medium · Enhanced ACT science
In oceanic basalt formations, the presence of certain olivine crystals can indicate the rate of cooling. Rapidly cooled 'quenched' basalt typically exhibits a microcrystalline structure, whereas basalt that cools slowly deep within the crust develops larger, well-defined crystals. A researcher observes a formation with large crystals at the center but a glass-like microcrystalline texture on the outer edges.
What is the most likely inference regarding the formation of this basaltic body?
- Larger olivine crystals are far denser than microcrystalline basalt, so they naturally sink toward the center of any molten body long before cooling truly begins.
- The entire formation actually cooled at just one uniform rate, yet the core's distinct chemical composition alone favored large crystal growth over the microcrystalline edges.
- The outer surface of the basaltic body was in direct contact with a significantly cooler medium, such as seawater, which accelerated the cooling process at the boundary.
- Basaltic formations deep within the crust always contain large crystals, because the immense pressure of overlying rock layers prevents any microcrystals from ever forming there at all.
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