Facts of the Matter
Facts of the Matter
Silicon fuels both
volcanic magma and
the digital worldVolcanoes, glass, white sand beaches, solar cells and computers may not appear to have much in common, but they do and it is silicon.
Silicon, the second most abundant element in Earth's crust, is a chemical relative of carbon that has a chemical "crush" on oxygen, the most abundant. Silicon and oxygen are also the two most abundant components of magma. Magma is formed when rock melts at depths of a few tens of miles to a few hundred miles beneath Earth's surface, then works its way up where it either solidifies underground or erupts. When it erupts it is called lava.
Magma is a mixture of many different kinds of atoms, but its properties are mainly determined by silicon and oxygen. It is the geometry of shapes formed by the marriage of silicon and oxygen that controls the explosiveness of volcanic eruptions.
Some volcanic eruptions (like the one in progress on the Big Island) are gentle, while others (such as Mount St. Helens and most volcanoes found around the Pacific rim, in the Italian Peninsula and in the Caribbean) are explosive.
Hawaiian volcanoes are known for their gentleness, typically producing fountains of lava and orange rivers that flow great distances. By contrast, continental volcanoes like Mount St. Helens typically erupt violently with very little lava, producing tremendous clouds of ash instead.
Silicon is chemically similar to carbon, but silicon, instead of forming chains like carbon atoms, bonds preferentially with oxygen. The silicon-oxygen bond is one of the strongest in nature and remains strong and stable even at very high temperatures.
The geometry and stability of the silicon-oxygen arrangement results from a coincidence of size and electrical properties of the atoms of the two elements. The shape that results when two or more atoms bond depends on the number of bonding sites, where on the atom the bonding sites occur and the sizes of the atoms. A silicon atom is much smaller than an oxygen atom (about the same as a baseball compared with a basketball). In fact it is just the right size to fit neatly in the space between three oxygen atoms that are just touching one another. A fourth oxygen atom fits nicely on top to form a pile of oxygen atoms with the silicon atom buried in the center. The pile is in the shape of a tetrahedron, a triangular pyramid with four sides. This silica tetrahedron is the basic geometric structure in magma and lava and all minerals based on silicon, including quartz, the most common constituent of beach sand.
The silica tetrahedron is responsible for the thickness or viscosity of magma. The tetrahedrons are already formed in the magma long before it is cool enough to solidify, and the more tetrahedrons there are in the magma, the more viscous it is, because the corners of the tetrahedrons tend to stick to one another. The more viscous the magma, the more explosive the eruption, because it is harder to extrude the lava from the volcanic opening during an eruption.
Continental-type volcanoes tend to have a greater abundance of silica tetrahedrons and thus have sticky magma and violent eruptions. Hawaiian volcanoes have a relatively low abundance of silica, so the fluid lava can be forced out of the volcanic opening under pressure producing eruptions that fountain or pour from the volcanic vent.
Quartz is a mineral made of pure silica. Quartz crystals are three-dimensional networks of tetrahedrons attached tightly by sharing oxygen atoms at their corners. They form as the silica-rich magma of continental volcanoes cools and solidifies. Quartz persists in the environment by resisting breakdown as the rocks are weathered and the quartz crystals are transported to the ocean by streams and rivers. Because quartz is both abundant and resistant, continental beaches are typically made of sand grains that are nearly pure quartz. In contrast, Hawaiian lavas contain such a small concentration of silica tetrahedrons in the first place that there is virtually no quartz in Hawaii rocks, and none on the beaches.
It is from this quartz that we get the silica from which we make glass and, when refined, the silicon for computer chips and solar cells. To make glass the quartz sand must be melted, and its high melting temperature makes it an energy-intensive and therefore expensive process.
To make chips and solar cells, the silicon must be extracted from the oxygen, breaking the strong bonds, then purified and grown into a pure crystal. The energy to break the bonds and the precision technology required to make computer chips and efficient solar cells make this an expensive process.
In both cases it is the processing that accounts for the cost, and not the rarity of the silicon.
That we have been able to understand the properties of silicon and to exploit them is noting short of amazing. That the same silicon we use to control information and electronic systems also controls the awesome power of volcanoes goes beyond amazing, all the way to awesome!
We could all be a little smarter, no? Richard Brill picks up
where your high school science teacher left off. He is a professor of science
at Honolulu Community College, where he teaches earth and physical
science and investigates life and the universe.
He can be contacted by e-mail at rickb@hcc.hawaii.edu