Hampden-Wilbraham Regional District

TASA The Wonders of Rocks and Minerals
Igneous Rocks Script

How Igneous Rocks Form

Recall from the discussion of the rock cycle that igneous rocks form when molten rock cools and crystallize. Igneous comes from the Latin word for “fire,” ignis.

Molten rock (magma) originates below the Earth’s surface as a result of melting solid rock.

Because the hot magma is less dense than the surrounding rock, it moves toward the surface, sometimes producing a volcanic eruption. (Click replay to see eruption a second time.)

Volcanic eruptions may produce various sized fragments such as ash, cinders, and volcanic bombs. (Click “Ash, Cinders, and Bombs to see photographs of each of these fragments.)

Volcanic eruptions can also produce extensive lava flows. Lava is the term we use for magma once it reaches Earth’s surface.

When the lava cools and hardens on the surface it is called extrusive igneous rock.

Most magma never reaches the surface. Rather it crystallizes very slowly below the Earth’s surface. This type of igneous rock is called intrusive. (Click replay to review crystallization.)

Although intrusive rocks form deep within the Earth, uplifting and erosion often expose them at the surface. (Click replay to review exposure of intrusive rock at surface.)

Intrusive igneous rocks are exposed in many places including Yosemite National Park… (Click globe to see map of location of park. Then click back arrow.)

… Stone Mountain, Georgia… (Click globe to see map of location of park. Then click back arrow.)

…and Grand Teton national Park. (Click globe to see map of location of park. Then click back arrow.)

To summarize, igneous rocks form when lava cools quickly at the surface…

…or at great depth over very long spans of time. Caption, “These rocks formed at great depth and were later uplifted and exposed by erosional forces.”

Because magma is ________ than the surrounding rocks, it works its way toward the surface, occasionally causing a volcanic eruption. Choose “less dense” or “more dense.” Next click “Is this right?”

Which of the following are not usually produced by volcanic eruptions? Choose “lava flows”, “Stalactites,” “Ash,” or “Cinders.” Next click “Is this right?”

Igneous rocks which form at the surface of the Earth are said to be ________ . Choose “extrusive” or “intrusive.” Next click “Is this right?”

Igneous rocks that form from magma that crystallizes beneath the surface are called _____. Choose “extrusive” or “intrusive.” Next click “Is this right?”

 

Textures of Igneous Rocks

The molten rock’s rate of cooling greatly affects the size of the mineral crystals that form a rock. We call this a rock’s texture.

Rapid cooling produces smaller crystals and slower cooling allows time for larger crystals to from. Click replay to review crystal formation.

Igneous rocks that form at the surface (rapid cooling) possess a fine-grained texture. Sample: rhyolite

Notice that the grains in this sample of rhyolite are too small for individual minerals to be seen without a magnifying glass.

When large masses of magma solidify at depth (slower cooling), they from igneous rocks that exhibit a coarse-grained texture. Sample: granite

In contrast to fine-grained rocks, the individual minerals in this sample of granite can be identified with the unaided eye. Arrows to quartz, feldspar and hornblende in granite.

Which of these igneous rocks formed more quickly? Click on the correct answer. Diorite or Basalt Diorite-This is a coarse-grained igneous rock that formed slowly. Basalt- Correct, the mineral grains are too small to see, so this igneous rock formed quickly.

Very rapid cooling of lavas may form a glassy texture as in this sample of obsidian. This occurs when cooling is so rapid that no mineral crystals forma at all. Sample: obsidian

During some volcanic eruptions, lava is ejected into the atmosphere where it is chilled quickly. Sample: pumice

These events can generate a frothy mass of glass fibers called pumice, which sometimes floats in water. Click replay to see float.

 Match the texture to the correct rock by dragging the names in the upper right to the appropriate rock samples. Fine-grained (rhyolite) , coarse-grained (granite), glassy (obsidian). Click, “Is this right?”

In general, the longer magma takes to crystallize, the _____ crystals will be. Choose : larger or smaller Next click “Is this right?”

A glassy textured rock (obsidian) is the result of ______ cooling. Choose: very slow or very quick. Next click “Is this right?”

Frothy igneous rocks that form when molten material cools very rapidly )often ejected into the air) are called _______ . Choose: granite or pumice. Next click “Is this right?”