Week 6

Question 1

What are shield volcanoes

Answer 1

• Low aspect ratio due to the fluidity of lava
• non-explosive eruptions are common
• broad and gentle slopes
  e.g Mauna Loa

Question 2

What is the magma type for shield volcanoes

Answer 2

• lavas that erupt in Hawaii are basaltic and contain about 50% silica
• They erupt at about 1200 degrees c
• very fluid and pasty
• effusive
• low silica
• low viscosity

Question 3

What is the eruption type for shield volcanoes

Answer 3

Hawaiian type eruption

Question 4

Hawaiian type eruption

Answer 4

• Lava is very fluid and hot
  -Bubbles can expand and escape easily
• relatively gentle
• low energy eruption

Question 5

What are stratovolcanoes

Answer 5

• Higher aspect ratio because of lava viscosity
• Explosive and non explosive eruptions
• e.g Mt rainier: subduction zone volcano

Question 6

What is the magma type for stratovolcanoes

Answer 6

• most lavas that erupt on volcanoes like mt st helens are closer to rhyolitic and are about 65 to 75% silica
• they erupt at about 900 degrees and are pretty stiff
• high viscosity
• high silica magma

Question 7

What is the eruption type for stratovolcanoes

Answer 7

Plinian type eruption

Question 8

Plinian type eruption

Answer 8

• Lava is sticky and cooler
• Bubbles can’t expand or escape easily
• Explosive
• starts with glowing pyroclastic flows
• forms lava domes with steep sides

Question 9

Characteristics of Plinian type eruptions

Answer 9

• Ash and smoke column that can extend to stratosphere
• large amounts of pumice
• powerful gas blasts
• large amounts of magma erupted
• caldera formation possible

Question 10

What are the three parts of a plinian column

Answer 10

• lower kilometer or two is the gas thrust region
• the next few kilometers is the convective region
• the upper part is the umbrella region

Question 11

What is calderas (ultra-plinian)

Answer 11

• formed by very voluminous and explosive eruption

Question 12

What are the key characteristics of calderas

Answer 12

• caldera formation
• large ejecta volumes
• high energy release
• massive plumes
• magma is rhyolitic

Question 13

What are cinder cones

Answer 13

• smallest in size
• e.g sunset crater, flagstaff , Arizona

Question 14

Comparison of shield and stratovolcanoes

Answer 14

Shield volcanoes are far larger than stratovolcanoes

Question 15

What are igneous rocks

Answer 15

-formed from the crystallization of magma
-extrusive: fine grained and quickly cooled
-intrusive: coarse grained and slowly cooled

Question 16

Classification of Igneous rocks

Answer 16

• mineral composition
• grain size
• texture

Question 17

Chemistry of silica

Answer 17

• magmas range from about 50 to 75% silica
• the more silica the stiffer the magma
• the less silica, the more fluid the magma
  -“basalt” is lava that is about 50% silica
  -“rhyolite” is lava that is about 75% silica

Question 18

Chemistry of water

Answer 18

• the more water, the more explosive especially for rhyolites
• the effect of water on the stiffness of magma
• high water content and high silica content makes magma erupt more explosively than a low water and low silica magma

Question 19

What are the three kinds of pyroclastic deposits

Answer 19

1. Pyroclastic fall/ airfall
2. Pyroclastic surges
3. Pyroclastic flows

Question 20

Pyroclastic fall/airfall

Answer 20

fallout from the plinian column blankets the ground with a nearly uniform mantle of fine particles

Question 21

Pyroclastic surges

Answer 21

clouds of debris-laden gas jetting out of the volcano and racing over the countryside form layers of irregularly bedded ash and blocks

Question 22

Pyroclastic flow

Answer 22

dense slurries of debris racing down the slops of the volcano deposit, which concentrate in valleys