LIFESCI 3C03 Lecture Notes - Asthenosphere, Aluminium Oxide, Dacite
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Chapter 3 – Volcanoes
3.1 Introduction to Volcanoes
Directly related to plate tectonics and most are located near plate boundaries.
Mid-ocean ridges and subduction zones are sites where molten rock, or magma, reaches the
2/3rd of all active volcanoes on land are located along the Ring of Fire, which surrounds the
Pacific Ocean (see figure 3.2).
Size, shape and behavior of volcanoes are closely related to their plate tectonic setting and to
their magma chemistry and gas content.
Magmas that form volcanoes mainly contain 8 elements: oxygen (O), silicon (Si), aluminum (Al),
iron (Fe), magnesium (Mg), calcium (Ca), sodium (Na), and potassium (K). oxygen and silicon are
the 2 most abundant
Different shapes and eruptive styles are formed depending on the chemistry and viscosity
(fluidity) of magmas. (cold honey doesn’t flow much so it has a high viscosity, warm honey flows
faster, therefore has a low viscosity)
Magma with high silica contents are cooler, more viscous, and have more dissolved gases than
mamas with relatively low silica contents.
Some examples of high-viscosity magmas include dacite and rhyolite. Low-viscosity magmas:
basalt and andesite
Rapid degassing of high-viscosity mamas triggers explosive eruption
Largest and tallest volcanoes in the world; gently sloping slides and broad summits
Generally have non-explosive eruptions of very hot, low viscosity, basaltic magmas.
Basaltic magmas don’t go through fractionation, a process that involves crystallization
of different minerals in a slowly cooling magma. As fractionation proceeds, the
remaining magma becomes progressively enriched in silica, alumina and gases. This
process basically changes the magma type (basalt/andesite etc)
Consist entirely of lava flow (erupts through the volcano vents) but tephra, which are
fragmented debris, can also be erupted. Accumulation of tephra is known as pyroclastic
Lava can flow many kilometers away from a vent through underground lava tubes
Composite Volcanoes aka stratovolcanoes
Erupt less frequently but are explosive (doesn’t just flow)
Lavas are relatively silica rich and viscous, and thus they rarely flow more than a few
kilometers from vents
Responsible for most of the deaths and destructions caused by volcanoes
Volcanic Domes are Steep-sided mounds of lava that form around vents from the eruption of
highly viscous, silica-rich magmas.
Cinder Cones aka scoria cones
Relatively small volcanoes made of nut- to boulder-sized pieces of red or black basalt.
Tephra from extinct cinder cones is the “lava rock” widely used in landscape
Found on the flanks of large volcanoes
Maars are produced during the violent interaction of magma and groundwater
Ice-contact Volcanoes (underwater volcanoes) aka tuyas
Usually melt large quantities of ice and thus producing high outburst floods termed
Pillows formed due to rapid cooling of lavas
most of the time, the eruption melts through the ice above, producing lava flows that
covered the pillow breccias
steep-sided flat-top volcanoes sometimes called tuyas
Volcanic Features (craters, calderas, volcanic vents, geysers and hot springs)
Craters are depressions at the tops of volcanoes that are formed by explosion or
collapse of the summit area. Couple of kilometers in diameter
Calderas are circular/oval depressions up to a few kilometers in diameter that form
during explosive ejections of magma and subsequent collapse (large craters). caldera-
forming eruptions rare but large and most deadly
Produced during the violent interaction of magma and groundwater
Volcanic vents are openings through which lava are pyroclastic debris erupt. Most
circular, others are elongated cracks called fissures. Extensive fissue eruptions have
produced huge accumulations of nearly horizontal lava flows called flood basalts
Hot springs: groundwater becomes heated when it comes into contact with hot rock.
The water, now hot, discharges as surface as hot springs
Geysers are created as groundwater boils in an underground chamber to produce
periodic, steam-driven releases of steam and hot water.
Resurgent Calderas are extremely violent are extremely large and violent calderas.
Around 10 of these have erupted during the last million years.
Oceanic crust, which is denser than continental crust and commonly covered by
thick, wet sediments, is carried into Earth’s mantle. Rising heat and pressure rey
out the subduction crust in a process called dehydration. The water expelled
from the descending crust rises and changes the chemical composition of the
overlying mantle. The presence of water lowers the melting temperature of the
overlying mantle rocks and causes them to melt, forming magma that rises
through the crust to erupt on the surface
Composite volcanoes (stratovolcanoes) occur at subduction zones
Plates moving away from one another
Occurs when basaltic magma derived directly from the asthenosphere rises to
the ocean floor to create new crust. Magma that is created is low in viscosity
Hot spots under the oceans
A series of volcanoes that form as plates continue to move. These volcanoes are created
where hot mantle material wells up beneath a plate at a stationary point rather than at the
boundary between 2 plates.
Hot spots beneath continents are the caldera-forming eruptions in this tectonic setting.
They can be extremely violent and explosive
3.4 Hazards associated with Volcanoes
Primary effects include lava flows, ash falls, volcanic bombs, pyroclastic flows, pyroclastic
surges, lateral blasts, and poisonous gases
Secondary effects include lahars, debris avalanches other landslides groundwater and surface
water contamination, floods, fires and tsunamis
The Volcanic Explosive Index (VEI) quantifies the size of eruptions by taking into account the
volume of material erupted, height of the eruption cloud, duration of the main eruptive phase
and other parameters to assign a number on a linear scale from 0-9
Directly related to plate tectonics and most are located near plate boundaries. Mid-ocean ridges and subduction zones are sites where molten rock, or magma, reaches the surface. 2/3rd of all active volcanoes on land are located along the ring of fire, which surrounds the. Size, shape and behavior of volcanoes are closely related to their plate tectonic setting and to their magma chemistry and gas content. Magmas that form volcanoes mainly contain 8 elements: oxygen (o), silicon (si), aluminum (al), iron (fe), magnesium (mg), calcium (ca), sodium (na), and potassium (k). oxygen and silicon are the 2 most abundant. Different shapes and eruptive styles are formed depending on the chemistry and viscosity (fluidity) of magmas. (cold honey doesn"t flow much so it has a high viscosity, warm honey flows faster, therefore has a low viscosity) Magma with high silica contents are cooler, more viscous, and have more dissolved gases than mamas with relatively low silica contents.