The Distribution of Tephra and the Location of Eruptions (Grades 7-8)
Aerial view of part of the area shown in the isopach map for this
activity. Photograph by J.D. Griggs, U.S. Geological Survey.
Geologists map the distribution and thickness of tephra in ancient
volcanic eruptions to locate the source of the eruption. The
distribution and thickness of tephra deposits is also mapped in Hawaii.
Isopachs are lines of equal thickness for a deposit of geologic
material. For most eruptions, the greatest thickness of tephra is near
the vent. The thickness of the deposit becomes progressively thinner
with increasing distance away from the vent. A geologist might make
several traverses across an area. At each location, she might locate
herself on the map and measure the thickness of the deposit. When she
returns to the office, she constructs an isopach map of the deposit. From
her map, she can interpret the location of the volcanic vent and the
factors that influenced the distribution of the tephra.
The purpose of this exercise is to have students understand
the concept of isopachs, to construct an isopach map for an eruption, to
locate the vent, and to suggest factors that influenced the distribution
of tephra. Activity 5 is a map of the summit area of Kilauea Volcano.
In historical time, eruptions have occurred around the margin and within
the caldera, at Halemaumau, at Kilauea Iki Crater, and at Keanakakoi
Crater. The numbers adjacent to each dot represent the thickness of
tephra at each location. Use the numbers to construct isopachs for the
deposit. To construct isopachs, simply draw a line through locations
with the same thickness (i.e., same number). The diagram is designed for
isopachs with a thickness of 0.1, 1, and 10 feet. Students may need to
extrapolate between some locations due to lack of data. Use the isopach
map to answer the following questions:
Click here for
the map and instructions for this activity.
- Was Halemaumau the vent for the tephra?
No. There is no tephra near Halemaumau.
- Was Puhimau or Kokoolau Crater the vent?
No. There is no tephra near Puhimau or Kokoolau Crater.
- Was Keanakakoi the vent for the tephra?
No. The tephra should be thickest close to the vent and thinner at
increasing distance away from the vent. At Keanakakoi, the tephra is
less than one foot and greater than one-tenth of one inch, and the
greatest thicknesses are to the northeast. The source was probably to
the northeast of Keanakakoi.
- Could Keanakakoi be the vent if the wind first blew the
tephra to the
northeast and later shifted and blew the tephra to the southwest?
No. If the wind blew to the northeast and then to the southwest, it might
distribute the tephra in the observed pattern. However, the thickest
deposit should be closest to Keanakakoi Crater, not to the northeast.
- Was the vent near Kilauea Iki Crater?
Yes. Puu Puai, or Gushing Hill, is a cinder cone. It is the thickest
deposit near the crater and was produced in 1959 by a high lava
fountain. The thickness of the deposit also decreases uniformly to the
southwest.
- Why does the tephra become thinner to the southwest?
During the Kilauea Iki eruption, the tradewinds were dominant. The
tradewinds blow across the islands from northeast to southwest. Wind
direction determined where the tephra would be deposited.
Congratulations! You just interpreted basic geologic data to determine
the location of a volcanic eruption.
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