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The active volcanoes form directly over a hot spot, where magma rises from the mantle. The hot spot is stationary. The plate moves - slowly - over the.

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Presentation on theme: "The active volcanoes form directly over a hot spot, where magma rises from the mantle. The hot spot is stationary. The plate moves - slowly - over the."— Presentation transcript:

1 The active volcanoes form directly over a hot spot, where magma rises from the mantle. The hot spot is stationary. The plate moves - slowly - over the hot spot. Volcanic islands form over the hot spot. As they move away from the hot spot, the volcanoes go extinct and the islands erode. There are several hotspots in the Pacific, and several chains of islands.

2 Oceanic lithosphere is made up of sediments and volcanic rock that contains water and other fluids. When oceanic lithosphere moves downward into the mantle at a convergent boundary, the fluids contact the surrounding rock. When the fluids enter the already hot mantle rock, the melting temperature of the hot rock decreases. As a result, the rock begins to melt.

3 Lava that flows at divergent boundaries forms from melted mantle rock and rich in iron and magnesium and poor in silica. Silica poor lava is called mafic magma and runny and sticky. This type of lava generally comes from nonexplosive eruptions.



6 Mafic magma is runny and sticky

7 Mid ocean ridges are underwater volcanic mountain chains that form where two tectonic plates are moving apart. As the plates move apart, magma from the mantle rises to fill cracks that form in the crust. Some of the magma and lava cool and become part of the oceanic lithosphere. This process is known as sea-floor spreading.


9 The mid Atlantic ridge is very active. Long linear cracks called fissures have formed where the Atlantic and Eurasian plates are moving apart. Basaltic magma rises to Earth’s surface through these fissures and erupts nonexplosively.

10 Krafla fissure eruptions in Iceland

11 As a general rule, basaltic lava erupts from fissure eruptions. A fissure is a seam in the earth's crust, from several meters long to kilometers long. If you have seen video of lava erupting that is bright red in color, you are probably looking at a basaltic eruption.

12 Hot spot volcanoes form over mantle plumes. These are columns of hot solid rock that rise through the mantle by convection. Continuous eruptions will eventually reach sea level and become islands.

13 There are four basic types of lava: 1. Aa lava (Hawaiian term pronounced 'ah-ah' for lava flows that have a rough rubbly surface composed of broken lava 2. Pahoehoe 3. Pillow lava 4. Blocky lava

14 Aa is lava that forms a thick, brittle crust. The crust is torn into jagged pieces as molten lava continues to flow underneath.

15 AA Lava as it flows

16 Mount Etna—Italy AA Lava flow.

17 Night time Aa lava flow.

18 Pahoehoe Lava is lava that forms a thin crust. The crust wrinkles as it is moved by molten lava that continues to flow underneath




22 Pillow lava forms when lava erupts underwater. This lava forms rounded lumps that are the shape of pillows.



25 If pillow lava was found in Idaho, does that mean that Idaho was once under water?

26 Blocky lava is cool, stiff lava that does not travel far from the eruption site. Blocky lava usually oozes from a volcano and forms jumbled heaps of sharp edged chunks.


28 When basaltic lava flows cool and solidify they contract, often developing fractures perpendicular to their surface. Individual columns tend to have a hexagonal (six-sided) cross section. Since columns are always at right angles to the cooling surface, vertical columns are seen in lava flows

29 Shield volcanoes usually form at hot spots. Shield volcanoes form from layers of lava left by many eruptions. Shield volcano lava flow is very runny so it spreads out over a wide area. The sides of this volcano are not very steep, yet this type of volcano can be very large. The base of this type of volcano can be more than 100 km in diameter.

30 Mauna Kea in Hawaii is the tallest mountain on Earth as measured from its base on the sea floor.




34 Magma Chamber: this is the magma that feeds the eruption pools from deep underground. The magma rises through cracks in the crust. This movement of magma can cause small earthquakes that are used to predict eruptions. Vents: lava is released through these openings. Lava may also erupt from fissures along the sides of a shield volcano. After eruption the lava moves down slope in long rivers of molten rock. Often this lava will cool and solidify on top while the interior continues to travel through long, pipelike structures called lava tubes.


36 A lava tube on the island of Hawaii, taken just above a lava falls. The floor is cauliflower pahoehoe, a rougher form of pahoehoe. Note the tree roots coming in from the ceiling. Lava tubes tend to be fairly close to the surface.



39 Magma at convergent boundaries are melted mantle rock and melted crustal rock. So, fluid mafic lava and lava rich in silica form at these boundaries. Lava rich in silica cool to form light- colored rocks. Silica rich magma tends to trap water and gas bubbles, which causes enormous gas pressure to develop within the magma. As the gas filled magma rises to Earth’s surface, pressure is rapidly released. This change results in a powerful explosive eruption.

40 Volcanic bombs are large blobs of magma that harden in the air


42 Lapilli are pebblelike bits of magma that harden before they hit the ground.


44 Volcanic ash forms when the gasses in stiff magma expand rapidly and the walls of the gas bubbles explode into tiny, glasslike slivers




48 Mount Pinatubo in the Philippines. The ash in this cloud had temperatures that reached 750°

49 Pyroclastic flows are produced when a volcano ejects enormous amounts of hot ash, dust, and toxic gases. This cloud of material can reach speeds of 200 km/hr. This is faster than most wind speeds of hurricanes. The temperature within the flow can reach 700° At this temperature the flow burns everything in its path thus, Pyroclastic flows are the most dangerous of all volcanic phenomena


51 Cinder cone volcanoes are the smallest type. They generally reach heights of no more than 300 meters. They are also the most common type of volcano. These volcanoes are made from Pyroclastic material and most often form from moderately explosive eruptions. These volcanoes have steep sides. They also have a wide summit crater. Unlike other volcanoes, cinder cone volcanoes usually erupt only once in their lifetime.


53 These volcanoes are also called Stratovolcanoes. These are the most recognizable of all volcanoes. They form from both explosive eruptions of Pyroclastic material and quieter flows of lava. This forms alternating layers of Pyroclastic material and lava. They have a broad base and sides that get steeper toward the summit.



56 Tambora, Indonesia. Most of the negative effects come from explosive eruptions which results in loss of life and property damage. This volcano erupted in 1815 and killed 92,000 people. Most were killed by falling debris (volcanic bombs).

57 The 1815 Tambora eruption put enough ash and gasses into the upper atmosphere that the average global temperature decreased by 3°C for one to two years. The lower temperatures is blamed for crop failures and starvation, particularly in New England and Europe. These effects led to the deaths of 82,000 people

58 Lahars are fast moving mudflows that bury everything in their path

59 Lahar mudflow





64 Mount Etna, Italy

65 Krakatau, Indonesia




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