Chapter 8 Section 2
Types of Volcanoes
-
The process of magma formation is different at each type of plate boundary.
-
Therefore, the composition of magma differs in each tectonic setting.
-
Tectonic settings determine the types of volcanoes that form and the types of eruptions that take place.
Volcanoes at Divergent Boundaries
-
At a divergent boundary, the lithosphere becomes thinner as two plates pull away from each other.
-
Hot mantle rock rises to fill these cracks.
-
As the rock rises, a decrease in pressure causes hot mantle rock to melt and form magma.
-
The magma that reaches Earth’s surface is called lava.
-
Lava that flows at divergent boundaries forms from melted mantle rock.
-
This lava is rich in the elements iron and magnesium. It is relatively poor in silica.
-
The term mafic describes magma, lava, and rocks that are rich in iron and magnesium.
-
Because it is low in silica, mafic lava is runny and not sticky.
-
This type of lava generally produces nonexplosive eruptions.
-
Mid-ocean ridges are underwater mountain chains that form where two tectonic plates are moving apart.
-
Some of the magma erupts as basaltic lava on the ocean floor.
-
The magma and lava cool to become part of the oceanic lithosphere.
-
As the plates continue to move, older oceanic lithosphere moves away from the mid-ocean ridge.
-
New cracks form, and new lithosphere forms in the rift zone.
-
The mid-ocean ridge called the Mid-Atlantic Ridge is unusually active.
-
This activity has built part of the ridge into a large island known as Iceland.
-
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.
-
Icelandic volcanoes, such as Krafla, are often associated with large, connected fissure systems.
-
Lava erupts frequently through these fissures. As a result, Iceland is continually getting bigger.
Volcanoes at Hot Spots
-
A hot spot forms in a tectonic plate over a mantle plume.
-
Plumes are thought to originate at the boundary between the mantle and the outer core.
-
When the top of a mantle plume reaches the base of the lithosphere, the mantle rock spreads out and “pools” under the lithosphere.
-
Because pressure on the rock is low at this shallow depth, the rock melts.
-
Large volumes of magma are released onto the ocean floor.
-
Continuous eruptions may produce a volcanic cone.
-
Because lava at hot spots comes from the mantle, it is mafic and fluid.
-
Most eruptions at hot spots are nonexplosive.
-
The type of rock that forms from this lava depends on the temperature, gas content, flow rate, and slope of the lava flow.
Shield Volcanoes
-
Shield volcanoes form from layers of lava left by many nonexplosive eruptions.
-
The lava is very runny, so it spreads out over a wide area.
-
Over time, the layers of lava create a volcanic mountain that has gently sloping sides.
-
The sides of shield volcanoes are not very steep, but the volcanoes can be very large.
Parts of a Volcano
-
Most volcanoes share a specific set of features.
-
At Earth’s surface, lava is released through openings called vents.
-
Before erupting as lava, magma rises from the magma chamber to Earth’s surface through cracks in the crust.
-
This movement of magma causes small earthquakes that can be used to predict an eruption.
-
Lava may erupt from a central summit crater of a shield volcano.
-
Lava may also erupt from fissures along the sides of the shield volcano.
-
A lava flow is a long river of molten rock.
-
Often the flow will cool and solidify on top while lava in the interior continues to flow.
-
Flowing lava in the interior travels through long, pipelike structures known as lava tubes.
Volcanoes at Convergent Boundaries
-
At a convergent boundary, a plate that contains oceanic lithosphere may descend into the mantle beneath another plate.
-
The descending lithosphere contains water.
-
As the lithosphere descends into the mantle, temperature and pressure increase.
-
The subducting lithosphere releases water into the surrounding mantle and overlying crust.
-
The magma that forms rises through the crust and erupts. These eruptions form a chain of volcanoes parallel to the plate boundary.
-
Magmas at convergent boundaries are composed of melted mantle rock and melted crustal rock.
-
Therefore, fluid mafic lava and lava rich in silica and feldspar form at these boundaries.
-
Lavas rich in silica and feldspar cool to form light-colored rocks.
-
The term felsic is used to describe magma, lava, and rocks that are rich in silica and feldspars.
-
Silica-rich magma tends to trap water and gas bubbles.
-
This 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 in pressure causes a powerful explosive eruption.
-
Pyroclastic materials are released during the eruption.
-
Pyroclastic material forms when magma explodes from a volcano and solidifies in the air.
-
Pyroclastic material also forms when powerful eruptions shatter existing rock.
-
Four types of pyroclastic material include volcanic bombs, lapilli, volcanic ash, and volcanic blocks.
-
Pyroclastic flows are produced when a volcano ejects enormous amounts of hot ash, dust, and toxic gases.
-
This glowing cloud of pyroclastic material can race down the slope of a volcano at speeds of more than 200 km/h.
-
This speed is faster than the speed of most hurricane-force winds.
-
The temperature at the center of a pyroclastic flow can exceed 700°C.
-
At this high temperature, a pyroclastic flow burns everything in its path.
-
Extreme winds and temperatures make pyroclastic flows the most dangerous of all volcanic phenomena.
Cinder Cone Volcanoes
-
Cinder cone volcanoes are the smallest type of volcano.
-
They generally reach heights of no more than 300 m.
-
Cinder cone volcanoes are made of pyroclastic material.
-
Cinder cone volcanoes most often form from moderately explosive eruptions.
-
They have steep sides and a wide summit crater.
-
Unlike other types of volcanoes, cinder cone volcanoes usually erupt only once in their lifetime.
Composite Volcanoes
-
They form from both explosive eruptions of pyroclastic material and quieter flows of lava.
-
This combination of eruptions forms alternating layers of pyroclastic material and lava.
-
Composite volcanoes have a broad base and sides that get steeper toward the crater.
-
These volcanoes may generate many eruptions. However, eruptions may occur at intervals of hundreds of years or more.
-
Mount Fuji in Japan is a famous composite volcano.
|