GEOLOGIC MAPPING
Superposition
Among the important tasks in developing understanding of Earth’s history, geologists create
detailed maps showing spatial and temporal relationships among different rock units. In this
exercise, you will practice developing a geologic map using only relationships of superposition
(that is, vertical succession of rock ‘layers’).
Fig. 1.
Google Earth image of successive lava flows on the north flank of Kilauea volcano, Hawaii.
LOCATION
LATITUDE
LONGITUDE
ALTITUDE
Kilauea, Hawaii
19.626188
-155.532571
3.10 km
1. Enter the coordinates above and zoom in or out until you are at an altitude of 3.10 km.
2. This view shows a series of successive lava flows on the north flank of Kilauea. On
Hawaii, a general sense of the relative age of lava flows can be deduced from
superposition (i.e., which flows overlie others) and, interestingly, by the color of the
flow.
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3. Fresh basalt flows on Kilauea are typically black. Over time, flows may progress through
a series of color changes wrought be progressive weathering of the lava (Holcomb,
1981). As the rock weathers over centuries, iron-bearing minerals hydrate and oxidize,
changing the overall appearance of the rocks. A common sequence of color changes is:
YOUNG (FRESH)
Black
OLD (WEATHERED)
Dark Gray
Brownish-Gray
(chocolate)
Reddish Brown
Tan
In addition to somewhat systematic color change, lava flows are often progressively
colonized by lichens and higher plants of various kinds related to the degree of
weathering and associated availability of ‘soil’ and nutrients (Holcomb, 1981). These
plants may also impart a sense of ‘color’ to the rocks when viewed in aerial imagery—
though the plants themselves are too small to be resolved, their presence will alter the
spectral character of individual image pixels.
4. Using these ideas, it is possible to create a relatively simple geologic map of the image
you are viewing in Google Earth. To do so, you will use the ‘Placemark’ tool (which has
an icon that looks like a push-pin) to tag flows of different ages. Use the simple rule
above relating lava color to age, with black being youngest and tan being oldest. On your
image, add a placemark for each lava flow you can distinguish. Label those placemarks
on the youngest lava flow with number ‘1’, and number each older flow sequentially. It is
possible on this image to determine which flow overlies others because you may be able
to see the terminus of a flow resting on the one beneath.
5. Once you have completed labeling your image, save it to your disk using the ‘Save
Image’ option under the ‘File’ menu, then print your image and turn it in to your
instructor.
LINKS TO RELATED SITES:
Wikipedia entry on Kilauea: http://en.wikipedia.org/wiki/Kilauea
Hawaii Volcanoes National Park: http://www.nps.gov/havo/index.htm
Hawaii Volcano Observatory: http://hvo.wr.usgs.gov/
Kilauea Volcano: http://hvo.wr.usgs.gov/kilauea/
Holcomb, R.T. 1981, Kilauea volcano, Hawaii: Chronology and morphology of the surficial lava
flows: Ph.D. Dissertation, Stanford University, Palo Alto, California, USA.
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