Week Five: Igneous Rock

Granite, Basalt, Obsidian, Pumice and many others

Igneous Rocks are records of the thermal history of Earth

Most common of all rocks is Granite: Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye.  It forms from the slow crystallization of magma below Earth’s surface.  Granite is composed mainly of quartz and  feldspar with minor amounts of mica, amphiboles and other minerals.   This mineral composition usually gives granite a red, pink, gray or white color with dark mineral grains visible throughout the rock.

Source: Geology.com

The most common igneous rock in Salt Lake Valley:

Quartz Monzonite: Quartz monzonite, a very close relative of, and locally known as "granite", is a gray, "salt and pepper" igneous rock exposed in the lower reaches of Little Cottonwood Canyon.

-Source: Utah Geological Survey

1. Where do igneous rocks occur in your region and why?

There are great reserves of granite in Little Cottonwood canyon. Most of the rock is covered by other type of rock: sedimentary and metamorphic. Since it occurs under a mountain range, magma collected and then cooled underneath layers of other rock.

2. Did indigenous peoples in your region use igneous rocks? 

The Ute people are the oldest residents of Colorado, inhabiting the mountains and vast areas of Colorado, Utah, Wyoming, Eastern Nevada, Northern New Mexico and Arizona.

Historically, Utes used obsidian rock for every day tools, since this rock is easy to shape and has sharp edges.

  

Obsidian is Extrusive Igneous Rock

Once magma reaches the earth’s surface it is considred lave, colling faster and forming fine-grained extrusive rock. Utah has large reserves of this fine rock.

3. How are igneous rocks used now? 

 Igneous rocks are used for construction, some types for building structures, some types for pavement. Pumice can also be used as a mild abrasive. Obsidian many times is used for decorative purposes in decorating and construction.

Uses:

Granite

A hard igneous rock consisting of clearly visible crystals of various minerals

Granite looks attractive when polished and is a very hard rock. It is often used for the facades (fronts) of buildings, expensive kitchen worktops and gravestones. 

Uses:

Basalt

A dark-coloured, fine-grained rock. Basalt is one of the main rocks to form the oceanic crust.

Basalt can be a rich source of iron and is commonly used as an aggregate in the building of roads. 

Uses:

Pumice

Pumice is formed in volcanic eruptions when gas is violently released from the still molten rock creating a rock with thousands of tiny bubbles in it. Pumice’s most obvious trait is that it is very light, so light in fact that it can sometimes float on water.

Day to day pumice use revolves around the removal of dead skin from peoples’ feet. It is also used in abrasive cleaning products and as an ingredient in lightweight construction materials.

Source: Mylearning.com

4. Describe two examples of uses of igneous rocks in your region (if applicable) or elsewhere.

In Utah, igneous rocks are used in construction, like construction of our State Capitol Building, LDS Main Temple, and LDS Administration Building. Native Americans in Utah region used igneous rock for every day tools, ranging arrowheads and weapons. In some cases, Obsidian was used and still used for decorative purposes.

Building One: LDS Temple. Location: Dowtown Salt Lake City.

Rock Type: Quartz: Monzonite from Little Cottonwood Canyon

Building Two: LDS Adminstrative Building. Location: Salt Lake City Downtown

Rock Type: Quartz: Monzonite from Little Cottonwood Canyon

Building Three: Utah Capitol, Main part of Salt Lake City

Rock Type: Quartz: Monzonite from Little Cottonwood Canyon