Volcanoes Flashcards
Magma
Magma is molten or partially molten rock beneath the Earth’s surface
We refer to magma as lava (from the Italian, lavare, meaning “stream”) when it erupts onto the Earth’s surface
Extrusive igneous rocks
produced from the cooling and solidification of magma or lava at the Earth’s surface (e.g. basalt)
Intrusive igneous rocks
produced by from the cooling and solidification of magma within the Earth (e.g. granite)
Decompression Melting
Occurs at divergent plate boundaries, continental rifts, and hot spots
Great pressures are created at depth due to the weight of overlying rock
Thinning and stretching of the crust at rifts and divergent boundaries causes the mantle to well up towards the surface where pressures are lower
Plumes of hot rock well up to shallow depths at hotspots
Mid-Oceanic Ridges
At spreading ridges, mafic magma derived from the asthenosphere rises to the ocean floor to create new crust
When magma erupts underwater, it forms pillow lava, formed by repeated oozing and quenching of mafic magma
A flexible glass crust forms around the newly extruded lava, forming an expanded pillow
Pressure builds until the crust breaks and new magma extrudes like toothpaste, forming another pillow
In Iceland, a spreading ridge occurs on land; active volcanism associated with this diverging plate boundary
Continental Rifts Valleys
Tectonic forces stretch the crust causing Earth’s surface to fracture into normal faults
As the crust thins, the hot mantle can rise closer to the surface, producing magma through decompression melting
The magma travels through fractures in the crust, often along normal faults
Felsic
Felsic magmas are produced by heating and hydration of the continental crust
Horst and Graben Topography
This results in blocks that tilt and alternatively drop and rise (horst and graben topography)
Hotspots
As rising plumes of hot mantle migrate upwards they begin to melt under low pressure (decompression melting) to form magma; the magma rises to the surface and forms a volcano
Hotspots are fixed positions, as the plate carrying the volcano moves away from the hot spot volcanism ceases and a new shield volcano forms in the position over the hot spot producing island arcs (oceanic) or volcanic arcs (continental)
Volcanism associated with hot spots occurs in both the Atlantic and Pacific Oceans but is more common in the Atlantic because it moves at a higher velocity
Oceanic Hotspots
Oceanic hotspots produce mafic magmas (e.g. Hawaii) whereas continental hotspots produce a mixture of mafic and felsic magmas (e.g. Yellowstone)
Stratovolcanoes
Occur at subduction zones around Pacific Rim
Oceanic Island Arcs
Form at oceanic-oceanic convergent plate boundaries
Continental island arcs
Form at oceanic-continental convergent plate boundaries
Assimilation
Occurs when the temperature of rocks exceeds the melting temperature of silicate rocks at that depth
This heat melts adjacent rocks which then melt and change the composition of the melting magma, a process called assimilation
Composition of Magma
Magmas are composed of melted silicate minerals (SiO2; referred to as silica) and dissolved gases
Differentiated on the basis of how much silica the magma contains (see next slide)
Viscosity of Magma (Resistance to flow)
Stickiness factor: low viscosity = high fluidity
Mafic magmas have lower viscosity (higher fluidity) due to their chemical composition (less silica) than felsic magmas (in which silica tends to form strongly bonded chains)
Differences in viscosity influence the mobility of the magma when it is erupted onto the surface as well as the style of the eruption (effusive vs explosive)
Basaltic magma
Basaltic magma: 1000 to 1200° Celsius
Basaltic lava has a low viscosity when it first is erupted onto the Earth’s surface; as the lava cools away from the vent, its viscosity increases
Andesitic Magma
Andesitic magma: 800 to 1000° Celsius
Rhyolitic Magma
Rhyolitic magma: 650 to 800° Celsius
Gases of Magma
The percentage and type of volatiles within a magma influence its buoyancy and explosivity
The main volcanic gases are H2O (water vapor) and CO2 (carbon dioxide)
The volatile content of magma increases with corresponding increases in silica content
andesitic and rhyolitic magmas are more prone to explosive eruptions because they contain more dissolved gas (2-5 wt%) than basaltic magmas (<1 wt%)
Volatiles
Dissolved gases
Magma chamber or reservoir
large underground pool of magma
Country rock
surrounding rock that may be heated and mix with the magma
Dike/Sill
conduits along which magma reaches surface