Year 9 Science – Seaview High School Mountains of Fire Year 9 Science – Seaview High School

Explosive evidence Most changes to the Earth’s surface are very slow to occur. Volcanic eruptions can be very explosive and are the most obvious evidence of these changes. For example, in 1883, the volcano Krakatoa exploded. The sound was heard over 4000 km away and the resultant tsunami drowned about 36000 people.

How are Volcanoes Formed? Volcanoes form when magma (molten rock) forces its way to the surface through a weakness in the Earth’s crust. Once the magma reaches the surface we call it lava. Lava is a mixture of magma and gases, including steam, carbon monoxide and hydrogen sulfide (‘rotten egg’ gas).

What Comes Out? As the pressure builds up the volcano begins to show signs of an imminent eruption. Smoke and ash billow from the vents. During the eruption lava flows from the vents and red-hot fragments of rock, dust and ash, steam and other gases shoot out of the crater. Exploding gases can destroy parts of the volcano. At times, larger fragments may be thrown from the volcano. These are called volcanic bombs, or lava bombs.

Dancing With The Lava

A Volcano Broken Down Lightning can sometimes be seen in the ash clouds. Colliding particles of ash and lava create static electricity An ash cloud explodes from the crater and up to 30 kilometres into the atmosphere. Main volcanic vent Ash settles on the volcano surface The volcanic crater is the point where the magma reaches the surface and becomes known as lava. Blocks of hot rock known as lava bombs are ejected Pressure from gas and magma mixing underground forces magma up the main vent and branch pipes. Avalanches and landslides are common during eruptions Branch pipe The initial heat blast from the eruption ‘boils’ trees by turning the water in plants and animals to steam. Melted snow and ice turn into destructive mudflows that travel down valleys. Magma gathers in the magma chamber before it is forced to the surface.

Types Of Volcanoes Lava that erupts from a volcano can be very runny, like honey, or very thick, like toothpaste. Runny volcano will move away from the volcano site creating large plains of basalt. Such plains exist around Melbourne. Thick lava will stick close to the volcanic site resulting in large, cone shaped volcanoes.

Types of Volcanoes Mauna Loa in Hawaii is an example of a shield volcano Popocepetl is an active composite volcano in Mexico Sunset Crater in Arizona, USA, is an example of a cinder cone volcano

Shield Volcanoes Some of the largest volcanoes on Earth. Forms from layers of very runny lava which can form puddles, rivers and streams (950˚C). Lava can flow for kilometres creating gently sloping sides. Usually don’t explode and are usually easy to avoid by car or even by foot. Occur at hotspots or mid-ocean ridges Skjaldbreidur Herbst, Iceland is famous for its shield volcanoes

Composite Volcanoes Also known as strato-volcanoes Formed by alternating layers of rock and lava This is why they are called composite. Often high altitude. Think of images of snow capped volcanoes such as Mt. Fuji Can appear to be extinct, but erupt explosively Occur along subduction zones Left: Mt Fuji, Fuji, Japan Right: Vesuvius, Napoli Italy Both are examples of composite volcanoes

Cinder Cone Volcanoes A steep conical hill that forms above a vent. Most common volcanic land forms on Earth. Usually form in groups around the edges of shield or composite volcanoes. Built from lava fragments called cinders which are ejected from the vent and fall back near the vent. Relatively small, 250 m tall and 500 m diameter Paricutín is a cinder cone volcano located in Mexico.

Types of Volcanoes http://dli.taftcollege.edu/streams/geography/Anim ations/VolcanoTypes.html Volcanic lightning as Mount Shinmoe-dake in Kyushu, Japan erupts.

Birth of a Volcano On a winter’s day, in Mexico, 1943, a small crack opened up in a corn field. As cinders shot out of the crack, a farmer attempted to fill it with dirt. By the next day, the crack had opened to over 2 metres in diameter. By a week later, a cone 150 metres tall had formed! Explosions from the crater continued, destroying the local village of Paricutin. Within a year, the cinder cone had reached 300 m in altitude. Eruptions stopped in 1952 with the cone reaching a height of 410 metres.

Subduction Zones Most of the world’s volcanoes occur near subduction zones (for example, the ‘Ring of Fire’). As the dense oceanic plate subducts beneath the less dense continental plate, the rock is pushed down into the mantle and is turned to molten rock. As this occurs, water is forced down with the subducting plate. This water turns to gas (steam) and escapes back towards the surface through any path it can find. The molten rock follows these paths and forces its way to the surface causing an eruption. http://static.howstuffworks.com/flash/volcano- eruption.swf

Hot Spots Volcanoes that occur away from the edges of the tectonic plates can be explained by hot spots. Hot spots occur at areas of crust directly above regions of particularly hot molten rock in the mantle. Volcanoes in western Victoria and Queensland have been formed this way. The Hawaiian island chain has also been formed this way. It is believed a large hot spot sits below Yellowstone National Park in central United States of America. http://sceyencestudios.com/movies/volcanism.swf