Vein
(c) 2006 Andrew Alden, licensed to About.com (fair use policy)
Veins are named for their resemblance to blood vessels, which are tubes. But the veins in rocks are actually sheets or tabular shapes, which look like tubes only because outcrops expose them in cross section. A vein is a fracture that is filled with mineral material.
This complex vein is exposed in the ceiling of an old mercury mine. The vein is clearly a fracture because it doesn't fit in with the structure of the rock around it. But the multiple stripes of white and dark minerals suggest that the fracture was active for some time while mineral fluids were rising from below. Each episode of widening and filling the vein may have corresponded to a large earthquake, this locality being near the San Andreas fault.
Fossils
Geologic Features and Processes
Glaciers and Ice
Landforms
Minerals
Rocks
Geology and Society
