Suppose you are on a hike when you find a dark-colored igneous rock (pictured be

Question

Suppose you are on a hike when you find a dark-colored igneous rock (pictured below). The rock has very small crystals that can only be seen under magnification. You bring the rock back to the lab and make a thin section of it so that you can examine its mineral composition using a petrographic microscope. In thin section, you are able to determine that the rock consists of plagioclase feldspar (mostly Ca-rich) and several ferromagnesian minerals. What does this tell you about the rock's origin? The rock formed under water by precipitation of iron and magnesium-rich minerals from solution. The rock formed from a mafic lava that cooled relatively rapidly on Earth's surface. The rock formed from a mafic magma that cooled relatively slowly deep with the earth. The rock formed from a felsic magma that cooled relatively slowly deep within the earth. The rock formed from a felsic lava that cooled relatively rapidly on Earth's surface.

Explanation / Answer

Presence of calcium plagioclase and ferromagnesium minerals indicates it's a mafic rock and presence of fine grained Crystal indicate that it has formed by rapid cooling conditions so option b is correct

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