Earth Science Unit 1 Lesson 6 Igneous RocksVocabulary: Igneous comes from the Latin word ignis, which means "fire" or "flame."

Igneous rocks form in a similar way as chocolate candy. Heat from Earth's core moves through the mantle to the crust. The extreme heat melts rock near the crust into magma. Pressure pushes hot plumes of magma through cracks in the crust. As the magma moves away from the mantle, it cools and hardens into igneous rock. In

this lesson, you will learn how these rocks are formed, classified, and used.

Lesson Outline

Igneous rock forms from cooling magma. Slow cooling forms large crystals. Quick cooling forms small crystals. Extrusive igneous rock forms above the surface, e.g., basalt, obsidian, rhyolite,

and pumice. Intrusive igneous rock forms below the surface, e.g., granite and gabbro. Coarse-textured rocks have large crystals. Fine-textured rocks have small

crystals. Glassy-textured rocks have few crystals. Vesicular rocks have holes and craters.

Granite is common below the continents. People use it for building. Basalt often forms on the ocean floor.

OBJECTIVES

Describe how igneous rocks form. Compare extrusive and intrusive igneous rock. Identify common characteristics of igneous rock.VOCABULARY

extrusive forming outside, on the surface of Earth's crust

intrusive forming inside, under Earth's crust

lava molten rock on Earth's surface

magma molten rock beneath Earth's crust

Formation

Formation of Igneous Rocks

You can find igneous rocks almost anywhere on Earth, but they are most common near the boundaries between plate tectonics.

Remember that at mid-ocean ridges, the tectonic plates pull apart from each other.

This allows magma to rise up to the ocean floor, and the cold ocean temperatures cool the magma and solidify it into rock.

Igneous rocks can also form along different types of convergent boundaries, as well as, fault lines where tectonic plates are pushed together.

Igneous rocks come in a wide variety of colors, chemical compositions, and textures.

The chemical composition of magma can affect the temperature at which it becomes solid.

Rocks that solidify at lower temperatures usually have high levels of elements such as silicon (Si), potassium (K), and aluminum (Al).

On the other hand, rocks that solidify at higher temperatures have high levels of elements, such as calcium (Ca), sodium (Na), iron (Fe), and magnesium (Mg).

The rate that magma cools will determine the texture and crystal size of igneous rock.

When magma cools quickly, crystals do not have much time to form. Igneous rocks with small or microscopic crystals form from quickly cooling magma. Obsidian forms when magma cools extremely quickly, and its crystals are so small they cannot be detected.

Magma that slowly cools allows crystal to grow, so larger crystals have time to form. Granite is an example of an igneous rock where magma cools at a slower rate that allows crystals to grow. Granite mountains are found in many places, and granite is mined as a building material- you might even have granite counters in your kitchen or bathroom. Igneous rocks come in many forms, and can be found all around us.

Extrusive VS. Intrusive

Extrusive and Intrusive Rocks

Geologists group igneous rocks into two categories: extrusive and intrusive. Have you ever seen a picture of an erupting volcano? Volcanic eruptions happen when magma pushes through weak spots in the crust.

Lava pours out of the volcano and when the lava cools, it will form extrusive igneous rock. These are rocks form from magma that reaches the surface.

[Since the temperature at Earth's surface is much cooler than in the mantle, extrusive igneous rocks cool quickly. [Igneous rocks with a lot of holes]These igneous rocks have very small crystals. Sometimes, gas bubbles trapped in the lava cool into holes.

For example, trapped gas bubbles cause the Swiss-cheese appearance of pumice.

The "airiness" of pumice makes it the only rock that floats in water. Pumice, obsidian, and basalt are examples of extrusive igneous rocks.

Not all magma reaches Earth's surface. Most magma stays beneath the crust. Intrusive igneous rock forms when magma is trapped and cooled inside the crust.

Sometimes, magma intrudes between rock layers. The magma spreads out parallel to the layers and forms a sill.

Dykes form when magma follows a fracture or break in rock layers. Both dykes and sills cool into intrusive igneous rock.

Intrusive magma slowly cools over thousands, or even millions, of years. [Sample of igneous rock] This slow cooling allows large crystals to form. The larger crystals give intrusive igneous rock a coarse texture, like sandpaper. You can find these rocks on many mountains, like Mount Rushmore. [in South Dakota]

Earthquakes and other geological processes can also expose intrusive igneous rocks to the surface.

Weathering, for example, can wear away rocks on the surface and expose the igneous rock below. Granite and gabbro are just two examples of intrusive igneous rocks.

Rock Textures

Geologists classify igneous rocks by their texture. There are four basic textures—coarse, fine, glass, and vesicular.

Large crystals that are easily visible to the naked eye characterize coarse texture. The crystals are usually 1/2 mm to several centimeters in size. The entire rock is made of these large crystals. Coarse-textured rocks form when magma cools very slowly, deep below the surface. Granite is a good example.

Fine-textured rocks are made entirely of small crystals less than 1/2 mm in size. The crystals in fine-textured rocks are too small to see with the naked eye. You must use a microscope to view the crystals. Fine-textured rocks form when magma cools quickly close to or above the surface. Rhyolite is an extrusive igneous rock with fine-texture.

Sometimes, rocks cool partially below ground and partially above. These rocks show a special kind of fine texture called porphyritic. In this texture, larger crystals appear randomly "stuck" in a mass of fine crystals.

Have you ever seen a piece of obsidian? This shiny rock looks like black glass. It forms when magma cools so quickly it has almost no crystals at all. This type of texture is called glassy.

Rocks with vesicular textures have almost the exact opposite look as those with a glassy texture. Vesicular-textured rocks are very rough and bumpy. They form when gas bubbles cool with the magma. The bubbles leave behind tiny holes and craters. Pumice is a vesicular rock.

The following table summarizes the types of textures of igneous rocks.

Texture Description Example

coarse large crystals; usually 1/2 mm to several centimeters in size granite

fine small crystals less than 1/2 mm in size rhyolite

glass almost no crystals obsidian

vesicular very rough with tiny holes and craters pumice

Granite and Basalt

Have you ever been to the Empire State Building? Maybe you have seen pictures of London's Tower Bridge or the Great Wall of China. What do these three structures have in common? The people who built them all used granite.

Granite is one of the most common igneous rocks. The crust below continents is almost entirely granite. Granite is made of feldspar, quartz, and other trace minerals. The mineral structure and chemical composition of granite make it an incredibly strong rock. Because of its strength, people make building, bridges, and other structures out of granite.

Basalt is another common igneous rock. It forms from lava cooling on Earth's surface. Basalt is made mainly of pyroxene and feldspar. Like granite, it can also contain trace minerals. Basalt is the most common rock type on Earth's crust. Most of the ocean floor is made of this extrusive igneous rock. The lava which forms basalt is very fluid and can flow long distances from its eruption site. This means you can find basalt both near and far away from volcanoes.