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How do we distinguish basalt which formed in an oceanic-floor flow from basalt formed in a terrestrial flow using large-scale observations?

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The basalt from ocean floors is called a MORB (mid-ocean ridge basalt) while the basalt emplaced in a continental setting is usually an alkali basalt. They can be easily distinguished by their mineralogy and their geochemistry. Now maybe you don't have access to such analyses. To distinguish a MORB from a continental basalt at a larger scale, you can look for pillow lavas and hyaloclastites, two features that are typical of submarine lavas.

It won't be 100% accurate though: some submarine lavas called "sheet flows" lack pillows and could look like continental lavas, while continental lavas can contain pillows and hyaloclastite if emplaced in a water-rich environment (lacustrine or glacial). For examples, most tuyas found in Iceland contain pillows and hyaloclastite but where formed on land, below ice.

Another way would be to look at the geodynamic history of the region: is it known for having had an ocean in the past? Have this ocean closed and part of the oceanic crust obducted? Have ophiolite outcrops been recognized in the area? If not, then the basalt is probably not submarine.

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