Rock Texture
In the late 1700s, while working in a field near his home in Scotland, James Hutton noticed coarse-grained granites cutting across and between layers of sedimentary rocks. Wondering how they penetrated the smooth fine sediments, Hutton thought they might have been forced into and between cracks as liquid magma.
The rock’s texture provided Hutton with clues that the different rock types came from a different beginning. As Hutton studied more and more about granites, he concentrated on sedimentary rocks that shared a border with the coarse granites, compared to sedimentary rock where no granite was present.
Hutton thought that the physical changes he saw in bordering sedimentary rocks must have come from an earlier exposure to high heat. This gave him the idea that molten magma from deep within the Earth had squeezed into areas of sedimentary rock and crystallized.
Grain size and color are the two main ways that geologists describe rock textures.
The size of the minerals or crystals that make up a rock’s texture is called grain size. Color can change depending on lighting, mineral content and other factors, so it is thought to be less dependable when describing a specific rock.
When a rock’s grains can be easily seen with the eye, roughly a few millimeters across, they are classified as coarse grain. When individual grains are not visible, the texture is considered to be fine.
Mineral grains or crystals have an assortment of different shapes and textures. They may be flat, parallel, needle-like, or equal in every direction like spheres or cubes. The shapes that crystals take, along with their grain size, combines to make rock samples unique. Think of it in terms of people and cultures of the world. Just as the combination of genetic inheritance and environment makes people individual and unique, the same thing happens with rocks!
Granites have a coarse grain size compared to obsidian with a very fine grain size. Granites are used for building materials because of their larger grain size and decorative pink or gray color. Obsidians are used for jewelry and art.
Intrusive Igneous Rocks
The cooled, crystallized magma that forces its way into the surrounding unmelted rock masses deep in the Earth is called intrusive igneous rock. It can be identified by its interlocking large crystals (grains 1mm or larger), which grow slowly as magma cools over time. These are the large crystals Hutton observed. Commonly, large mineral grain igneous rock is known as phanerites. When this type of rock is formed several kilometers below the surface of the Earth, it is known as plutonic rock. All masses of intrusive igneous rock, whether large or small, are called plutons. They are created on the slow boat of movement to the surface and have a lot of time to solidify and develop their individual unique compositions.
Plutons are given specific names depending on their size and shape. Dikes are areas of intrusive igneous rock that thrust up through other rock layers. They are generally perpendicular to the layers above, found at any depth, and trace the last push of a finger of upward rising magma. A volcanic neck is different from a dike in that it is discordant. It forms the feeder pipe just below a volcanic vent. The 400m upthrusting mass of igneous rock known as Shiprock, New Mexico, United States is an ancient volcanic neck.
Sills are areas of intrusive igneous rock that are parallel to the layering of other intruded rock layers. They form flat, horizontal pockets between piles of rock layers at shallow depths and lower overlaying pressure. A laccolith is a mass of intrusive igneous rock that has pushed up between rock layers and been stopped to form a dome-shaped mound that looks like a blister.
Dikes and sills are often found together as part of a larger pluton network of intrusive igneous rock called a batholith.
Large plutons with outcrop exposures (rock sticking up through the ground) of greater than 100km2 are called batholiths and are huge compared to dikes and sills. They cover thousands of square kilometers and stretch across big parts of states and even between countries. One such monstrous batholith is the Coast Range Batholith that stretches from southern Alaska down the western coast of British Columbia, Canada to end in the state of Washington. It is roughly 1500km in length.
When a batholith has outcrop exposure of less than 100km2 in length, it is called a stock. A stock of igneous rock is often found as a minor collection of rock located near the main batholith or as part of a mostly worn-away batholith.
Batholiths are huge masses of intrusive igneous rock, usually granite, with an exposed surface of larger than 100km2 and formed in the subduction zone along continental plate borders.
Granite, an example of intrusive igneous rock that crystallized slowly from magma below the Earth’s surface, makes up a large portion of plutons andbatholiths. Geologists’ measurements of large batholiths have recorded depths of between 15 and 30km thick.
A single magma can crystallize into an assortment of igneous rock types. It doesn’t solidify into one compound like water does when it freezes into ice. When magma solidifies, it forms many different minerals, which all crystallize at different temperatures. The different crystals solidify from the liquid magma, when their crystallization temperature is reached. Like a row of dominoes, as the temperature drops, crystals form one after another. Cooling magma then contains some fluid rock and some rock that has already hardened into crystals. When this happens, the concentration of certain minerals in the remaining magma is increased. Sometimes when crystals are forming from an isolated chamber of magma, they are denser than the surrounding fluid. When this happens, they sink to the bottom of the chamber and form a separate crystalline layer with characteristics different from the remaining magma.
As cooling, crystallization, and sinking of minerals continues, many crystal layers with different compositions are formed.
In the early 1900s, geologist Norman Bowen was the first person to understand the importance of the temperature and the formation of separate crystals from magma. His studies at the Geophysical Laboratory inWashington, DC, which focused on the melting and crystallization properties of minerals, showed that as magma cooled at different temperatures, the composition of later formed crystals was very different. Bowen found that early forming crystalline rock had a lot more calcium, than later formed crystalline rock. As time went on, other geologists got interested in Bowen’s ideas and the process of separating crystalline fall out from liquid magma became known as magmatic differentiation by fractional crystallization.
