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October 4, 2017

This EarthWord is probably what you’ve been doing thanks to our fantastic puns...

EarthWords is an on-going series in which we shed some light on the complicated, often difficult-to-pronounce language of science. Think of us as your terminology tour-guides, and meet us back here every week for a new word!

Image shows an oil refinery with mountains in the background
Anacortes Refinery (Tesoro), on the north end of March Point southeast of Anacortes, Washington, United States. Credit: By Walter Siegmund (talk) - Own work, CC BY 2.5, domain.)

The EarthWord: Cracking


  • It may sound like it’s related to fracking, but cracking actually has little to do with hydraulic fracturing. Instead, it’s when larger molecules in petroleum break apart due to heat and pressure into smaller ones.

  • Cracking happens naturally in petroleum deposits, but it can also be triggered in industrial plants called Crackers.


  • Cracking comes from the Proto-Germanic krakojan, which, among other meanings, referred to a “splitting sound,” and eventually led to “a split,” or “an opening.”

Use/Significance in the Earth Science Community:

  • Cracking is where the lighter petroleum products like natural gas, butane, and propane come from. The degree to which petroleum has cracked from oil to natural gas is heavily correlated to the deposit’s thermal maturity.


  • USGS oil and gas resource assessments look at how much oil is present vs. how much gas is present, meaning the degree of cracking that has taken place is an important part of the estimate.

  • As a result, USGS incorporates research on the conditions under which oil cracks into natural gas into its assessment methodology.

  • By using the same, publicly available methodologies for all of its assessments, USGS ensures that each basin can be compared to the others and provide a trusted, apples-to-apples estimate for the Nation’s energy resource endowment.

Next EarthWord: No, this EarthWord isn’t a trait of adult petroleum basins, but it is related to how old they are...

Hungry for some science, but you don’t have time for a full-course research plate? Then check out USGS Science Snippets, our snack-sized science series that focuses on the fun, weird, and fascinating stories of USGS science.

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