Chapter 10 Flashcards
Magma
Molten rock or liquid composed of mostly silica under the surface of Earth.
Pyroclasts
Rock fragments formed when magma solidifies quickly as it explodes by gases into the air.
Pyroclastic debris aka Tephra
Size-based names:
Dust (< 1/8 mm)
Ash (1/8 to 2 mm)
Cinder or Lapilli (2 to 64 mm)
Blocks or Bombs (> 64 mm)
Lava
Magma that is on the surface of Earth.
Extrudes out of fissures in the ground as lava flows.
Extrusive Rock
Collective description of the surface rock formed by volcanic activity (pyroclastic debris and rock formed by solidification of lava).
Volcano
A hill or mountain formed by the extrusion of lava or ejection of rock fragments from a vent.
Volcanos not the only “volcanic” landforms… lava can flow out of Earth, floods an area, and solidifies into a horizontal layer of extrusive rock.
If these layers build up, they can form a lava plateau.
Viscosity
Resistance to flow of lava.. determines how easily the gases escape.
More viscous the lava, the greater the volume of gas trying to escape, the more violent the eruption.
Three factors influence viscosity:
(1) Silica SiO2 content of the lava.
(2) Temperature of the lava.
(3) Gases dissolved in magma.
Greater the dissolved gas content, the more fluid the lava.
Lava solidifies between 700 degrees Celsius for silica rocks, and 1200 degrees C for mafic rocks.
Volcanic Explosivity Index
Indicates how powerful volcanic eruptions are.
On a scale of 0-8 and based on number of factors including: volume of erupted pyroclastic material, height of eruption column, and length of eruption time.
Felsic (Silicic) Rocks
Are silica rich (65% or more SiO2).
Ex: Rhyolite(most abundant)
Mafic Rocks
Silica deficient rocks (close to 50%).
Ex: Basalt (most abundant).
Intermediate Rocks
Have a chemical content between that of felsic and mafic rocks.
Ex: Andesite(most abundant)
Pyroclastic Flow
Mixture of gas and pyroclastic debris that is so dense that it hugs the ground as kt flows rapidly into low altitude areas.
Texture
Refers to a rock’s appearance with respect to size, shape, and arrangement of its grains or constituents.
Grain size is rock’s most important textural characteristic.
Extrusive rocks for most part are fine grained, or else made of glass.
Fine-grained Rock
Rock in which most of the mineral grains are smaller than 1 millimeter. (Grains only distinguishable with microscope).
Obsidian
Volcanic glass that is usually silicic.
Extrusive rock due to glassy texture.
One of the few rocks that is NOT composed of minerals.
Porphyritic Rock
Rock in which larger crystals are enclosed in a ground-mass of much finer-grained minerals or obsidian.
Larger crystals are called phenocrysts.
Porphyritic texture indicates two stages of solidification…
Vesicles
Cavities in extrusive rock resulting from gas bubbles that were in the lava as it solidified.
Ex: Scoria - a highly vesicular basalt.
Pumice - frothy glass with so much vesicular space that it floats in water. Powdered pumice can scratch metal and glass.
Flank Eruption of Volcano
Lava pours from a vent on the side of a volcano.
Caldera
A volcanic depression much larger than the original volcanic crater (diameter of 1km minimum)
Shield Volcano
Broad, gentle slopes (between 2degrees- 10degrees from horizontal) constructed of solidified lava flows.
During eruption, Lava spreads widely and thinly due to low viscosity.
Composed of basalt, layers of solidified lava flows.
Cinder Cone
A volcano constructed of pyroclastic fragments ejected from a central vent.
Formed exclusively of pyroclastic material accumulating around vent, unlike shield volcanos(lava flows). Form because of buildup of gases, independent of composition.
Commonly have slopes of about 30 degrees. And are much smaller than shield volcanos (~500m).
Composite Volcano (Stratovolcano)
Volcano constructed of alternating layers of pyroclastic fragments and solidified lava flows.
Long lives=large size
Slopes are intermediate steepness compared to cinder cones and shield volcanos.
Built over long periods of time.
Eruptions are intermittent with long periods of dormancy, followed by few years of intense activity.
Less vulnerable to erosion than cinder cones due to solidified lava acting as a protective cover over the loose pyroclastic layers.
Volcanic Domes
Steep-sided, dome-shaped, spine-shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent.
Plateau Basalts
Produced during the geologic past by vast outpourings of lava from fissures.
Columnar Jointing
Basalt layers, instead of stacked up slabs or tablets of solid, unbroken rock, the individual layers may appear to be formed of parallel, vertical columns(6-sided).
