What is the difference between continental and oceanic crust?

Question

There are numerous characteristics that distinguish these two types of crust.

Continental crust is generally composed of granite. That means that it formed intrusively (inside the earth), cooled slowly, and grew crystals ranging in size from 1 - 10 mm. Continental crust is felsic in composition (lots of silica and aluminum), is low in density, and light in color. It contains quartz and potassium feldspar as the distinguishing minerals.

In comparison, oceanic crust is composed of basalt. It formed extrusively (above the earth's surface at a mid ocean ridge), cooled quickly, and has fine grained crystals less than 1 mm. Oceanic crust is mafic in composition (lots of iron and magnesium), is higher in density and dark in color. The key minerals in a basalt are pyroxene, olivine, and biotite mica.

Answer 1

This looks mostly pretty good! There are one or two bits that are a little off, so let's go over them.

Continental crust is indeed comprised of granite, an intrusive felsic rock. You are right that it contains quartz and potassium feldspar (also called alkali feldspar) as major rock forming minerals. However, granites may also have plagioclase feldspar.

Oceanic crust is correctly identified here as an extrusive mafic--generically basalt--that is formed at mid-ocean ridges. However, the mineral composition would be pyroxene, olivine, and plagioclase feldspar, rather than biotite mica. It's a good guess: biotite does tend to have a lot of iron, and mafic does mean iron and magnesium. However, you're much more likely to see biotite as a secondary mineral in granites or in pegmatites, not so much in oceanic basalts.

In the future, Bowen's reaction series--an series that describes crystallization from common magmas--is a very helpful way to remember the primary rock forming minerals in common rocks. Mafic minerals that would make up a basalt or gabbro--olivine, pyroxene--typically form at higher temperatures, and thus form first as a magma chamber cools. Felsic phases in a granite or rhyolite--quartz, alkali feldspar--usually form at lower temperatures, and would crystallize later in the process. Plagioclase feldspar can be found in mafic and felsics, as it exists in a solid-solution series that spans a wide range. We see minerals like amphibole as biotite as secondary phases in some felsics, or in intermediate rocks like andesite.