Compare these two basalts. One is an example of a basalt flow, the other of a pillow basalt. Pillow basalts erupt underwater and cool instantaneously around their margins, while basalt flows cool more gradually, and fractures may break the flow up into discrete cooling columns.
3D model of Outcrop A
3D model of Sample B
Which basalt formed as part of a flow? (A or B)
Which basalt is more likely to be associated with oceanic sediments? (A or B)
Now take a look at this outcrop (which we can call outcrop C) from Washington state’s Columbia River flood basalt province.
3D model of Outcrop C:
What characteristics does it display?
How would you interpret the story of its formation?