IGNEOUS ROCKS AND THE ROCK CYCLE

INTRUSIVE IGNEOUS ROCK

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Formed under the surface of the earth

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Formed from cooling magma

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Cools slowly

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Forms medium mineral grains

Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. Granite is composed mainly of quartz and feldspar with minor amounts of mica and other minerals. This mineral composition usually gives granite a red, pink, gray or white color with dark mineral grains visible throughout the rock. Granite is used to make many objects that we encounter in daily life. These include counter tops, floor tiles and paving stones.

EXTRUSIVE IGNEOUS ROCK

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Formed on the surface of the earth

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Formed from cooling lava

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Cools quickly

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Forms tiny mineral grains

Basalt is a dark-colored, fine-grained, igneous rock. Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is also an abundant rock on the Moon. Much of the Moon's surface is underlain by basaltic lava flows and flood basalts. Crushed basalt is used for road base, concrete aggregate, asphalt pavement aggregate, railroad ballast, filter stone in drain fields and may other purposes. Basalt is also cut into dimension stone. Thin slabs of basalt are cut and sometimes polished for use as floor tiles, building veneer, monuments and other stone objects.

VOLCANIC GLASS IGNEOUS ROCK

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Formed on the surface of the earth

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Formed from cooling lava

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Cools so quickly that no mineral grains form

Pumice is a light-colored, extremely porous igneous rock. The pores are actually gas bubbles that were trapped in the rock during the rapid cooling of a gas-rich frothy magma. During explosive volcanic eruptions, a rush of gas from the vent shreds the magma and blows it out as a molten froth. The froth rapidly solidifies as it flies through the air and falls back to Earth as pieces of pumice. This material can range in size from tiny dust particles to large block of pumice the size of a house. Pumice is use to make lightweight concrete, as landscaping stones and as an abrasive in conditioning "stone washed" denim for jeans, an abrasive in bar and liquid soaps such as "Lava Soap“, and an abrasive in pencil erasers.

Scoria forms when magma containing abundant dissolved gas flows from a volcano or is blown out during an eruption. As the molten rock emerges from the Earth the pressure upon it is reduced and the dissolved gas starts to escape in the form of bubbles. If the molten rock solidifies before the gas has escaped the bubbles become small rounded or elongated cavities in the rock. You can find scoria all over North America: The red variety of scoria is commonly used as landscaping pebbles at Taco Bell. Landscapers know this rock as lava rock.

Obsidian is found in many locations worldwide. It is confined to areas of geologically recent volcanic activity. Most obsidians have a composition similar to granite. Granites can form from the same magma as obsidian and are often geographically associated with the obsidian. Rarely volcanic glasses are found with a composition similar to basalt.

Obsidian older than a few million years is rare because the glassy rock is rapidly destroyed or altered by weathering, heat or other processes. The first use of obsidian by people probably occurred when a sharp piece of obsdian was used as a cutting tool. People then discovered how to skillfully break the obsidian to produce cutting tools in a variety of shapes. Obsidian was used to make knives, arrow heads, spear points, scrapers and many other weapons and tools.

PLUTONIC IGNEOUS ROCK

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Formed deep under the surface of the earth

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Formed from cooling magma

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Takes millions of years to cool

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Large mineral grains form

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Usually found after years of erosion uncovers it.

WHERE CAN WE FIND THEM IN THE UNITED STATES?

The Plains of Abraham is a huge expanse covered with ash and pumice, both light gray in color. Scattered randomly across this pale backdrop are black basalt boulders, many the size of cantaloupes but some as big a Volkswagen Beetles. All were deposited in a brief, terrifying rainfall when Mt. St. Helen’s exploded in Washington in 1980. From the Plains is a view of Mount Rainier, also located in the Cascade Range in Washington.

In the Grand Canyon, the Ruby Pluton (background) consists mostly of diorite, an igneous rock with a different composition than granite. The Ruby Pluton formed as a magma chamber for the volcanoes of an island arc, prior to their collision with the main continental landmass. Other igneous rocks, like these of pink granite (running through the rocks in the foreground) formed during the actual collision between the island chains and the continent.

Obsidian glass and Paulina Lake Newberry Volcanic National Monument Oregon, USA

The geologic landscape of Joshua Tree National Park in California has long fascinated visitors to this desert. How did the rocks take on such fantastic shapes? What forces sculpted them? When the rocks formed, rectangular joint fractures occurred, As ground water percolated down through the joint fractures, it began to transform some hard mineral grains along its path into soft clay, while it loosened and freed grains resistant to dissolving. Rectangular stones slowly weathered to spheres of hard rock surrounded by soft clay containing loose mineral grains. Over time, flash floods began washing away the protective ground surface. As they were exposed, the huge eroded boulders settled one on top of another, creating those impressive rock piles we see today.

The largest plutons are batholiths, such as these plutonic rocks of Colorado’s Pike's Peak, which are part of a 1,300 square-mile batholith.

DEVIL’S TOWER NATIONAL MONUMENT

Wyoming Devils Tower is composed of a grayish rock studded with feldspar crystals. As the magma cooled underground, it formed hexagonal or six-sided columns although columns have from four to seven sides. The last large column fell about 10,000 years ago. Devil’s Tower was the important geologic feature in the 1977 movie “Close Encounters of the Third Kind”.

Colorado's Rocky Mountains boast spectacular views of numerous plutonic (intrusive) rocks. These rocks were formed long ago as magma rose from deep sources and solidified before making it all of the way to the surface. As this magma rose, in many places, it brought with it precious minerals such as gold, silver, lead, and molybdenum (used in hardening steel). After millions of years, erosional processes stripped off the overlying rocks, exposing them as we see today.