Asphalt

What is it with all these black and somewhat toxic substances? After coal, now asphalt! A friend's inquiry as to why the roads start off pitch black, then gradually lighten prompted my small query.

Asphalt is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits. Asphalt is composed almost entirely of bitumen. The term asphalt is often used as an abbreviation for asphalt concrete.

The largest use of asphalt is for making asphalt concrete for road surfaces, which accounts for approximately 80% of the asphalt consumed in the United States. Roofing shingles account for most of the remaining asphalt consumption. Other uses include cattle sprays, fence post treatments, and waterproofing for fabrics.

While Americans are accustomed to thinking about recycling their newspapers and beverage containers, the most widely recycled product in terms of both percentage and tonnage is actually asphalt road surfaces.

http://en.wikipedia.org/wiki/Asphalt

I must note that that friend told me aged roads can vary in color because the rock used in the asphalt concrete is often local to the road (as so much is needed I guess). I remember seeing green-ish and red-ish road around here, but I never asked myself why! Woe!