Eruptive Stage and Composition of Rocks in Hawaii (Grades 9-12)
Two types of basalt common in Hawaii can be distinguished, based on
their alkali element (Na2O+K2O) content at the same silica (SiO2)
content. Tholeiitic basalts have low alkali element contents relative to
alkali basalts at the same silica content. Each type of basalt can
differentiate (or evolve) to a higher silica content through such
processes as settling of olivine crystals or mixing with more evolved
magmas. The relationship between eruptive stage and composition of rocks
from specific volcanoes is explored in this activity. A plot of silica
versus alkali element content is shown in activity 9. The plot is from
Peterson and Moore (1987) and represents all the volcanoes of the Island
of Hawaii (and Loihi).
Click
here
for the handout for activity 9.
Students must interpret the plot to answer the following questions:
- Which volcanoes are tholeiitic? Mauna Loa and Kilauea have
tholeiitic
rocks.
A few rocks from Kohala and Hualalai are also
tholeiitic.
- Which volcanoes are alkalic?
Hualalai, Mauna Kea,
and Kohala have alkalic rocks.
- Is Loihi tholeiitic or alkalic?
Loihi has both
alkalic and tholeiitic rocks.
- Which volcano(es) is (are) in the preshield eruptive
stage?
Loihi is in the preshield eruptive stage.
- Which volcano(es) is (are) in the shield eruptive
stage?
Kilauea and Mauna Loa are in the shield eruptive stage.
- Which volcano(es) is (are) in the postshield eruptive
stage?
Hualalai, Mauna Kea, and Kohala are in the postshield eruptive stage.
- Which eruptive stages are characterized by low eruption rates
and low eruption volumes?
Preshield and postshield eruptive
stages are characterized by low eruption rates and low eruption volumes.
- Which eruptive stage is characterized by high eruption rates and
high
eruption volumes?
The shield eruptive stage is
characterized by high eruption rates and high eruption volumes.
- Based on these observations, outline the relationships between
eruptive stage, eruptive rate, eruption volume, and chemical composition
of Hawaiian eruptive products?
During the preshield and
postshield
eruptive stages, alkalic lavas are erupted at low rates and volumes.
During the shield eruptive stage, tholeiitic lavas are erupted at high
rates and high volumes.
- How does the position of the volcano, relative to the hot spot,
relate to eruptive stage and chemical composition of eruptive
products?
A volcano on the southeast edge of the hot spot, like Loihi, is in the
preshield eruptive stage and erupts low volumes of alkalic lava at low
rates. Shield volcanoes, like Kilauea and Mauna Loa, are directly above
the hot spot and erupt large volumes of tholeiitic lava at high rates.
Hualalai and Mauna Kea have passed beyond the hot spot and erupt low
volumes of alkalic lava at low rates.
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