Isopach


http://www.geo.utexas.edu/faculty/barker/kempter/rbtephra.html

This is the map of the Rio Blanco Tephra Deposit after it erupted.  Isopachs indicate where rock, or rock-matter, are of equal thickness.  Notice as the lines move outward, the space in between them grows wider and the rock density becomes lessened.  This map would  indicate how rock matter becomes seperated as it is blown farther from its point of origin before the eruption.