Estimate the height of the original volcano. Assume that the present gradient of the Okmok Caldera (calculated in problem 1) is the same as that of the volcano before it collapsed. Given the complex history of the Okmok Caldera, this is a very simplistic assumption, but it can be used to provide a crude estimate of the volcano’s previous height.

Question

The following questions are based on Map T-2, the “Umnak, Alaska,” topographic map (scale 1:250,000; contour interval 200 feet). In this problem, you will estimate the height of the original Okmok volcano before the caldera was formed. (You may also determine the elevation changes and distances needed to make these calculations by using Google Earth™.)
Gradients can be used to estimate an elevation increase over a given distance. (See Exercise 34 for a review of gradients.) For example, if a mountain has a gradient of 1000 feet per mile, we can estimate that over a horizontal distance of five miles, the elevation will increase by 5000 feet.

5 miles * 1000 ft>mi = 5000 foot elevation increase over five miles


(a) Following line EF, what is the horizontal distance from the coastline (closest to Point E) to the center of the caldera?
_________ miles
(b) Multiply the horizontal distance (from problem 2a) by the gradient of the present caldera (problem 1). This is an estimate of the original elevation of the volcano before its collapse.
_________ feet
(c) Approximately how much higher was the original volcano than the present caldera rim?

Answer 1

(a) about 8.2 miles
(b) 4100 to 5945 feet
(c) Between 600 and 2445 feet