Add-on:US Major calderas and impact structures

Outlines of major calderas and impact structures in USA.

3 colors :
 * red : Caldera Boundary
 * green : Caldera Boundary, uncertain
 * blue : Extent of large buried impact structure


 * World Wind Forums Thread
 * Download US Major calderas and impact structures v1.0
 * Add-On website
 * USGS Website