Presentation on theme: "Lava Video Volcanoes are weak spots in the crusts where molten material, or magma, comes to the surface. When magma reaches the surface it is called."— Presentation transcript:
Volcanoes are weak spots in the crusts where molten material, or magma, comes to the surface. When magma reaches the surface it is called lava. The lava released during volcanic activity builds up Earth’s surface, it is a constructive force.
There are about 600 active volcanoes on land, many more lie beneath the sea. Most volcanoes occur along volcanic belts which lie along plate boundaries. Most volcanoes occur along diverging plate boundaries, such as the mid-oceanic ridge. The biggest volcanic belt is the Ring of Fire that lies along the edges of the Pacific Ocean.
Many volcanoes occur on islands, near plate boundaries where two oceanic plates collide. When the magma breaks through the ocean floor it creates volcanoes. The resulting volcanoes create a string of islands called island arcs. Examples of island arcs are Japan, New Zealand, Indonesia, and the Caribbean Islands. Birth of a Volcano Birth of a Volcano
Some volcanoes result from hot spots in Earth’s mantle. A hot spot is an area where magma from deep within the mantle melts through the crust like a blow torch. Hot spots often lie in the middle of plates far from any plate boundaries. A hot spot volcano in the ocean floor can gradually form a series of volcanic mountains. (HAWAII) Hot spots can also form under the continents. (YELLOW STONE NATIONAL PARK) Last volcanic eruption there occurred 75,000 year ago. Under Water Hot Spots Under Water Hot Spots
Lava begins as magma in the earth’s mantle. Because magma is less dense than the surrounding solid material, magma flows upward into any cracks in the rock above. Magma rises until it reaches the surface, or until it becomes trapped beneath layers of rock. All materials go from areas of high pressure to areas of low pressure. Magma reacts in the same way.
During a volcanic eruption, the gases dissolved in magma rush out, carrying the magma with them. Picture to the right is Mount St. Helens.
Beneath a volcano magma collects in a pocket called a Magma Chamber. The magma moves through a Pipe, a long tube in the ground that connects the magma chamber to the Earth’s surface. The volcanoes Vent is the point on the surface where magma leaves the volcanoes pipe. Lava can collect in the Crater the bowl shaped area that forms around the volcanoes vent.
Some types of magma are thick and flow very slowly. Other types of magma are fluid and flow almost as easily as water. The hotter the magma, the more fluid it is. The amount of silica in magma also helps determine how easily magma flows. Silica is a material formed the from the elements of oxygen and silicon. The more silica magma contains the thicker it is. Obsidian is high in silica
A volcano erupts quietly if its magma flows easily. Thin, runny lava oozes quietly from the vent. Quiet eruptions produce two different types of lava known as “pahoehoe” which is lava that flows like a solid mass of wrinkles, billows, and chunks. The second type of lava is called “aa.” Which translated means “painful to walk on.”
If a volcanoes magma is thick and sticky, a volcano erupts explosively. The thick magma does not flow out of the crater and down the mountain. Instead it slowly builds up in the volcano’s pipe, like putting your finger over a garden hose. Dissolved gases cannot escape from the thick magma. The trapped gases build up pressure until they explode.
Explosive volcanoes create volcanic ash, cinders, and large pieces of lava called bombs. These bombs can be as big as cars. Pyroclastic flows occur when an explosive eruption hurls out ash, cinders, and bombs. Pyroclastic flows are quick they can travel between 100 and 200 mph. You will be cooked if you are around one of these. Pyroclastic Flow Video Pyroclastic Flow Video
During a quiet eruption, lava flows pour from vents, setting fire to and then burying everything in their path. During an explosive eruption, a volcano can throw out hot, burning clouds of volcanic gases as well as cinders and bombs. Volcanic ash can bury entire towns, damage crops, and collapse the roofs of homes.
The activity of a volcano may last from less than a decade to more than 10 million years. An active, or live, volcano is one that is erupting or has shown signs that it may erupt in the near future. A dormant, or sleeping, volcano may be awaken in the future and become active. (Mt. Taranaki-New Zealand Pictured to the right) An extinct, or dead, volcano is unlikely to erupt again. Dating Lava Flows Dating Lava Flows
Hot springs and geysers are two examples of volcanic activity that do not involve the eruption of lava. These features may occur in any volcanic area-even around an extinct volcano. A hot spring forms when groundwater heated by a nearby body of magma rises to the surface and collects into a natural pool. Japanese Macaque is relaxing in a hot spring to the right in a cold winter day
Sometimes, rising hot water and steam become trapped underground in a narrow crack. Pressure builds until the mixture suddenly sprays above the surface as a geyser. A geyser is a fountain of water and steam that erupts from the ground.
Rock and other materials formed from lava create a variety of landforms including shield volcanoes, composite volcanoes, cinder cone volcanoes, calderas and lava plateaus.
At some places on Earth’s surface, thick layers of lava pour out of a vent and harden on top of previous layers. Such lava flows gradually build a wide, gently sloping mountain called a shield volcano.
Cinder Cone Volcanoes are steep, cone shaped hills or mountains. If a volcanoes lava is thick and stiff, it may produce ash, cinders, and bombs. These materials pile up around the vent in a steep, cone shaped pile.
Sometimes, lava flows alternate with explosive eruptions of ash, cinder, and bombs. This creates a Composite Volcano which are generally tall, cone shaped mountains in which layers of lava alternate with layers of ash. Example is Mount Fuji or Mount St. Helens.
Enormous eruptions may empty the main vent and the magma chamber beneath the volcano. The mountain becomes a hollow shell with nothing to support it. This hollow shell is known as a Caldera. Crater Lake is pictured to the right
Instead of forming mountains, some eruptions of lava form high, level areas called Lava Plateaus. This is usually caused by thin, runny lava traveling far before cooling and solidifying.
Before an eruption, magma moves into the area beneath the volcano and collects in a magma chamber, or reservoir. As it comes closer to the surface, the magma releases gases. These events can offer valuable clues about the likelihood of an eruption. For example, the movement of magma produces small earthquakes and vibrations (seismicity).
Magma gathering in a chamber causes slight swelling of the volcano's slopes. Gases released near the volcano can be measured for changes in quantity and makeup. A number of tools can be used to record these warning signs. Seismographs can detect small earthquakes, while tiltmeters and geodimeters can measure the subtle swelling of a volcano. Correlation spectrometers (COSPECS) can measure amounts of sulfur dioxide--a telltale gas that is released in increasing quantities before an eruption.
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