6 Igneous Rocks and Dad’s Heart Attack

Rocks, a combination of minerals, are the words that we use to create the story of the Earth. Their composition and physical properties determine how they are separated and where on Earth they end up, such as on the continents or in the ocean bottoms.

A year and a half before I was born, my father had a heart attack. My father’s doctor put dad on a strict diet—low fat and low cholesterol. The pigs and chickens my parents raised at the time were sold, and the vegetable garden was expanded. When my mom cooked any meat, she would boil it first, then cool it and skim off the fat. My mom was using a process similar to what created the continents, using the lower melting point and density of fat relative to the meat to separate the two. Such differences in physical properties can also separate rocks.

Most of the minerals that form rocks in the mantle and crust are silicates. Silicates may be distinguished by the amount of silica each contains. Olivine and pyroxene have large percentages of iron and magnesium. These form rocks referred to as mafic (or ultramafic) rocks. Rocks with 70% silica or more are felsic. Felsic rocks are lighter in color, less dense, and have a lower melting point than mafic rocks. Felsic rocks behave similarly to the fat, while mafic rocks behave like the meat.

Water boils at 100 degrees C, above the melting point of fat but below that of meat. The melting point of most felsic minerals is around 700 degrees C, while that of mafic minerals is around 1300 degrees C. Thus, if we heat a rock to about 1000 degrees C, then the felsic minerals will melt while the mafic ones will not. Because the felsic material is less dense, it will tend to be buoyed upward, similar to the fat in the boiling water. This separation brings the lighter colored, low density, felsic rocks to the surface to form the continents.

New igneous rock is constantly being formed as volcanoes erupt and intrusions cool. Magma from the mantle is constantly erupting at midoceanic spreading centers to form new oceanic crust. This new rock is mafic and is the most common rock on the Earth’s surface: basalt, the extrusive form, and gabbro, the intrusive form. (Look at a map. You’ll see a lot more ocean than continent.) Mantle hot spots, such as underlie Hawaii, erupt basalt. In addition, rifting of a continent allows mafic rock to rise to the surface, such as where the Middle East is splitting off from Africa. At subduction zones, melting oceanic crust is mafic, but sediments that are subducted with the plate are often felsic. An intermediate composition may result, such as is found at many of the volcanoes in Washington and Oregon. Occasionally, an eruption of felsic material, such as at Yellowstone National Park, may occur as a continent passes over a hot spot and melts its felsic rock.

Distinguishing felsic from mafic rocks gives us clues about where and how the rocks originated. As we study geology, we can better understand our world and develop more choices about how we live our lives. For example, we have learned how slowly soils form and how quickly humans are causing them to erode. We have learned how the chemicals we release into the air destroy the ozone and speed global warming. As to heart attacks, we have learned that our own choices—food, exercise, smoking—can reduce our chances of a heart attack. Dad chose to stop eating foods that were hurting him and had no further heart problems. Perhaps we can choose to stop doing so many things that hurt the Earth.

Notes on Rocks:

In summary, felsic minerals (like fat) have a lower melting point, a lower density, and lighter color—lower, lower, lighter. They form the scum that floats to the top and stick up—the continents, which are much thicker than the oceanic plates.

Mafic minerals (like meat) have a higher melting point, greater density, and darker color. They form the plates of the ocean basins.  When we talk about felsic and mafic rocks, we are discussing igneous rocks, rocks that form from melted rock that recrystallizes.

Two other rocks types, together with igneous rocks, form the rock cycle:

igneous: form from magma and lava (magma that has come to the surface), previously existing rock that has melted and then recrystallized.

metamorphic: form from preexisting rocks that are squeezed and heated to transform them but have not melted.

sedimentary: form from preexisting rocks that have been broken apart, transported, deposited, and cemented back together.

We’ll sketch the rock cycle together in class.