1.3 - Igneous Rocks
Igneous rocks are formed through crystallization from melt. All igneous rocks, with the exception of volcanic glass (obsidian), are made up of interlocking crystals. In rocks that cool slowly, deep below the surface of the earth, these crystals can be quite large and visible to the naked eye. Common coarse-crystalline rocks that cooled slowly in a magma chamber are granite or gabbro. In rocks that cool rapidly from lava after a volcanic eruption, these crystals may be too small to be visible with the naked eye, but a hand lens or microscope easily reveals the crystalline nature of these rocks. Basalt is an example of a fine-crystalline rock that cooled rapidly at the surface of the earth. Igneous rocks can be thought of as the ultimate parent material for most soils. The igneous rock may have been ground up and transported innumerable times between when it first solidified and what we see today.
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Image by C. Geiss
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Image by C. Geiss
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Image by C. Geiss
Granite Properties
Specimen of granite. Image by C. Geiss
Type: Igneous
Distinguishing Features: Completely crystallized; large, well formed interlocking crystals, visible to the naked eye; no preferred crystal orientation; fairly light in color (white, gray, pink).
Main Minerals: quartz, feldspar, biotite
Weathering Behavior: Physical weathering can break the rock along its crystal boundaries; chemical weathering preferentially removes feldspars and biotite, leaving behind a residue of quartz and other weathering resistant minerals. Because the mineral grains are randomly orientated, granite tends to weather into characteristic well rounded shapes as shown below.
Weathered granite (note the rounding). Image by C. Geiss
Impact on Soils: Since granite is a coarse, crystalline rock, it breaks down fairly slowly. The quartz-rich material tends to produce poorly buffered, acidic soils of poor nutrient status.
How it Forms: Granite is an igneous rock that forms by slow cooling of silica-rich magma, deep within the earth. Exhumation through uplift and erosion of the overlying rocks brings granite to the surface.
Gabbro Properties
Specimen of Gabbro. Image by C. Geiss
Type: Igneous
Distinguishing Features: Completely crystallized; large crystals, easily visible with the naked eye; dark in color.
Main Minerals: plagioclase feldspar, olivine, pyroxene, amphibole
Weathering Behavior: Rocks containing olivine, pyroxene, and plagioclase generally weather faster.
Impact on Soils: Weathering of gabbro releases nutrient elements, such as calcium and potassium, which produce initially alkaline soils.
How it Forms: Gabbro forms by slow cooling from magma of basaltic composition. Most basaltic magma erupts from oceanic volcanoes; gabbro is generally associated with oceanic rocks.
Basalt Properties
Specimen of Basalt. Image by C. Geiss
Type: Igneous
Distinguishing Features: Completely crystallized; very small crystals, generally invisible to the naked eye; black or very dark gray in color; sometimes contains gas bubbles (as shown above); weathering of iron minerals can turn the surface tan or reddish brown (see below).
Weathering of iron creates a tan or reddish brown color. Image by C. Geiss
Main Minerals: feldspars, olivine, amphibole, pyroxenes
Weathering Behavior: Depending on crystallinity and mineral composition, basalt can weather fairly rapidly, due to chemical and physical weathering processes.
Impact on Soils: Weathering of volcanic rocks high in basic cations tends to generate fertile, alkaline soils; the black color of basalt causes the soil to warm quickly. Many vineyards are located in soil formed from basaltic rocks.
How it Forms: Basalt is an igneous rock that forms through rapid cooling at the surface of the earth. The island of Hawaii is almost entirely made from basaltic lava flows.