• A vent, hill or mountain from which molten or hot rocks with gaseous material have been ejected
• Also craters, depressions, hills or mountains formed by removal of pre-existing material or by accumulation of ejected materials

The Philippines sits on a unique tectonic setting ideal to volcanism and earthquake activity. It is situated at the boundaries of two tectonic plates – the Philippine Sea Plate and the Eurasian plate –  both of which subduct or dive beneath the archipelago along the deep trenches along its east and west seaboard.

• Lava flow
• Tephra fall or ashfall and ballistic projectiles
• Pyroclastic density currents or PDCs (pyroclastic flow, pyroclastic surge, base surge)
• Lateral blast
• Volcanic gas

• Lahar, flooding
• Debris avalanche, landslide
• Volcanic tsunami
• Ground deformation (subsidence, fissuring)
• Secondary explosion
• Secondary PDCs and ashfall

Lava flows are rivers of incandescent of molten rock or lava moving downslope or away from an eruption vent. Lava flows composed of low silica magma have low viscosities and tend to flow at high speeds (kilometers per hour), while those composed of high silica magma have high viscosities and tend to move slowly (kilometers per day).  Steep slopes encourage faster and longer flows than gentle slopes or terrain.

Tephra (fragmented volcanic particles) or ash (fragmented volcanic particles less than 2 mm in diameter) propelled through the atmosphere in an eruption plume or an eruption column eventually fall or gravitationally settle over areas downwind of an erupting volcano, forming blankets of tephra fall or ashfall. Ashfall can also be rained out from the ash clouds of PDCs. Large particle tephra called volcanic bombs that are too heavy to transport in eruption columns are ejected straight out of the volcanic vent as ballistic projectiles that fall near the vent. Tephra or ashfall can accumulate as thick blankets of material, causing infrastructural damages, roof collapse, contamination of water resources and burial. Ashfall is a health hazard and a danger to aircraft and other industrial machinery, such as those for transportation and power generation.

PDCs are mixtures of fragmented volcanic particles (pyroclastics), hot gases and ash that rush down the volcanic slopes or rapidly outward from a source vent at high speeds. PDCs range from pyroclastic flows to pyroclastic surges depending mainly on particle concentrations, pyroclastic flows being denser, and therefore ground-hugging currents and pyroclastic surges being more dilute, more mobile currents. PDCs can be generated by the gravitational collapse of the base of eruption columns, or explosion of a lava dome, or by spalling or gravitational collapse of a lava dome or of lava flow margins (called nuée-ardéntes).  A special class of PDC called base surges, mobile and water-vapor-rich pyroclastic surges, are generated by explosive phreatomagmatic eruptions. PDCs are the most lethal of all volcanic hazards and can cause incineration, asphyxiation, abrasion, dynamic pressure impact and burial in hot volcanic material.

Lateral blasts are laterally-directed thrusts of hot gas and ash that can be generated from an exploding dome on the summit vent or inside the edifice when sudden mass failure of the volcanic flanks occur. Lateral blasts travel at subsonic speeds at the onset, flattening everything in their paths and causing impacts similar to those of PDCs.

Volcanic gases form a dissolved component of magma that is released to the atmosphere in large quantities during eruptions.  The principal volcanic gases are water vapor, hydrogen sulfide, sulfur dioxide, carbon dioxide, carbon monoxide, hydrogen chloride and hydrogen fluoride. Minor amounts of nitrogen, methane, argon and helium can also be degassed from magma. Volcanic gases form aerosols that can both cool down the earth’s atmosphere and deplete its ozone concentration for a period of time. Some volcanic gases such as hydrogen fluoride are toxic and can endanger livestock that ingest contaminated vegetation.  Non-toxic species such as carbon dioxide can also be lethal when released in large quantities, displacing air and causing asphyxiation in human and animal populations.

Lahars (an Indonesian term), sometimes called volcanic mudflows or debris flows, are slurries of volcanic sediment, debris and water that cascade down a volcano’s slopes through rivers and channels. Lahars in tropical areas are mainly generated by torrential rainfall on unconsolidated deposits from a past eruption. These can also be triggered by the sudden draining of a crater lake or a collapsed natural or man-made dam or the movement of a PDC into a river or lake and eventual mixing with water.

Debris avalanche or sector collapse is the mass failure of the flanks of a volcano edifice due to magma intrusion, a strong earthquake or the movements of faults beneath the edifice. Debris avalanche events form a horseshoe-shaped scar or amphitheater, from which the collapse mass has detached from the edifice to form a field of hummocks or small hills downslope of the amphitheater. Debris avalanches are rare phenomena but are extremely hazardous when they do occur, endangering localities far beyond the usual extents of more frequent volcanic hazards.

Volcanic tsunami occur in caldera lakes when water is displaced by deformation of the lake floor caused by rising magma or the entry of PDCs or landslides into the lake, or in seas when water is displaced by PDCs or debris avalanches from volcanoes. Such tsunamis are unlike those generated by large magnitude offshore earthquakes, which are long-period waves generated by fault displacement or deformation of the seafloor.

Ascending magma can cause the volcano edifice to swell before and during an eruption, causing the ground to break up into fissures, typically along weaknesses in the rock such as fractures or faults. After magma has been erupted, its removal from the subsurface can cause the ground to sink and subside and further fissuring to occur. Ground subsidence and fissuring are typically accompanied by earthquakes, and altogether can cause infrastructural and house or building damages, loss or degradation of land surface and re-routing of waterways and rivers.

Secondary explosions can be generated in still hot volcanic deposits such as those of PDCs and lava flow when these come into contact with water by erosion, rising groundwater or rainfall. These can cause remobilization of volcanic material to generate small-scale PDCs and minor ashfall.