2007-01-23 08:51:43 · 3 answers · asked by MICHAEL C 1 in Science & Mathematics ➔ Earth Sciences & Geology
From the crystallization of melted minerals. This can happen at the surface of the earth, resulting in extrusive igneous rocks (ex. pumice, obsidian) or within the earth, forming intrusive igneous rocks (granite, etc.) Intrusive rocks have larger grain sizes, whereas in extrusive rocks, grains may not be visible without a hand lens if they are visible at all.
2007-01-23 09:34:58 · answer #1 · answered by kiddo 4 · 0⤊ 0⤋
Just to add on to the previous answer. The reason why intrusive rocks have larger crystals than extrusive rocks is because intrusive rocks take longer to cool whereas extrusive rocks cool more rapidly and don't have enough time to crystallize the larger crystals.
2007-01-23 12:58:05 · answer #2 · answered by geocadet07 2 · 0⤊ 0⤋
They are formed when molten rock (magma) cools and hardens.
2007-01-23 08:59:47 · answer #3 · answered by einzelgaenger08 3 · 0⤊ 0⤋