Chap 5 Volcanoes and other igneous activity

Question 1

NATURE OF VOLCANIC ERUPTIONS:

Answer 1

Temperature: hotter magmas less viscous

Composition: higher silica = more viscous

Dissolved gases: gases expand as they near earth’s surface – violence related to how easily they
escape the magma.

Question 2

TYPES OF LAVA FLOWS:

Answer 2

1. Basaltic flows (thin and fluid mafic lava)
2. Andesitic flows (flow slowly around vent)

Question 3

Basaltic flows (thin and fluid mafic lava)

Answer 3

o Pahoehoe: lava spreading in sheets underground and a thin elastic shin congeals on
surface as it cools – as lava flows below surface, the skin drags and forms roped coils.

o Aa- lava: lost its gases (more viscous) therefore the thick ski breaks into jagged blocks
that ride on the viscous interior, piling up a steep front of angular blocks.

o Pillow lavas: basalt cools instantly underwater and forms blobs called pillow lava.

Question 4

Andesitic flows (flow slowly around vent)

Answer 4

o Rhyolitic: highest silica (most viscous), often plugs vent domes.

Question 5

Pyroclastic debris:

Answer 5

fragments blown out of volcanoes, including:

a) Ash : powdery glass shards

b) Pumice : porous rock from frothy lava

c) Lapilli : pea-to-plum sized material

d) Blocks : pre-existing rock torn from volcano

e) Bombs : streamlined fragments of ejected lava

f) Tephra : deposits of pyroclastic debris
– can be agglomerate (lailli and bombs
near the vent) or air fall tuff (accumulations of ash that fall like snow) or welded-tuff (tuff that is
deposited while hot)

g) Lahar : mudflow deposits of volcanic
ash that move rapidly with wet

Question 6

VOLCANIC GASES:

Answer 6

o Most abundant being: water, then carbon
dioxide then sulphur dioxide

o S02 + H20 = Aerosol sulphuric acid

o Low viscosity lavas (basalt) have easy gas
escape = mellow eruptions

o High viscosity (rhyolite) have difficult gas
escape = violent eruptions

Question 7

VOLCANIC TYPES:

Answer 7

1. SHIELD VOLCANO
2. COMPOSITE CONE
3. CINDER CONE

Question 8

SHIELD VOLCANO:

Answer 8

• Broad, slightly dome shaped
• Largest and composed of basaltic lava
• Mild eruptions of large volumes of lava

Question 9

COMPOSITE CONE:

Answer 9

• Large, classic shaped
• Composed of layers of lava flows and
  pyroclastic debris
• Most violent volcanic activity
• My produce pyroclastic flow and lahar
• Most in Pacific Ocean

Question 10

CINDER CONE:

Answer 10

• Built from ejected lava fragments
• Steep slope, small size
• Occur in groups