Chapter 10

Question 1

Magma

Answer 1

Molten rock or liquid composed of mostly silica under the surface of Earth.

Question 2

Pyroclasts

Answer 2

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)

Question 3

Lava

Answer 3

Magma that is on the surface of Earth.

Extrudes out of fissures in the ground as lava flows.

Question 4

Extrusive Rock

Answer 4

Collective description of the surface rock formed by volcanic activity (pyroclastic debris and rock formed by solidification of lava).

Question 5

Volcano

Answer 5

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.

Question 6

Viscosity

Answer 6

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.

Question 7

Volcanic Explosivity Index

Answer 7

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.

Question 8

Felsic (Silicic) Rocks

Answer 8

Are silica rich (65% or more SiO2).

Ex: Rhyolite(most abundant)

Question 9

Mafic Rocks

Answer 9

Silica deficient rocks (close to 50%).

Ex: Basalt (most abundant).

Question 10

Intermediate Rocks

Answer 10

Have a chemical content between that of felsic and mafic rocks.

Ex: Andesite(most abundant)

Question 11

Pyroclastic Flow

Answer 11

Mixture of gas and pyroclastic debris that is so dense that it hugs the ground as kt flows rapidly into low altitude areas.

Question 12

Texture

Answer 12

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.

Question 13

Fine-grained Rock

Answer 13

Rock in which most of the mineral grains are smaller than 1 millimeter. (Grains only distinguishable with microscope).

Question 14

Obsidian

Answer 14

Volcanic glass that is usually silicic.

Extrusive rock due to glassy texture.

One of the few rocks that is NOT composed of minerals.

Question 15

Porphyritic Rock

Answer 15

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…

Question 16

Vesicles

Answer 16

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.

Question 17

Flank Eruption of Volcano

Answer 17

Lava pours from a vent on the side of a volcano.

Question 18

Caldera

Answer 18

A volcanic depression much larger than the original volcanic crater (diameter of 1km minimum)

Question 19

Shield Volcano

Answer 19

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.

Question 20

Cinder Cone

Answer 20

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).

Question 21

Composite Volcano (Stratovolcano)

Answer 21

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.

Question 22

Volcanic Domes

Answer 22

Steep-sided, dome-shaped, spine-shaped masses of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent.

Question 23

Plateau Basalts

Answer 23

Produced during the geologic past by vast outpourings of lava from fissures.

Question 24

Columnar Jointing

Answer 24

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).

Question 25

Pillow Basalt

Answer 25

Rocks, generally basalt, occurring as pillow shaped, rounded masses that are closely fitted together.

Usually formed when basalt erupts underwater.

Question 26

Compare the hazards of lava flows to those of pyroclastic flows.

Answer 26

Pyroclastic flows are a mix of gad and debris that is so dense that it hugs the ground as it flows rapidly into lower areas. These are much faster and thus more dangerous than lava flows.

Question 27

What roles do gases play in volcanism?

Answer 27

Gases provide the eruptive force for eruptions in volcanism. Also they make lava less viscous so it can flow faster.

Question 28

Name the minerals and percentage of each that you would expect to be present in each of the following rocks::
Andesite
Rhyolite
Basalt

Answer 28

Rhyolite: 30% each of quartz, plagioclase and alkali feldspar, 10% each of muscovite, biotite.

Basalt: 60% plagioclase, 30% pyroxene, 10% olivine.

Andesite: 60% plagioclase, 20% amphibole, 20% biotite.

Question 29

What determines the viscosity of lava?

Answer 29

Temperature:
Colder means more viscous (thicker)
Hotter means less viscous (thinner)

Composition:
Felsic=sticky (more silicon)
Mafic=fluid

Question 30

What determines whether a series of volcanic eruptions builds a shield volcano, a composite volcano, or a cinder cone? Describe each type of volcanic cone.

Answer 30

Shield:
Largest
Shallow slope
Low viscosity

Composite:
Mid size
Steep slope
Composed of mix of lava and pyroclastic flows
Mostly from intermediate lava
Can be either quiet flows or major eruptions.

Cinder Cone: 
Smallest
Steepest slope
All pyroclastic 
Any composition, but late stage mafic is common.

Question 31

Explain how a vesicular porphyritic andesite might have formed…

Answer 31

Intermediate magma began to cool underground erupted to the surface and solidified before the gas could escape, creating vesicles throughout the rock.