Lecture Thirty - Volcanoes Flashcards
What are the different kinds of rocks that come from volcanoes?
Silicate composition:
Kormatiite - ‘Ultramafic’ = high percentage Fe and Mg-rich (dark) minerals, very low percentage of SiO2.
Basalt - ‘Mafic’ = low percentage SiO2.
Andersite - Intermediate.
Rhyolite - ‘Felsic’ = low percentage of De and Mg-rich minerals, high percentage light coloured minerals, high percentage SiO2.
What eight minerals are igneous rocks mainly made out of?
Describe Komatiite.
Ultramafic.
Very high magnesium and iron content.
Very low silica content (SiO2).
Very unusual rocks.
Very low viscosity (high viscosity = slow flow, low viscosity = high flow).
Volcanic equivilent of periodotite.
Describe Basalt.
Mafic composition.
Magnesium, ironss, low silica, dark colour, low viscosity.
Volcanic equivilent of gabbro.
Describe Andersite.
Intermediate composition.
Plageoclase Feldspar.
Amphibole.
Mixture of light and dark minerals.
Intermediate viscosity.
Volcanic equivilent of diorite.
Describe Rhyolite.
Felsic composition.
Feldspar.
Silica (quartz).
Ligth colour minerals.
High viscosity.
Volcanic equivilent of granite.
Where do volcanoes occur?
Plate boundaries, intra-plate volcanic provinces and hot spots.
Divergent plate boundaries - mid oceans ridges (mafic) and continental rift zones.
Convergent plate boundaries - island arcs and continental arcs caused by subduction.
Do not occur at transform plate boundaries.
What are the different types (shapes) of volcanoes?
Essentially, the more runny the lava flow, the more spread out and flat the volcanoe will be.
What controls the shape and size of volcanoes?
Nature of material that makes up the volcano.
- Lava vs. tephra (explosive volcanic products).
- Gas content.
- Composition - determines whether you get an effusive (lava flow) = low viscosity = low Si, or explosive eruption = high viscosity = high Si.
- Original amount of volatiles within a magma.
- What magma comes into contact with - water + subsurface magma = explosion.
What is the anatomy of a volcano (diagram)?
How does Caldera or Maar formation work?
What are the two kinds of magmatic eruptions?
Effusive:
- Lava flow.
- Low viscosity magma produces londer flows.
- Low gas pressure in the magma.
Explosive (pyroclastic):
- Clouds and avalanches of pyroclastic debris.
- Basalt lava, e.g. pahoehoe, Aa and Pillow lava.
- Andersite lava.
- Rhyolite lava - Hazard: move slowly, but very hot.
What is magma?
Hot fluid or semi-fluid rock below or within the earth’s crust from which lava and other igneous rock is formed on cooling.
+/- crystals.
+/- bubbles = preserved in volcanic rocks.
What causes explosive eruptions?
Gas pressure:
- magmatic.
- Phreatomagmatic (the gas comes from water outside of the magma - lake, groundwater, etc.).
Forms ‘pyroclastics’ = broken by fire.
What are pyroclastic deposits?
Two types of deposits:
1) Pyroclastic fall deposits.
2) Pyroclastic flow deposits.
Pyroclastic fall deposits:
Hot gas, fragments of ash, pumice, and rock debris are carried up into the erruption collumn.
Dispersed by the wind.
Ash falls out, blanketing the landscape.
Hazard to aeroplanes, vehicles, crops and the climate.
Pyroclastic deposits are fragmented, transported and deposited by pyroclastic (i.e. explosive) processes.
Pyroclastic flow deposits:
When an eruption collumn becomes overloaded with solids part or all of the debris will collapse, and flow downslope, producing pyroclastic flows.
Turbulent, ground hugging mixtures of gas, ash and rock debris.
Pyroclastic flows may travel 100’s of km/h.
