Chapter 4 Volcanism

what is a caldera?

a volcanic crater that is more than 2km in diameter.

where are most volcanic craters formed?

above magma chambers

Tectonic setting: Composite Volcano

almost all are at subduction zones

what is a mantle plume

an ascending column of hot rock (not magma) that originates deep in the mantle

what is a volcano

any location where magma comes to the surface

In terms of area there is very likely more pillow ____ on the sea floor than any other type of rock on Earth

basalt

As magma approaches the surface the pressure exerted on it ___________

decreases

what magma is deep beneath the surface and under high pressure from the surrounding rocks, the gases remain ______

dissolved

what are the three main plate-tectonic settings where magma is formed?

divergent boundaries (decompression melting) convergent boundaries (flux melting) mantle plumes (decompression melting)

when is Tephra produced?

during explosive eruptions, and accumulates in the vicinity of cinder cones

An erruption that involves a steady non-violent flow of magma is called

effusive

because mantle plume and spreading-ridge magmas tend to be consistently mafic, ________ eruptions are the norm

effusive

subaqueous eruptions

eruptions on the ocean floor or under the water of a lake

subaerial eruptions

eruptions that take place on land

true/false - felsic magma allows gases to escape easily

false

true/flase - felsic magma flows easily

false

At subduction zones, where magma ascends through significant thickness of crust, interaction between magma and crustal rock leads to increases in ______ character of the magma

felsic

partial melting of country rock and country-rock xenoliths increases the overall _____ character of mama

felsic

this kind of magma tends to be more viscous because they have more silica

felsic magma

this kind of magma tends to have higher levels of volatiles; that is components that behave as gases during volcanic eruptions

felsic magma

If a magma is felsic, and therefore is too viscous for gases to escape easily, or if it has a high gas content, it is likely to be under ____ pressure

high

what are the two main causes of casualties in volcanic eruptions?

hot, fast moving pyroclastic flows and lahars

at subduction zones, the average magma composition is likely to be close to

intermediate

Pyroclastic eruptive material is/is not very strong

is not

large mudflows triggered by rapid melting of ice and snow on a volcano

lahars

Another name for Tephra

lava fragments

size: Cinder cone

small (10s to 100s of m) and steep (>20degrees)

Composite volcanoes are almost all associated with

subduction at plate boundaries

Pahoehoe is

lava that flows as non-viscous lava, that flows gently forming a skin that gels then wrinkles because of ongoing flow of the lava below the surface.

the rigid part of the mantle is called the

lithosphere

pillows are

little blobs resulting from hot lava oozing out into cold seawater

Cinder cones are made up almost exclusively of what? This is why they can relatively quickly and easily be eroded away

loose fragments

At divergent boundaries and oceanic mantle plumes, magma tends to be consistently

mafic

Mantle plume and spreading-ridge magmas tend to be consistently

mafic

Aa forms when

magma is forced to flow faster than it is abel to (down a slope for example)

Most shield volcanoes are associated with what?

mantle plumes

size: Composite volcano

medium (1000s of m) and moderate steepness (10-30 degree)

cinder cones that formed during a single eruptive phase are said to be

monogenetic

What is an example of Large Igneous Provinces

the Columbia River Basalt Group

best known shield volcanoes make up what

the Hawaiian Islands

size: Kimberlite

the remnants are typically 10s to 100s of m across

Because the rock that makes up Mt. St. Helen ranges in composition from rhyolite to basalt it can be inferred that what

the types of eruptions have varied widely in character

Tectonic setting: Kimberlite

upper-mantle sourced

tectonic setting: Cinder cone

various; some form on the flanks of larger volcanoes

Large igneous provinces is related to

very high volume but relatively short duration bursts of magma from mantle plumes

If the gas content is low or the magma is runny enough for fases to rise up through it and escape to the surface, the pressure will/will not become excessive

will not

size: sea-floor volcanism

Large areas of the sea floor associated with spreading ridges

Tectonic setting: Large igneous provinces

Associated with "super mantle plumes"

Tectonic setting: Sea-floor volcanism

Generally associated with spreading ridges but also with mantle plumes

size: Shield volcano

Large (up to several 1000m high and 200 km across) not steep (typically 2 to 10 degrees)

this kind of eruption produce lava flow

Mafic eruptions (and some intermediate eruptions)

What is the largest shield volcano on Earth?

Mauna Loa

Tectonic setting: Shield Volcano

Most are at mantle plumes; some are on spreading ridges

described as a "pile of junk" by Patrick Pringle

Mt. St. Helen

Largest LIP eruption on the seafloor

Ontong Java

what is the most abundant volatile in magma?

Water (H2O)

Aa is also known as

blocky lava

how is the level of the crater floor influenced?

by the amount of pressure exerted by the magma body

in a geological context, what kind of volcanoes tend to form relatively quickly and do not last very long?

composite

Mt. St. Helen is an example of

composite volcano

the two main types of textures created during effusive subaerial eruptions are

pahoehoe and aa

Under these conditions, hot lava oozes out not the cold seawater quickly cools on the outside and then behaves like toothpaste resulting in blobs known as

pillows

this kind of debris is produced when pressure builds up until a conduit opens and an explosive eruption results from the gas rich upper part of the magma chamber

pyroclastic debris

Rock that makes up Mt. St. Helen ranges in composition from ____ to _____

rhyolite & basalt

Pahoehoe is also known as

ropy