The correct option is
C. Igneous.
Detailed Explanation:
- Definition of Igneous Rock:
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Igneous rocks are formed from the cooling and solidification of molten material, which can either be magma (when it is beneath the Earth's surface) or lava (when it erupts onto the surface). This process can occur slowly, allowing large crystals to form, or quickly, resulting in smaller crystals.
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Formation Process:
- Magma: When magma cools slowly underground, it forms intrusive igneous rocks (also known as plutonic rocks). An example of this is granite, which has large crystals due to the slow cooling process.
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Lava: When lava cools quickly on the Earth's surface, it forms extrusive igneous rocks (also known as volcanic rocks). An example of this is basalt, which has small crystals because of the rapid cooling.
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Key Characteristics:
- Igneous rocks are characterized by their crystalline structure, which is a result of the solidification of molten rock. The mineral composition can vary widely, leading to different types of igneous rocks.
Why Other Options Are Incorrect:
- A. Sedimentary:
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Sedimentary rocks are formed from the accumulation and compaction of mineral and organic particles, or from the precipitation of minerals from water. They do not form from molten material. Examples include sandstone and limestone.
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B. Metamorphic:
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Metamorphic rocks are formed from existing rocks (either igneous, sedimentary, or other metamorphic rocks) that have been transformed by heat, pressure, or chemically active fluids. They do not originate from molten material. Examples include schist and marble.
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D. Organic:
- The term "organic" in geology typically refers to sedimentary rocks that are formed from the remains of living organisms, such as coal (from plant material) or limestone (from shell fragments). These rocks do not form from molten material either.
Summary of Key Points:
- Igneous rocks are formed from the cooling and solidification of molten material (magma or lava).
- They can be classified into intrusive (formed underground) and extrusive (formed on the surface) types.
- Sedimentary and metamorphic rocks are formed through different processes and do not involve molten material.
- Organic rocks are a subset of sedimentary rocks formed from biological materials.
Revision Summary:
- Igneous rocks form from cooling and solidification of magma or lava.
- Intrusive igneous rocks cool slowly underground; extrusive igneous rocks cool quickly on the surface.
- Sedimentary and metamorphic rocks are formed through different geological processes, not from molten material.
- Understanding the formation processes helps distinguish between the different types of rocks.