explain about the igneous rock? 8 marks
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Answer:
Igneous rocks (from the Greek word for fire) form when hot, molten rock crystallizes and solidifies. The melt originates deep within the Earth near active plate boundaries or hot spots, then rises toward the surface.
Explanation:
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Explanation:
What are Igneous Rocks?
Igneous rocks are defined as types of rocks that are formed when molten rock (rock liquefied by intense heat and pressure) cools to a solid-state.
Lava is molten rock flowing out of fissures or vents at volcanic centers (when cooled they form rocks such as basalt, rhyolite, or obsidian). Pyroclastic deposits are accumulations of fragmented material (e.g. ash, bombs, tuffs, and volcanic breccias) ejected during volcanic eruptions.
Igneous rocks can be easily identified with their texture, density, color, and mineral composition. Its texture depends on the shape, size, time period to cool down and solidify, and the arrangement of crystals in the rock.
Types of Igneous Rock
Following are the two types of igneous rock:
Intrusive igneous rock: These rocks crystallize below the earth’s surface resulting in large crystals as the cooling takes place slowly. Diorite, granite, pegmatite are examples of intrusive igneous rocks.
Extrusive igneous rock: These rocks erupt onto the surface resulting in small crystals as the cooling takes place quickly. The cooling rate is for a few rocks is so quick that they form an amorphous glass. Basalt, tuff, pumice are examples of extrusive igneous rock.
Characteristics of Igneous Rocks
1) The igneous form of rocks does not include any fossil deposits. If there are any chances of fossil deep inside the crust, it erupts out of the Earth’s surface and gets destroyed due to the sheer heat these rocks produce.
2) Most of the igneous forms include more than one mineral deposit.
3) They can be either glassy or coarse.
4) These usually do not react with acids.
5) The mineral deposits are available in the form of patches with different size.
Types of Igneous Rock Textures
Aphanitic: fine-grained, less than 1 mm, grains not seen with the unaided eye
Phaneritic: “coarse-grained”; visible crystals; 1 to 10 mm
Pegmatitic: “very coarse-grained”; > 1 cm
Porphyritic: composed of both large and fine-grained crystals, and the large crystals are called phenocrysts, and the background is the matrix
Vesicular: rocks that have vesicles, resembling a sponge (e.g. scoria and pumice)
Pyroclastic: fragmented, angular grains ejected during the eruption (e.g. volcanic breccia)
Glassy: when lava cools quickly, there is not enough time for large mineral crystals to form (e.g. obsidian).
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