QUIZ LAB 7: VOLCANOES & MAGMA VISCOSITY
GEOL110 Physical Geology
NAME________________________
1. (8 points) Fill in the blanks.
A lava containing more phenocrysts is ________________ (more or less?) viscous than a
lava containing little or no phenocrysts.
A lava containing a lot of water is __________________(more or less?) viscous than a
lava containing little or no water.
A lava containing more vesicles is ________________ (more or less?) viscous than a lava
containing little or no vesicles.
A lava at high temperatures (hotter) is ________________ (more or less?) viscous than a
lava that is relatively cool.
2. (3 points) During a volcanic eruption, magma is depressurized as it moves from a deep magma
chamber through a conduit to the surface of the Earth. Many changes can occur simultaneously and affect
other magma properties during such an eruption process. Based on what you learned in lab last week,
which of the magmas listed below is most likely to actually make it to the surface and cause an explosive
eruption (A or B) ? Why?
Both magmas erupt at the same temperature, and are the same composition (a rhyolite lava).
Choice A: A magma containing a large (8%) amount of water. Vesicles form as some of the water comes
out of the magma, making steam bubbles. The high water content of the magma prevents phenocrysts
from forming.
Choice B: A magma containing low water content (1%). A few vesicles form, and many phenocrysts grow.
3A. (2 points) A typical rhyolite has a viscosity that is most like which food? (circle one)
Maple syrup
Peanut butter
Water
Ketchup
Molasses
3B. (2 points) A typical basalt has a viscosity that is most like which food?
Maple syrup
Peanut butter
Water
Ketchup
Molasses