A contact is a sharp boundary between two geologic units. Here we see a clear boundary where sand has slid down to partially cover lava flows on a steep slope.

A contact is a sharp boundary between two geologic units. Here we see a clear boundary where sand has slid down to partially cover lava flows on a steep slope. Image courtesy of Bill Chadwick, Oregon State University, National Science Foundation, and Woods Hole Oceanographic Institution.

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