Obsidian and coal are not rocks. To be rock, a substance must be an inorganic crystalline solid. Coal is organic. Obsidian is a volcanic glass that solidified too fast for a crystalline structure to form.
The U-S Park Service says the older Old Faithful gets, the more the geyser at Yellowstone National Park slows down. In the 1950s, it erupted every 62 minutes. Lately, it's been erupting every 77 minutes.
Coyotemike
USA
May 2006
AUG 03, 2010 07:42 PM