[MUSIC] So far we've discussed hazards in mountains that are caused by steep slopes and gravity. Let's shift and discuss hazards associated with volcanic mountains because they're quite unique. First though, let's quickly review how volcanos are formed. Volcanos, you might remember, can form at convergent plate boundaries. Here, subduction zones often form where one plate is driven beneath another into the mantle of the earth. The heat of the mantle melts the crust of the subducting plate. Turning it into magma, and magma, because it's less dense than the surrounding mantle, rises towards the surface of the earth. If the magma erupts through the over running plate, produces a volcano. Divergent plate boundaries, where plates are separating, may also expose the hot mantle, which melt and upwells to form volcanoes. And finally, volcanoes can form along hotspots where disturbances in the mantle produce hot plumes that rise to the surface. Remember, hotspots are not necessarily associated with plate boundaries and often occur within a plate. Different types of volcanoes are formed by different types of magma. Silica is one of the most abundant elements in magma, and magma types are defined by their silica content. The amount of silica in magma determines its viscosity, or its ability to flow. Magma formed my upwelling, melted mantle, has low silica content and low viscosity. Meaning that it flows easily. This type of magma, called basaltic magma, contains high amounts of iron and magnesium, making it dark in color. Basaltic magma reaches the Earth's surface without passing through continental crust. And when basaltic magma reaches the Earth's surface as lava, it cools to form a type of rock called basalt. Basaltic magma occurs at divergent plate boundaries because magma derived from melted mantle rises to directly fill the gaps separating the plates. Now, hot spots on oceanic plates also consist of basaltic magma. There are some spectacular formations as a result of basaltic magma eruptions, including this Svartifoss waterfall, in Skaftafell National Park in southern Iceland. At the other extreme, rhyolitic magma has high silica content and low levels of iron and magnesium, making it lighter color. Rhyolitic magma is formed when basaltic magma rises through the continental crust, which is mainly composed of silicone rich granite rock. As the magma rises it melts the surrounding granite, increasing it's silica content and viscosity, making it thick and sticky. When the Rhyolitic magma reaches the Earth's surface as lava It cools to form rhyolite rock. Rhyolitic magma is most likely produced by volcanos that arise in subduction zones and hot spots on continental plates, because in both of these cases the magma must pass through a continent plate before reaching the earth's surface. How do different types of magna produce different types of volcanoes? The answer lies in the differences in viscosity. Viscosity determines how well lava flows once it's erupted, influencing the shape of the volcano. Viscosity also influences how easily gas that is trapped in the magma can escape, which influences the explosiveness of the eruption. Magma that's contained in gas that can't escape easily leads to more explosive eruptions. Volcanoes produced by basaltic magma are called shield volcanoes. Because they're shaped like a warrior shield. The low viscosity of the basaltic magma means that the resulting lava is relatively fluid. Allowing it to slide easily down slope and travel over a large area before cooling and solidifying into basalt rock. This creates broad, gentle sloping volcanoes. The low viscosity of basaltic magma also allows gas to escape fairly easily, so eruptions tend to be mild. Shield volcanoes are found worldwide. The Hawaiian and the Galapagos Islands are good examples. Another example might be Tamu Massif, an extinct submarine shield volcano, located in northwestern Pacific Ocean. The possibility of its nature as a single volcano was announced recently in 2013, and if corroborated, would make Tamu Massif the largest known volcano on earth. In contrast, a stratovolcano is produced by rhyolitic magma. The high viscosity of rhyolitic magma means that the resulting lava does not spread far before cooling. As a result, the lava piles up, forming the steep conical shapes that we often attribute to volcanos. The high viscosity of the thick rhyolitic magma also strongly traps gas, causing internal pressure to build and build until the point when the gas is explosively released upon eruption. Two famous stratovolcanoes are Krakatoa best known for it's catastrophic eruption in 1883. And Vesuvius, famous for it's destruction of the towns of Pompeii and Herculaneum in 79 A.D. both eruptions claimed thousands of lives. In modern times, Mount Saint Helens and Mount Pinatubo have erupted catastrophically, shield volcanoes and stratovolcanoes are two extreme cases of volcanic formation. Because there's a gradient in the silica content and the viscosity of magma, there exist other, intermediary types of volcanoes, such as cinder cone volcanoes. The most famous cinder cone, Paricutin, grew out of a cornfield in Mexico in 1943 from a new vent. Eruptions continued for nine years, built the cone to a height of 424 meters, produced lava flows that covered 25 kilometers. Volcanoes pose a diversity of hazards with potential consequences from local to global scales. Probably the first volcanic hazard that comes to mind is lava flows, when lava pours from a volcano and moves downslope. Lava flows destroy everything in their path and they're almost impossible to stop. For example, the eruption of Kilauea, which forms part of Hawaii, has been ongoing since 1983. Since then, the continuous flow of lava has resurfaced over 125 square kilometers of land. Buried about 14 kilometers of main highway and destroyed over 200 homes. The intense heat of lava flows can even burn areas not in their direct path. But lava flows are considered the least hazardous volcanic process because they're usually not life threatening. Usually, lava flows at a rate that is slow enough that people can be evacuated before it reaches communities. Another hazard, volcanic ash, is produced by explosive eruptions. When gas explodes out of magma, the magma is shattered and propelled into the air where it cools and solidifies into various small shards of glass and rock, producing volcanic ash. Therefore volcanic ash is heavy and abrasive, unlike the familiar ash of wood-burning fires. If volcanic gas if projected high enough, it can reach the stratosphere, the upper layer of the Earth's atmosphere. Here it can travel thousands of kilometers, having far-reaching effects. In particular, it can produce ash clouds that block incoming solar radiation, it can have a temporary cooling effect on the planet. Volcanic ash can coat just about everything. Destroying infrastructure and crops, often with large economic impacts. It becomes very heavy when wet, turning into a thick sludge which can collapse roofs. Volcanic ash also poses a danger to the aviation industry because it can clog airplane engines. For example, in April 2010, the eruption of Eyjafjallajokull in Iceland severely affected air traffic in Europe. It grounded airplanes across Europe for a week stranding millions of airline passengers. And costing the aviation industry almost $3 billion. Volcanic ash also has negative consequences for the health of humans and animals. If inhaled it causes breathing problems and can ultimately lead to suffocation. In addition to volcanic ash, an explosive eruption can create pyroclastic flows. When hot masses of gas and rock fragments are ejected and moved down slope. Compared to lava flows, pyroclastic flows are much more dangerous. They travel down slope extremely fast, reaching speeds moving away from a volcano of up to 700 kilometers per hour. They're also very hot, reaching hundreds of degrees Celsius. As a result, they are very difficult to escape from and are often deadly, destroying everything in their path. For example, the deadly 1902 eruption of Mount Pelee on the coast of Martinique in the Caribbean created pyroclastic flows that buried the sound of Saint-Pierre killing 30 thousand people. One of the greatest dangers a volcano presents is not the direct result of material that's ejected. The height of many volcanos means that they're often covered in snow and ice and the heat of an eruption can cause them to rapidly melt with catastrophic consequences. The melting of snow and glaciers can create a volcano triggered version of debris flow called a lahar. A lahar is triggered when large amounts of water released from the melting snow and ice mix with the loose volcanic rock and ash on the flanks of the volcano. This mixture pours into creeks and rivers that flow downslope causing their channels to overflow. The consistency of a lahar has been described as wet cement. And they're often very hot, causing burn injuries to their victims. Lahar are not as fast as pyroclastic flows but like other down slope hazards they bury and destroy everything in their path and they can be extremely dangerous because people of course tend to build their communities near a rivers and creeks. In addition lahars don't require a large eruption to be triggered. One of the greatest lahar disasters occurred in 1985. When Nevado Del Ruiz erupted in Columbia. Now although not a large eruption, it melted the volcano's summit glaciers. It triggered a series of lahars that ran down the rivers that originated in the region. These lahars destroyed several communities built along the rivers and killed approximately 23,000 people. Remember that mountains and glaciers are often the source of major rivers and waterways. Lahars thus have the potential to have far-reaching impacts. Other potential hazard when volcanos erupt near water is the displacement of large volumes of sea water. Which can generate large waves called tsunamis. The famous eruption of Krakatoa in Indonesia in 1883 is a classic example. When that volcano erupted, it collapsed into the surrounding ocean and created massive tsunamis that inundated the surrounding coastal areas, killing more than 36,000 people. Perhaps the greatest impact that volcanic eruptions have had through history is the release of gas into the atmosphere. Most of the gas that's released in a volcanic eruption is relatively harmless. It's water vapor, for example. However, two gasses that are ejected from volcanic eruptions, carbon dioxide and sulphur dioxide, have important impacts on the earth's climate. The bigger the eruption or the longer that it continues the more gas is ejected. Let's first talk about carbon dioxide. Carbon dioxide that's released from volcanoes is an important component of the earth's climate because it's a greenhouse gas. We learned previously that the earth itself is the source of most of the heat on our planet. It traps incoming solar radiation and radiates heat back into the atmosphere. The heat that's radiated from the earth is then absorbed by greenhouse gases in the atmosphere which block in the heat. Greenhouse gases like carbon dioxide regulate the earth's climate. Without them, the Earth would be too cold to support life. However, the concentration of greenhouse gases in the atmosphere is very delicate balance, and volcanoes have been essential to maintaining this balance for over 4.5 billion years of Earth's history. Carbon dioxide is taken up by plants through photosynthesis. Where Carbon is then stored in their tissues. And over long periods of time as plants and animals are buried into the earth, Carbon is sequestered or stored for a long time in coal beds and rocks. And this process continued on unchecked carbon dioxide levels will get so low that the earth would essentially freeze. However, volcanic activity recycles carbon dioxide back into the atmosphere. When the sediments that have sequestered carbon dioxide are subducted into the mantle, carbon dioxide is released into magma, where it will eventually erupt back into the atmosphere. Sulfur dioxide also affects the Earth's climate, but on a shorter timescale than carbon dioxide. Once erupted, sulfur dioxide reacts with water vapor to produce sulfuric acid. Sulfuric acid then falls to earth as acid rain which can have harmful impacts on plants and animals, human health. In addition, the corrosive nature of sulfuric acid causes the outside of buildings to deteriorate and to disintegrate. Like volcanic ash If an eruption is explosive enough, Sulphur dioxide can reach the stratosphere where it can travel to widespread locations. Here it can stay aloft in the atmosphere for several years. Importantly the high albedo of sulphuric acid reflects incoming solar energy back into space, which can cause temporary cooling of the Earth's climate. Decreased global temperatures caused by sulfuric acid and to a lesser extent volcanic ash is called a Volcanic Winter. One such event occurred recently following the 1991 eruption of Mount Pinatubo in the Philippines. During the two years following the eruption, the average global temperature fell by almost a degree Celsius. Interestingly, the volcanic winters also tend to produce stunning sunsets and it's believed that the fantastic skies seen in the background of Edvard Munch's famous painting, The Scream, were inspired by the sunsets created by the 1883 eruption of Krakatoa. People have inhabited mountain regions for centuries and have learned to cope with the numerous hazards and risks associated with steep relief, extreme weather and unstable geology. Mountains are dynamic landscapes and so are more frequently affected than other environments by destructive natural processes like earthquakes and volcanic eruptions, slope failures or glacial lake outbursts. Hazards such as avalanches and landslides occur almost exclusively in mountains. Human activities such as the destruction of mountain forests can also accelerate erosion and increase the risk of landslides, floods and avalanches. However, with careful planning and a great deal of respect for the forces of nature. It's still possible to live, work, and play in mountains. Let's pause here for you to return to your mountain world, see if you can locate some of those big volcanoes that we've visited in this lesson. Given today's theme, Matt Peter's also standing by with a pertinent tech tip on how to make travel decisions in or near avalanche terrain. And then, of course, there's your end-of-lesson quiz. So good luck and join us next time as we shift gears to a lesson on biodiversity and the special adaptations made by plants that enable them to thrive in those alpine environments.