1. Prepared by: Group II IX- Archimedes

2. There are several kinds of events caused from volcanic action that can be harmful to life and property. These include lava flows, lahars, ash falls, debris avalanches, and pyroclastic density currents

3. Earthquakes related to volcanic activity may produce hazards which include ground cracks, ground deformation, and damage to manmade structures. There are two general categories of earthquakes that can occur at a volcano: volcano-tectonic earthquakes and long period earthquakes. Earthquakes produced by stress changes in solid rock due to the injection or withdrawal of magma (molten rock) are called volcano-tectonic earthquakes.These earthquakes can cause land to subside and can produce large ground cracks. These earthquakes can occur as rock is moving to fill in spaces where magma is no longer present. Volcano-tectonic earthquakes don't indicate that the volcano will be erupting but can occur at anytime.

5. In a typical volcanic eruption, an eruption column consisting of pyroclastic material is ejected into the atmosphere. The base of this column is known as the gas thrust zone where material is shot ballistically from the volcanic vent. Above this zone is the convective thrust zone. This zone is where heat buoys pyroclastic material upward toward the top of the troposphere. Once the eruption column reaches the stratosphere, shearing occurs and the material is spread out in an umbrella shape (Francis, 1993). The May 18, 1980 eruption of Mount St. Helens did not initially produce such an eruption column. Instead, the initial eruption was a directed blast. This blast was a result of depressurization triggered by an earthquake initiated landslide on the north flank of the volcano. The area affected by the directed blast extended greater than 19 miles from the volcano..

8. When a volcano erupts it will sometimes eject material such as rock fragments into the atmosphere. This material is known as tephra. The largest pieces of tephra (greater than 64 mm) are called blocks and bombs. Blocks and bombs are normally shot ballistically from the volcano. Because these fragments are so large they fall out near their source. Blocks and bombs as large as 8-30 tons have fallen as far away as 1 km from their source. Small blocks and bombs have been known to travel as far away as 20-80 km. Most particles greater than a millimeter in size will fall out within 30 minutes of the time they are erupted. The smallest particles which are less then.01 mm can stay in the atmosphere for two or three years after a volcanic eruption. Sometimes these particles produce fantastic sunsets such as was seen after the eruptions of Krakatau in 1883 and Pinatubo in 1991. Some scientists believe that these particles may contribute to global warming.

9. The size of particles that fall out is largest near the volcano and gets progressively smaller further from the volcano. The thickness of material usually decreases the further away from the volcano. Occasionally, as occurred in Ritzville, Washington when Mount St. Helens erupted in 1980, secondary thickening will occur. Secondary thickening means that ash deposits are thicker in a particular area than in surrounding areas. This results when ash particles and water form clumps which produce larger particles that have higher terminal velocities and so fall out of the ash cloud. When these particles hit the ground they break apart and produce a thicker deposit of ash than in surrounding areas

11. An erupting volcano will release gases, tephra, and heat into the atmosphere. The largest portion of gases released into the atmosphere is water vapor. Other gases include carbon dioxide (CO2), sulfur dioxide (SO2), hydrochloric acid (HCl), hydrogen fluoride (HF), hydrogen sulfide (H2S), carbon monoxide (CO), hydrogen gas (H2), NH3, methane (CH4), and SiF4. Some of these gases are transported away from the eruption on ash particles while others form salts and aerosols. Volcanic gases are also produced when water is heated by magma. Gases also escape from pyroclastic flows, lahars, and lava flows, and may also be produced from burning vegetation.

14. Lava flows are the least hazardous of all processes in volcanic eruptions. How far a lava flow travels depends on the flows temperature, silica content, extrusion rate, and slope of the land. A cold lava flow will not travel far and neither will one that has a high silica content. Such a flow would have a high viscosity (a high resistance to flow). A basalt flow like those in Hawai'i have low silica contents and low viscosities so they can flow long distances. Such a flow can move as far away as 4 km from its source and have a thickness of 10 m.These flows can move at rates of several kilometers per hour. More silica-rich flows can move as far away as 1.3 km from their sources and have thicknesses of 100 m.These flows can move at rates of a few to hundreds of meters per hour. If a lava flow is channelized or travels underground in a lava tube then the distance it travels is greatly extended.

17. A debris avalanche is formed when an unstable slope collapses and debris is transported away from the slope. Large scale avalanches normally occur on very steep volcanoes. There are two general types of debris avalanches: those that are "cold" and those that are "hot".

18. Landslide is a general term for mass movement. It implies a gradual movement rather than the more sudden movement of an avalanche. The perfect example of possible large scale landslides currently taking place is on the south side of the island of Hawai'i.

19. Tsunamis are large sea waves that have long wave periods. When these waves reach coastal areas, they can go far inland. If such a wave were produced by the collapse of a large portion of the south side of the island of Hawai'i then it would put people in danger who were living on the coast of Hawai'i and surrounding islands. It could even travel across the Pacific to coastal areas thousands of miles away. Tsunamis aren't always produced by collapsing land masses. They can also be produced by volcanic earthquakes and explosions, atmospheric shock waves due to rapidly moving volcanic material, and lahars or pyroclastic flows that have entered the sea

21. Pyroclastic flows are fluidized masses of rock fragments and gases that move rapidly in response to gravity. Pyroclastic flows can form in several different ways. They can form when an eruption column collapses, or as the result of gravitational collapse or explosion on a lava dome or lava flow. These flows are more dense than pyroclastic surges and can contain as much as 80 % unconsolidated material. The flow is fluidized because it contains water and gas from the eruption, water vapour from melted snow and ice, and air from the flow overriding air as it moves downslope.

23. One of the greatest volcanic hazards is lahars. Lahars are similar to pyroclastic flows but contain more water. Lahars form 1) from debris avalanches that contain water from snow and ice which, when released, mixes with loose debris to form a lahar, 2) from pyroclastic flows and surges which release water that mixes with debris, 3) from pyroclastic flows which dilute themselves with river water as they travel downslope, 4) from natural dam failure from rainfall on loose material such as ash. Lahars that contain 20 to 60% sediment are usually very turbulent. Lahars that contain greater than 80% sediment usually flow more smoothly (laminar flow). These smooth flowing lahars usually travel much faster than their turbulent counterparts and can float boulders, cars, buildings, and bridges.