Extrusive Igneous Rocks: Formation and Features — Reading Comprehension

Grades

Standards

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

About This Reader

This educational passage describes extrusive igneous rock formation through rapid cooling of lava on Earth's surface. It covers key features like fine-grained textures, glassy appearances (obsidian), and porous structures (pumice), with examples including basalt, obsidian, pumice, and rhyolite. Aligned with NGSS MS-ESS2-3 (Earth's Systems) for understanding rock formation processes and CCSS.ELA-LITERACY.RI.6.7 for interpreting scientific information. The informatyional text explains how these volcanic rocks provide evidence of past eruptions and geological activity. It highlights the relationship between cooling rates and rock textures, while connecting to practical uses in construction and industrial applications. The content supports earth science curriculum standards about rock cycles and volcanic processes.

CONTENT PREVIEW

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 rocks

Which of these is NOT an example of extrusive igneous rock?

GraniteBasaltObsidianPumice

What texture do most extrusive igneous rocks have?

Large crystalsFine-grained or glassyLayered bandsRough and chunky

Why 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 rock

What causes pumice to have holes and float in water?

Trapped plant fossilsDissolved mineralsGas bubbles in fast-cooling lavaErosion by wind

Which extrusive rock is dark-colored and makes up much of the ocean floor?

RhyoliteObsidianBasaltPumice