Volcanic hazards
PHIVOLCS sorts volcanic hazards into two groups: the ones directly associated with an eruption, such as lava flows, ashfall and pyroclastic flows, and the ones indirectly associated with it, such as lahars. It calls pyroclastic density currents the most lethal of all volcanic hazards.
The alert levels, and what to do before, during and after an eruption and its ashfall, are on the volcanic eruption guide.
Written for the Philippines. In an emergency here, the number is 911.
The two groups
Directly associated
- Lava flow
- Tephra fall or ashfall, and ballistic projectiles
- Pyroclastic density currents: pyroclastic flow, pyroclastic surge and base surge
- Lateral blast
- Volcanic gas
Indirectly associated
- Lahar and flooding
- Debris avalanche and landslide
- Volcanic tsunami
- Ground deformation: subsidence and fissuring
- Secondary explosions
- Secondary pyroclastic density currents and ashfall
Source: PHIVOLCS, Introduction to Volcanoes. Read 23 October 2026.
Pyroclastic flows and surges
PHIVOLCS groups these as pyroclastic density currents: mixtures of volcanic fragments, hot gases and ash that rush down the slopes, or outward from a vent, at high speed. A pyroclastic flow is the dense kind that hugs the ground. A pyroclastic surge is more dilute and more mobile. A base surge is a surge rich in water vapor, produced by an explosive eruption where magma meets water.
They form when the base of an eruption column collapses, when a lava dome explodes, or when a lava dome or the edge of a lava flow breaks away and collapses, the case known as nuées ardentes. PHIVOLCS calls them the most lethal of all volcanic hazards and names what they do: incineration, asphyxiation, abrasion, the force of the impact, and burial in hot volcanic material.
A lateral blast is close kin: a sideways thrust of hot gas and ash from an exploding summit dome, or from the flank of the volcano failing suddenly. PHIVOLCS says it flattens everything in its path, with effects like those of a pyroclastic density current.
Source: PHIVOLCS, Introduction to Volcanoes. Read 23 October 2026.
Lahar
Lahar is an Indonesian word. PHIVOLCS defines lahars, sometimes called volcanic mudflows or debris flows, as slurries of volcanic sediment, debris and water that pour down a volcano's slopes through its rivers and channels.
In the tropics, PHIVOLCS says, lahars are mainly set off by torrential rain falling on loose deposits left by a past eruption. So a lahar can come with heavy rain after the eruption that left the deposits is over. PHIVOLCS names three other triggers: a crater lake draining suddenly, a natural or man-made dam collapsing, and a pyroclastic density current running into a river or lake and mixing with the water.
Source: PHIVOLCS, Introduction to Volcanoes. Read 23 October 2026.
PHIVOLCS publishes lahar hazard maps for Mayon, Kanlaon and Bulusan, and a lahar and flood hazard zone map for Pinatubo. The full list is further down this page.
Source: PHIVOLCS, Volcano Hazard Maps. Read 23 October 2026.
Ashfall and ballistic projectiles
Tephra is the fragmented material a volcano throws out, and ash is tephra smaller than 2 mm across. It rises in the eruption plume and settles over the areas downwind as a blanket of ashfall. Ash can also rain out of the clouds above a pyroclastic density current.
Pieces too heavy for the eruption column, which PHIVOLCS calls volcanic bombs, are thrown straight out of the vent as ballistic projectiles and land near it.
Ash builds up in thick layers. PHIVOLCS lists the damage as harm to infrastructure, roofs collapsing, water supplies contaminated and things buried. It is a health hazard, and a danger to aircraft and to machinery such as the equipment used in transport and power generation.
PHIVOLCS's steps for before, during and after an ashfall, including clearing ash off the roof, are on the volcanic eruption guide.
Source: PHIVOLCS, Introduction to Volcanoes. Read 23 October 2026.
Lava flows and volcanic gas
Lava flows are rivers of glowing molten rock moving downslope or away from a vent. How fast depends on the magma. Low-silica lava is runny and can move at kilometers an hour; high-silica lava is thick and moves at kilometers a day. Steep slopes make flows faster and longer than gentle ones do.
Volcanic gas is dissolved in magma and released in large amounts during an eruption. These are the main gases PHIVOLCS names:
- Water vapor
- Hydrogen sulfide
- Sulfur dioxide
- Carbon dioxide
- Carbon monoxide
- Hydrogen chloride
- Hydrogen fluoride
Hydrogen fluoride is toxic and can endanger livestock that eat plants it has contaminated. Carbon dioxide is not toxic, and PHIVOLCS warns that it can still kill when released in large amounts, because it pushes the air out and people and animals suffocate.
Source: PHIVOLCS, Introduction to Volcanoes. Read 23 October 2026.
The other indirect hazards
- Debris avalanche, or sector collapse
- The flank of the volcano fails all at once, from magma pushing in, a strong earthquake or faults moving beneath it. It leaves a horseshoe-shaped scar and a field of small hills below. PHIVOLCS says these are rare and extremely hazardous, reaching places far beyond the usual extent of the more frequent hazards.
- Volcanic tsunami
- Large waves from an underwater volcano exploding, from pyroclastic material pouring into the sea, or from a slope or caldera collapsing into the water.
- Ground subsidence and fissuring
- Rising magma makes the volcano swell and the ground crack. Once the magma has erupted the ground can sink and crack further, usually with earthquakes. PHIVOLCS lists damage to houses and buildings, lost land and rivers changing course.
- Secondary explosions
- Deposits that are still hot, from a pyroclastic density current or a lava flow, can explode when water reaches them through erosion, rising groundwater or rain. They can set off small pyroclastic density currents and some ashfall.
Source: PHIVOLCS, Introduction to Volcanoes. Read 23 October 2026.
The hazard maps PHIVOLCS publishes
These are the maps on PHIVOLCS's Volcano Hazard Maps page, volcano by volcano. It points to its GIS site for more.
| Volcano | Maps published |
|---|---|
| Mayon | Pyroclastic flow, lahar, lava flow and ashfall |
| Taal | Base surge, volcanic tsunami and fissuring, and ballistic projectiles, with lists of the barangays prone to base surge, volcanic tsunami and ballistic projectiles |
| Kanlaon | Lava flow, lahar and pyroclastic flow |
| Bulusan | Pyroclastic density current, lahar and lava flow |
| Pinatubo | The lahar and flood hazard zone, and hazard maps of Zambales and of Pampanga |
Source: PHIVOLCS, Volcano Hazard Maps. Read 23 October 2026.
For a single address, HazardHunterPH assesses lava, lahars, pyroclastic density currents, ballistic projectiles, base surge and seiches for selected active volcanoes, and gives the distance to the volcano and to its danger zone. How to use HazardHunterPH.
Source: PHIVOLCS, Press release: Hazard Hunting Made Easier with HazardHunterPH Mobile. Read 23 October 2026.
When a volcano near you stirs
PHIVOLCS sets an alert level from 0 to 5 for a restive volcano, and each level carries its own recommendations. The guide reads each one in PHIVOLCS's words.
The volcanic eruption guide The 24 active volcanoes Evacuation plan template