Volcanism and plutonism, Lectures 6,7,8 Flashcards
Igneous rocks
Material products of eruptions
Form when magma cools and solidifies
Magma
Material products of eruptions
molten rock below the Earth’s surface
Lava
Material products of eruptions
Molten rock at the Earth’s surface. Magma erupts as lava flows and pyroclastic material.
Example of a volcano with lava flows
E15, Iceland 2010 eruption lava fountaining and lava flows no flight distruption due to little ash tourist attraction
Pyroclasts
Material products of eruptions
Form when magma is explosively ejected from a volcano as fragments with volcanic gases.
Pyroclastic rocks
Material products of eruptions
Form when pyroclastic sediments consolidate
Three classifications of pyroclasts
ash
lapilli
volcanic bombs
Ash
<2mm diameter
Lapilli
pyroclast classification
2-64mm diameter
little stones in Latin
Volcanic bombs (pyroclast classification)
> 64mm diameter
fragemenrs hurled from the vent which land close (normally within 100s metres) of the vent, can be house sized!
Four components of ash
Material products of eruptions
pumice/scoria
lithics
crystals
glass shards
Pumice/scoria
components of ash
fragments of vesicular lava
Lithics
components of ash
rock fragments
Crystals
components of ash
individual and broken
Glass shards
components of ash
chilled magma, vesicle walls
Pumice structure
spherical vesicles
Tube pumice structure
elongate vesicles
Components of lapilli
2
Pumice/scoria
lithics
Pumice
Light coloured vesicular igneous rock which forms through rapid solification of melt.
What causes the structure of pumice?
gas trapped in the melt at the time solidification
Processes and deposits of eruptions
6
pyroclastic fall pyroclastic fall deposit pyroclastic density current (aka pyroclastic flow) lahar debris avalanche phreatic eruption lava flow
Material products of eruptions
lava
pyroclasts
pyroclastic rocks
Pyroclastic fall
sedimentation of particles from a volcanic plume
What produces a pyroclastic fall deposit
pyroclastic fall
Pyroclastic density current/pyroclastic flow
gravity flow of hot gases and particles which flow down the flanks of a volcano
What produces a pyroclastic density current deposit?
Pyroclastic density current/pyroclastic flow
Pyroclastic density current deposit
material deposited by a pyroclastic density current/pyroclastic flow in the valleys
What can a pyroclastic density current/pyroclastic flow result from?
the collapse of a lava dome
Lahar
flow of mixture of water and volcanic material - a volcanic mudflow
Lahar produces
a lahar deposit
Debris avalanche
Flow of landslide material down the flanks of a volcano
Debris avalanche produces a
Debris avalanche deposit
Phreatic eruption
steam eruption
Does a phreatic eruption produce a deposit/ its name
no deposit produced!!
Lava flow
Flow of hot lava down the flanks of a volcano
Lava flow produces a…
lava
Three techniques to find information about igneous intrusions
- fieldwork
- geophysical surveying
- underground mapping
Pluton
and its spatial scale
connected intrusions
a few to 10s of km
Batholith
and its spatial scale
larger connection of intrusions than a pluton
up to 100km long and 100s of km across
Dike
when magma pushes up through older rocks and forms a vertical rock column
(discordant)
Sill
flat intrusion of igneous rock that forms between pre-existing layers of rock (but not through one layer)
Example of a batholith
the Cornubian granite,
Cornwall,
a solidifed magma chamber that crystallised 300Ma ago
Example of a pluton
Ross of Mull,
Isle of Mull, Scotland,
crystallised 418 Ma ago
Three ways igneous rocks are classified
- silica content
- grain size
- proportions of minerals
Classifying igneous rocks
Silica content
weight % silica
Classifying igneous rocks
grain size
fine-grained, volcanic, extrusive
coarse-grained, plutonic, intrusive
Plutonic rock
not been above the surface,
allows time for crystals to develop
Classifying igneous rocks
proportions of minerals
explanatory
Types of magma
4
(table in notes and lab book)
Ultramafic
Mafic
Intermediate
Felsic
Is there just one or many types of granite?
many types
Two types of lava (surface characteristics)
Aa
Pahoehoe
Aa lava
(blocky lava)
Irregular scoriaceous fragments; form during cooling at high flow rates due to internal shearing
Pahoehoe lava
(ropy lava)
patterns of ropes and whorls; form when lava cools at low flow rates
Felsic
made up of
feldspar
silica
Mafic
made up of
Magnesium
ferric (iron)
Volcanic geomorphological features
7
- calderas and craters
- composite volcanoes
- lava domes
- shield volcanoes
- fissures
- cinder cones
- hot springs, geysers and fumeroles
Caldera
Topographic depression greater than 1km in diameter
Crater
Topographic depression less than 1km in diameter
Example of caldera
Sollipulli, Chile ,
ice-filled caldera,
4km diameter
Formation of calderas
- magma chamber filled with fresh magma triggers an eruption of lava and ash
- the magma chamber becomes partially depleted as eruptions of lava and pyroclastic flows continue
- the summit collapses into the chamber which is now empty, pyroclastic flows blanket the collapse
- a lake forms in the caldera, hot springs and geysers exist as a minor form of volcanic activity
Another word for composite volcanoes
stratovolcanoes
Composite volcanoes
conical, steep profile, many strata, periodic explosive and effusive eruptions, intermediate-felsic composition
Example of a composite volcano
Mount Mayon, Philippines
Lava domes
- viscous intermediate- felsic composition
- common/dangerous feature of composite volcanoes
- material blocks the column of the volcano, can overspill the crater walls and go down the volcano flanks
- ^ plugs the central vent, often occurs before major explosive events
Examples of lava domes
2
Volcan de Colima, Mexico
Mount St Helens, USA
Shield volcanoes
- low, rounded profiles
- composed of low viscosity, mafic lava flows: basalt
- magma can seep out of the flanks of the volcano
Example of shield volcanoes
Mauna Loa, Hawaii, USA
Fissures
Mafic lava flows rapidly away from fissures, rather than constructing a volcanic mountain
Example of a fissure
Kilauea, Hawaii, USA
Cinder cones
- steep, conical, symmetrical peaks <400m high with a summit crater
- composed mainly of mafic pyroclastic material (scoria)
- formed by explosive eruptions or lava fountains
- common at fissures and composite volcanoes
Hot springs
produced by the emergence of geothermally heated groundwater
Geyser
a vent that periodically ejects a column of hot water and steam
Example of a geyser
Old Faithful, Yellowstone national park, Wyoming, USA
Fumaroles
Volcanic vents that emit gases
