the engine and can melt inside at the high engine operating temperatures. The abrasion severely damages the engine fan blades and the melting ash disrupts the mixing of air and fuel in the combustion chamber, leading to engine shutdown. The ash also abrades the exterior of the aircraft, ‘frosting’ cockpit windows (shown below) and landing lights. Ingested ash can badly damage aircraft navigation and hydraulic systems. The threat from airborne ash in Canadian airways constitutes the most important short-term impact of volcanoes on the Canadian public.

Ash also has deleterious effect on fish because it abrades their gills. Studies of steelhead and salmon in streams west of Mount St. Helens showed that fish populations suffered severely after the 1980 eruptions. There were many other lethal, secondary effects, such as the loss of fish-spawning habitat and riparian cover. Thick accumulations of ash and mudflows can kill the riparian cover (the normal trees and bushes that grow along steams and rivers), which keeps stream waters cool and liveable for fish.

Thick accumulations of ash can significantly affect forestry and agriculture (shown above). Ash can rip the leaves from trees and bury small plants. Ash-covered trees and crops are difficult to work because of the potential damage to harvesting and cultivation equipment and vehicles caused by the ash. Wind and moving equipment also redistribute the ash, prolonging the problems. However, the long-term impact of ash can actually be beneficial, as ash enriches the soil and, as with tephra from the 1980 Mount St. Helens eruption, can act as a mulch, increasing soil moisture and promoting growth.

'Lahar' is an Indonesian word that describes a slurry of water and rock particles produced directly or indirectly by volcanic activity. Lahars typically have the consistency of wet concrete and consist of particles of many different sizes, ranging from flour-sized particles to blocks as large as houses. Although extremely destructive, they are topographically controlled, and usually follow river valleys where they are confined to valley bottoms. They are most common and voluminous as a result of explosive eruptions on snow-clad volcanoes, because pyroclastic flows can instantaneously melt ice and slow, creating large-volume lahars.

The term 'lahar' is often applied to events generated years after a volcanic eruption; these can pose a significant secondary volcanic hazard.

Volcanoes produce large quantities of gases, mostly water (H₂O), but also significant amounts of carbon dioxide (CO₂), hydrogen sulphide (H₂S), sulphur dioxide (SO₂), and hydrofluoric acid (HF) as well as some chlorine (Cl) and nitrogen (N) compounds. Each of these gases has caused loss of life and damage. Sulphur dioxide vented from the Laki volcano in Iceland in 1783 damaged crops and killed livestock and people. Survivors faced starvation and many more died as a result. Sulphur dioxide causes other problems as well. Exposure to this gas can lead to more numerous incidents of acute asthma and bronchitis. When SO₂ reacts with water in the atmosphere, it forms sulphuric acid and acid rain. Acid rain formed from volcanic clouds can damage crops and, in Hawaii, the increased acidity of water collected in cisterns leaches heavy metals into drinking water. The long-term health affects of this problem are not completely known. In Iceland, fluorine killed and disfigured livestock after the 1845 and 1970 Hekla eruptions. When CO₂ gas, which is heavier than air, is released by a volcano, it collects in low areas; concentrations can become high enough to be deadly.

This hazard, known by several names, involves the collapse of part of a volcano. If a large part of the edifice collapses, this is termed a 'sector collapse'.

As magma is pushed into a volcano from the magma chamber below via the magma conduit, the volcano inflates, much like blowing up a balloon, causing slopes to become oversteepened and unstable; this can result in larger, more explosive eruptions than would otherwise be expected. Such was the case in the 18 May 1980 eruption of Mount St. Helens, where catastrophic depressurization of the volcano caused by a massive landslide resulted in an eruption of significantly different character than what had been predicted. Ground shaking associated with stream venting and magma ascent can also trigger a collapse.

Landslides generated during volcanic eruptions at snow and ice-covered volcanoes can be especially dangerous: As a landslide moves downslope, the large blocks break into smaller and smaller pieces, which, when combined with water from melting snow or ice, can transform the landslide into a debris avalanche, carrying it farther than a similar-sized rock avalanche.

Volcanic earthquakes may be small events detectable only by instruments, or larger events associated with explosive eruptions. They may serve as a useful indicator that a volcano is coming to life, and can be used to help predict when a volcano may erupt. Earthquakes large enough for humans to feel may damage property.

Volcanic eruptions along coastlines can generate tsunamis (often inaccurately called “tidal waves”) due to seismic activity or landslides into water. For example, an 1883 eruption of Augustine volcano in Alaska involved a collapse of a portion of the volcano into Cook Inlet, resulting in a localized tsunami that was about 15 m high. Far greater damage, including about 36,000 deaths, was caused by the 1883 eruption of the Indonesian volcano Krakatau, which caused massive tsunamis to strike the coasts of nearby Java and Sumatra. Fortunately for Canada, all our volcanoes are located inland, and none of them pose a tsunami hazard.

Secondary volcanic hazards are those that are not directly associated with an eruption, but rather result from the environment created by the volcano. They include landslides, debris flows, mudflows, groundwater contamination, and surface water contamination. All these hazards may persist at a volcano for decades after an eruption and even long after the volcano is considered extinct.

Stratovolcanoes are particularly susceptible to landslides (rock failures), debris flows, and mudflows. Heat from cooling magma can cause hydrothermal alteration of the rocks at a stratovolcano, turning sections of them into clay. The presence of clay weakens the rocks and increases the risk of slope failures. Large landslides are a hazard at many of Canada's volcanic centres, even those long extinct, because they are located in extremely rugged regions of high relief and are underlain by unstable rocks. Such events have occurred at or near the Canadian stratovolcanoes Mount Meager, Mount Cayley, and Mount Garibaldi. One of these, at Mount Meager in southern British Columbia, killed four people. Rock failures can also temporarily dam local rivers, leading to dam failure and catastrophic flooding. Debris flows and mudflows can occur days, weeks, or years after an eruption. Explosive eruptions and, to a lesser extent, effusive eruptions can denude areas around a volcano and disrupt drainage patterns, leading to long-term flooding problems. In addition, vast areas around a volcano may be covered by loose, unconsolidated tephra, which is easily mobilized and can be washed away by heavy rainfall, forming mudflows and debris flows.

Another secondary hazard at volcanoes is contamination of groundwater and surface water. By virtue of the composition and physical attributes of their component rocks, volcanoes are much more susceptible to weathering than many other types of rocks. Weathering leads to increased particulate matter in streams draining the edifices and higher concentration of elements easily leached by percolating groundwater. Thermal springs are often associated with volcanoes. The water in some of these springs is acidic. Hot, acid waters will enhance the breakdown (or leaching) of the rock and will often carry dissolved metals. Because of this, streams and rivers draining volcanic areas may have a significantly different trace-element chemistry than nearby streams draining other regions underlain by more chemically stable rock types.