Isopach

Isopach Maps are maps that use contour lines to show equal thickness over a certain region for a period of time.  For this particular Isopach map, you can see the thickness of ash that fell in 1995 between November 19th and December 2nd. 
http://www.geo.mtu.edu/volcanoes/central_america/nicaragua/cerro_negro/gvn/cerro_negro.v20n11_12.html