geo hazards

• Natural hazards are unavoidable aspects of our life. The Philippine Archipelago is particularly prone to earthquakes, volcanic hazards, typhoons and storm surges, tsunami, mass movement, and floods.
• Humans can, by their own actions, either aggravate or mitigate the destructive effects of these natural phenomena.
• Identifying geological hazards is very important and is a prerequisite to the proper management of disasters.
• There should be adequate information and technologies available in minimizing the effects of natural hazards and to some extent, in proper management and prevention of disasters.
• Zoning restrictions and improved structures must be in place. Proper prediction and monitoring should always continue. Warning and excavation systems should be operational.
• Disasters have a tremendous impact on development both among humans and societies.
• Pyroclastic flows (Nuée ardente)


  Fast-moving volcanic materials generally at gas-aided speeds reaching to more than 100 kph destroying natural barriers of about 10 meters high, composed of gas and partially molten volcanic fragments that can burn everything in their path
• Lava flows of Mt. Pinatubo


  Highly viscous (e.g. with silica content of about 64%) suggesting that the lava flows very slowly and can only travel relatively shorter distances, differentiated from those of the Hawaiian Volcanoes where the lava moves rapidly and can travel tens of kilometers away
• Noxious gases


  A variety of poisonous gases and volatiles (e.g. CO, H2S) which can be irritable to organisms and to some extent, be fatal
• Tephra falls


  • Ash and rock fragments ejected in vertical columns that are hazardous in at least three different ways: volume of tephra that may be deposited, inhalation of ash-sized particles of quartz crystals leading to respiratory ailments, and clogged jet engines of planes flying into ash clouds
• Phreatic explosions


  Violent extrusions of hot, steaming water with accompanying volcanic fragments that can cause ground subsidence and volcanic earthquakes
• Lahars


  Rapid flowing mixtures of volcanic debris and water which can achieve speeds of up to 40 kph, with unusually high sediment load reaching to about 91% in concentration of sediments and about 86% in volume, ranging in sizes from clay to boulders
• The whole region surrounding Taal, for example, is at considerable volcanic risk. This includes five towns around the lakeshore, two cities, and nine towns along the caldera rim, as well as two large power stations. Metro Manila will also be extremely vulnerable to pyroclastic flow and lahars should eruptions of prehistoric magnitudes recur.
• There is no definitive way of determining in the long term, where and when the next eruption will occur, what kind of eruption it will be, or how devastating will be its effects.
• The recognition of such hazards is often overlooked by developers, planners, project proponents, and the general public except when a dramatic and devastating incident occur that caused great loss of life and destruction to property.
• The objective of the National Lahar Mapping Program in the Philippines is to identify areas surrounding volcanic landforms that are most likely to be threatened by lahar encroachment and burial.
• DENR implemented the Department Administrative Order (DAO) No. 2000-28 which contains the implementing guidelines on Engineering Geological and Geohazard Assessment as additional requirement for ECC applications.
• Volcanoes have helped shape the lithosphere, triggered the formation of the early atmosphere and oceans, and yield energy through geothermal sources.
• Volcanoes are also natural wonders that open opportunities for tourism, as seen in Tagaytay City and Taal Volcano Island.
• Mass movement


  Earth processes where large masses of rocks and/or earth materials spontaneously move downward either slowly or quickly because of gravity
• Types of slow mass movements

  • Creep
  • Debris flow
• Types of fast mass movements

  • Mudflows
  • Debris avalanches
  • Landslides
• Slump


  Intermittent backward rotational movement of rock mass over short distances, characterized by a curved or concave surface rupture
• Debris or rockslide


  Planar surface rupture, movement of rock masses by sliding and rolling motion along a bedding slope or fault surface without any backward rotation
• Slump deposit


  Dislocated, usually backward tilted fragments of source rock or surface material
• Slump


  • Original structure of the source material is still recognizable
  • Manifested by the presence of landslide toes, ledges, and terraces
  • Separated from the bedrock by rotational shear zones
• Debris or rockslide (translational slide)

  • Characterized by a planar surface rupture
  • Movement of rock masses by sliding and rolling motion along a bedding slope or fault surface without any backward rotation
• Debris or rockslide deposit

  • Dislocated fragments of the source rock or surface material
  • Original structures are still recognizable with landslides toes, ledges, and terraces
  • Separated from the bedrock by planar shear surface
• Debris or rock fall


  Characterized by freefall of individual boulders or rock fragments along steep slopes, vertical cliffs, and overhangs under the influence of gravity
• Debris or rock fall deposit

  • Generally blocky and lumpy without visible sorting
  • Great variation in grain sizes depending on the type of source material
  • Irregular blocks accumulated at the foot of a steep slope or a cliff
• Debris or sand run

  Rapid downslope movement of non-cohesive particles due to slope angle exceeding the angle of repose
• Debris or sand run deposit

  • Made up of detrital, fine-grained to blocky materials with a certain downslope sorting in which largest sizes are generally deposited farthest away downslope
  • Manifested with talus cones and scree cover
• Topple


  Movement of a block of rock that is detached by tilting or rotating forward on a pivot
• Subsidence (or collapse)


  Vertical downward movement of the ground surface primarily due to different weathering processes where the competence of the rock structure is compromised and cannot contain the lithostatic load above it
• Causes of subsidence


  • Dissolution of carbonate rocks
  • Excessive extraction of groundwater
  • Mining
  • Earthquakes
• Sinkholes


  Depressions formed by the collapse of the upper subsurface rocks due to the formation of caverns from the dissolution of limestone
• Removal of interstitial waters in confined aquifers would lessen the hydrostatic pressures resulting in the inability of the aquifer to equalize the lithostatic load pressure above, leading to the collapse of the surface
• Subsidence or collapse can also occur during earthquakes due to liquefaction associated with the formation of supersaturated, fine-grained sediments during ground movement
• Tropical cyclone


  A weather system that forms in geographic areas that are hot and humid, with cyclonic, counterclockwise direction of wind circulation in the northern hemisphere and clockwise in the southern hemisphere
• Names of tropical cyclones

  • Hurricane (North Atlantic Ocean)
  • Typhoon (Northwest Pacific Ocean)
  • Severe tropical cyclone (Southeastern Indian and Southwest Pacific Oceans)
  • Severe cyclonic storm (North Indian Ocean)
  • Tropical cyclone (Southwest Indian Ocean)