Asphalt is the heaviest fraction of petroleum, left behind when the more volatile compounds evaporate (more about petroleum). It flows slowly during warm weather and may be stiff enough to shatter during cold times. Geologists use the word "asphalt" to refer to what most people call tar, so technically this specimen is an asphaltic sand. Its underside is pitch-black, but it weathers to a medium gray. It has a mild petroleum odor and can be crumbled in the hand with some effort. A harder rock with this composition is called a bituminous sandstone or, more informally, a tar sand.
Traditional peoples used asphalt as a mineral form of pitch, to seal or waterproof items of clothing or containers. In the 1800s, asphalt deposits were mined for use on city roads, then technology advanced and crude oil became the source for tar, manufactured as a byproduct during refining. Now natural asphalt only has value as a geological specimen. This specimen came from a petroleum seep near McKittrick, in the heart of California's oil patch. It looks like the tarry stuff that roads are built of, but it weighs much less and is softer.
For more photos see the Sedimentary Rocks Gallery.
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Geologic Features and Processes
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