Activity 2—Volcanic Landforms
Background Information

Topographic maps are a two-dimensional representation of a three-dimensional land surface. They use lines (or, more precisely, curves) of equal elevation, called contour lines, to show the elevation of the land. A topographic map shows the relief (variation in elevation) of the land surface. Contour lines can be thought of as boundaries that separate areas above that are higher in elevation from areas below that are lower in elevation. The contour interval of a contour map is the difference in elevation between adjacent contour lines. Topographic maps also show many features on the surface, including water bodies, vegetation, roads, buildings, political boundaries, and place names.

Very high temperatures are needed to melt rock. Magma is molten rock beneath the Earth’s surface that has been formed by melting of some of the minerals of a preexisting rock. Ordinarily, only some of the mineral material of the rock is melted, and then the liquid part moves away and collects somewhere else, leaving a residue of harder-to-melt minerals.

Magmas vary widely in composition, depending on the composition of the original rock. Magma composition is the most important factor that determines both the shape and the eruptive style of volcanoes. There are two aspects to this: magma viscosity, and content of volatiles.

The most common chemical elements in magmas are silicon and oxygen, the two most abundant elements in the Earth’s crust. Silicon atoms and oxygen atoms become very strongly bonded together to form an arrangement of one silicon atom and four oxygen atoms in the shape of a tetrahedron. (A tetrahedron is a regular geometrical figure with four sides that are equilateral triangles. It has the appearance of a triangular pyramid.) The four oxygens are at the corners of the tetrahedron, and the silicon is in the center. These are called silica tetrahedra. In most of the important rock-forming minerals, these tetrahedra are joined together, by sharing of oxygen atoms, to form complex chains, sheets, and networks. This joining of silica tetrahedra into larger structures is called polymerization. As the magma cools, the silica structures become bonded with individual atoms of other chemical elements to form silicate minerals. The most important of these other elements are iron, magnesium, calcium, sodium, and potassium. Keep in mind, however, that silica polymerization develops in the magma even before any minerals crystallize from the magma.

The percentage of silica (silicon and oxygen) in magmas varies widely. Magmas that are relatively low in silica, which form rocks like basalt or gabbro as they cool, have relatively high melting temperatures, and the degree of polymerization of the silica in the magma is relatively low. In contrast, magmas that polymerization of the silica in the magma is relatively high.

The ease of flow of a magma, as described
by its viscosity, depends largely on the degree of polymerization of the silica in the magma. Magmas that contain large networks of silica flow much less easily than magmas with small and simpler silica structures. The viscosity of the magma affects the type of volcanic landform that develops and also the style of eruption. Low-viscosity magmas, those that are relatively low in silica and have less polymerization of the silica, are erupted as basaltic lavas that flow relatively freely and far during an eruption. Volcanoes built from such flows are very wide relative to their height, and have gentle side slopes. Such volcanoes are called shield volcanoes. The buildup of hundreds or thousands of flows over time can create a massive volcanic structure, as on the big island of Hawaii. Less viscous magmas, those that are relatively high in silica and have more polymerization of the silica, flow less readily. Such magmas erupt as lavas that ooze out onto the surface close to the volcano, like toothpaste being squeezed out of a tube. The resulting landform tends to be a steep-sided volcanic dome, such as the one that has been forming at Mt. St. Helens since the major eruption of 1980. There are many such volcanic domes in Yellowstone as well.

The cooler, silica-rich magmas tend to have a much higher content of what are called volatiles: compounds like water or carbon dioxide that have low boiling points. At the great pressures deep in the Earth, such volatiles remain dissolved in the magma, but as the magma approaches the surface and becomes erupted as lava, the volatiles come out of solution to form bubbles of gas, just as in carbonated beverages. Because of this, silica-poor magmas tend to be erupted as quiescent flows of liquid, whereas silica-rich magmas tend to be erupted explosively, because of the buildup and the sudden release of gas pressure caused by the outgassing of the volatiles. The 1980 eruption of Mt. St. Helens is a good example (although relatively small!) of such explosive eruptions.

The EarthComm web site also contains a variety of links to web sites that will help you deepen your understanding of content and prepare you to teach this activity.