lecture 8 - volcanoes Flashcards
volcanoes
most located near plate boundaries
approx 65% located along the ring of fire, surrounding the pacific ocean
subduction zones and mid ocean ridges allow molten rock to reach the surface
2 types of molten rock and where are they found
magma - found deep within crust and upper mantle
lava - found flowing from an erupted volcano
–> lava is magma on the earths surface
what is magma
The most abundant elements in magma are silicon and oxygen; when combined it is referred to as silica.
Volcanic rocks are named based on the amount of silica present.
Types of volcanic rocks:
basalt, andesite, dacite, rhyolite
Low silica <—————-> high silica
Viscosity:
Definition:
The resistance to flow
An everyday liquid that has a high viscosity: syrup
Magma also contains small amounts of gases (water vapour and carbon dioxide).
Volcanoes have different shapes based on the chemistry and viscosity of their magma.
Magma viscosity is determined by silica content and temperature.
Magma and silica content
Magma with high silica content:
Cooler, more viscous, more gases
Magma with low silica content:
Hotter, less viscous, fewer gases
Volcanoes with high silica magma produce the most explosive eruptions.
As magma approaches the surface, the pressure lowers allowing gases to bubble up and escape.
This is a clue of a pending eruption.
magma
Rhyolitic and dacitic magmas produce explosive eruptions.
Basaltic and andesitic magmas produce eruptions that tend to flow rather than explode.
Cooler temperatures: not as much rock, not as runny or not as much liquid.
4 types of volcanoes
The classification is based on their shape, appearance, and style of eruptions.
Types of volcanoes:
Shield
Composite
Volcanic dome
Cinder cone
Shield volcanoes:
These are the largest volcanoes on Earth and are shaped as broad arcs (like warrior shields) built from lava.
Very liquid like but it hardens in that shape
They are associated with basaltic magma
Eruptions are non-explosive and consist of gentle flows.
Some eruptions can contain tephra.
Accumulations of tephra are referred to as pyroclastic deposits.
If compacted together, these deposits are called pyroclastic rocks.
These volcanoes are common in Hawaii, Iceland and around the Indian Ocean.
tephra
fragmented material blown out during an eruption.
Composite volcanoes:
These volcanoes are cone-shaped and are built from a combination of lava flows and pyroclastic deposits.
Typical thinking when we think of a volcano.
They are also called stratovolcanoes; this term comes from the layers of lava and deposits.
Composite volcanoes:
Eruptions are more dangerous and explosive but less frequent than shield volcanoes.
These volcanoes are common along the west coast from Alaska to Northern California.
Volcanic domes:
These volcanoes contain highly viscous rhyolite magma.
They are steep-sided mounts that form around vents.
Maars:
Definition:
A circular volcanic crater produced by an explosive eruption and filled with water.
They are caused by groundwater encountering magma creating an explosion.
Maar derives from the Latin mare meaning sea and resembles a large lake
Cinder cone volcanoes:
These are relatively small volcanoes composed of small pieces of tephra.
Small, found in Mexico or central America
They are round to oval-shaped and typically contain a crater at the top.
These volcanoes are found in Mexico.
Caldera:
A circular to oval depression formed during the collapse of a volcano.
They can be up to 25km in diameter.
Eruptions that form calderas are the largest and most deadly eruptions on Earth.