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Intrusive Igneous Rocks: Formation and Features — Reading Comprehension

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Grades

Standards

MS-ESS2-3
RI.6.1
RI.7.4
RI.8.2

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This passage explains the formation of intrusive igneous rocks through slow cooling of magma beneath Earth's surface. It details key characteristics like coarse-grained textures and large crystal formations, with examples including granite, diorite, gabbro, and pegmatite. The content aligns with NGSS standard MS-ESS3-2 (Earth's Systems) by describing how Earth's internal processes form rock structures, and CCSS.ELA-LITERACY.RI.6.1 for informational text comprehension. The passage emphasizes the geological significance of intrusive rocks in understanding Earth's crust composition and mountain-building processes. It also connects to real-world applications in construction and decoration due to the rocks' durability and aesthetic qualities.

CONTENT PREVIEW

Intrusive igneous rocks form when magma cools slowly beneath the Earth's surface. This slow cooling allows large mineral crystals to grow, giving these rocks their characteristic coarse-grained texture. Unlike extrusive rocks that cool quickly, intrusive rocks remain hidden underground until erosion exposes them over millions of years.

Examples of Intrusive Igneous Rocks

Common intrusive igneous rocks include:

● Granite – A light-colored rock with visible quartz, feldspar, and mica crystals, often used in countertops and buildings.

● Diorite – A grayish rock with a mix of light and dark minerals, similar to granite but darker.

● Gabbro – A dark, heavy rock rich in iron and magnesium, forming much of the oceanic crust.

● Pegmatite – An extremely coarse-grained rock with exceptionally large crystals, sometimes containing rare minerals.

Characteristics of Intrusive Igneous Rocks

Coarse-Grained Texture – Slow cooling creates large, interlocking crystals visible to the naked eye.
High Density – Most intrusive rocks are dense and hard due to their mineral composition.
Varied Colors – Colors range from light (granite) to dark (gabbro), depending on mineral content.
Massive Formations – They often occur in large bodies like batholiths (mountain-sized formations) or dikes (sheet-like intrusions).

Intrusive igneous rocks make up much of Earth's continental crust. Their study helps geologists understand deep Earth processes and ancient mountain-building events. Because of their durability and beauty, many intrusive rocks are valued in construction and decoration.

How do intrusive igneous rocks form?

Rapid cooling of lava on the surfaceSlow cooling of magma undergroundCompression of sedimentary layersHeating of metamorphic rocks

Which is NOT an intrusive igneous rock?

BasaltGraniteGabbroDiorite

What texture do intrusive rocks typically have?

GlassyFine-grainedCoarse-grainedPorous

Why does granite have visible crystals?

It forms from fast-cooling lavaMagma cools slowly undergroundIt contains many gas bubblesIt undergoes rapid erosion

What makes pegmatite unique?

Its smooth textureIts extremely large crystalsIts ability to floatIts layered appearance

Where are intrusive rocks typically found?

Only in riverbedsOn the ocean floorBeneath Earth's surfaceIn desert sand dunes