What is a difference between extrusive and intrusive igneous rocks? View Available Hint(s) Intrusive igneous rocks are made of large minerals, while extrusive igneous rocks are made of small minerals. Intrusive igneous rocks generally have large crystals, whereas extrusive igneous rocks have no crystals at all or small crystals. Extrusive igneous rocks have large crystals, but intrusive igneous rocks have no crystals. Extrusive igneous rocks are glassy, while intrusive igneous rocks have crystals 1 mm in diameter or larger.
The correct answer and explanation is:
The correct answer is:
Intrusive igneous rocks generally have large crystals, whereas extrusive igneous rocks have no crystals at all or small crystals.
Explanation:
Igneous rocks are formed from the cooling and solidification of magma or lava. The key difference between intrusive and extrusive igneous rocks is where they form and how quickly they cool.
Intrusive Igneous Rocks
Intrusive (or plutonic) igneous rocks form below the Earth’s surface when magma cools slowly over thousands or millions of years. Because of the slow cooling process, minerals have time to grow, resulting in large, visible crystals. Granite is a common example of an intrusive igneous rock. The texture of intrusive rocks is typically coarse-grained (phaneritic), meaning the mineral grains can be seen without a microscope.
Extrusive Igneous Rocks
Extrusive (or volcanic) igneous rocks form on or near the Earth’s surface when lava erupts from a volcano and cools rapidly. Because the cooling happens quickly, there is little time for crystals to form, resulting in small or no visible crystals. Some extrusive rocks, like basalt, have a fine-grained (aphanitic) texture with small crystals. Others, like obsidian, cool so fast that they become glassy with no crystal formation at all. Pumice is another example, which has a frothy texture due to trapped gas bubbles.
Summary:
- Intrusive rocks → Slow cooling, large crystals (e.g., granite).
- Extrusive rocks → Fast cooling, small or no crystals (e.g., basalt, obsidian).
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