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Extrusive Igneous Rocks: Formation and Features
Extrusive igneous rocks form when molten lava erupts onto the Earth’s surface and cools quickly. Unlike intrusive rocks, which solidify slowly underground, extrusive rocks have fine-grained or glassy textures because rapid cooling prevents large crystals from forming. These rocks are also called volcanic rockssince they come from volcanic activity.
Examples of Extrusive Igneous Rocks
Some common examples include:
● Basalt – A dark, fine-grained rock found in lava flows and oceanic crust.
● Obsidian – A smooth, glassy rock formed from fast-cooling lava with high silica content.
● Pumice – A lightweight, porous rock filled with gas bubbles, often floating in water.
● Rhyolite – A light-colored rock with high silica, similar to granite but with a finer texture.
Characteristics of Extrusive Igneous Rocks
1. Fine-Grained Texture – Most extrusive rocks have small crystals due to rapid cooling.
2. Glassy or Foamy Appearance – Some, like obsidian and pumice, cool so quickly that they lack visible crystals.
3. Gas Bubbles – Lava with trapped gases forms porous rocks like pumice or vesicular basalt.
4. Dark or Light Colors – Composition affects color; basalt is dark, while rhyolite is light.
Extrusive igneous rocks cover large areas, such as lava plains and volcanic islands. They help scientists study past eruptions and Earth’s geological processes. Their unique textures also make them useful in construction and industry.
How do extrusive igneous rocks form?
Slow cooling of magma undergroundRapid cooling of lava on Earth's surfaceCompression of sedimentary layersHeating of metamorphic rocksWhich of these is NOT an example of extrusive igneous rock?
GraniteBasaltObsidianPumiceWhat texture do most extrusive igneous rocks have?
Large crystalsFine-grained or glassyLayered bandsRough and chunkyWhy does obsidian have a smooth, glassy texture?
It forms from slow-cooling magmaIt contains large mineral grainsIt cools too quickly for crystals to formIt is a type of sedimentary rockWhat causes pumice to have holes and float in water?
Trapped plant fossilsDissolved mineralsGas bubbles in fast-cooling lavaErosion by windWhich extrusive rock is dark-colored and makes up much of the ocean floor?
RhyoliteObsidianBasaltPumiceWhat determines the color of extrusive igneous rocks?
The speed of coolingThe amount of silica presentThe age of the rockThe location where it formedWhy are extrusive igneous rocks important to geologists?
They are always found in riverbedsThey provide clues about past volcanic activityThey are the rarest type of rockThey cannot be used for construction
