What special geologic conditions would cause these spectacular eruptions of molten rock at the earth’s surface? Measurements taken in deep mines and oil wells show that the normal increase in the earth’s temperature with depth is about 1°F per 100 feet. This heat is generated by the decay of radioactive elements—chiefly uranium, thorium, and potassium—which are present in at least small amounts in virtually all rocks of the earth’s crust. Ordinarily, enough heat is conducted to the earth’s surface so that the deeply buried rocks do not become hot enough to melt. In some places, however, the heat is not carried off fast enough, and the temperature rises slowly toward the melting point of the rock. Such hot spots may develop (1) because the rocks in those places contain more than an average amount of radioactive elements; (2) because hotter material moves upward from still deeper levels in the earth; or (3) because drastic changes in pressure are brought about by the alternate squeezing and relaxing of mountain-building forces, which in turn substantially affect the melting point of the rocks. Whatever the cause, the eventual result is the accumulation of a huge body of molten rock, called _magma_, enclosed in a deep underground chamber. Magma, being a mixture of hot liquids and gases that is lighter in weight than the solid rocks surrounding it, tends to rise toward the earth’s surface. Forcing its way upward, some of the molten material solidifies before reaching the surface and forms bodies of various kinds of _intrusive igneous rocks_ (fig. 15). Some of the magma, however, reaches the surface and either pours out as lava or is blown out explosively as rock fragments, ash, and pumice to form _extrusive igneous rocks_. [Illustration: INTRUSIVE AND EXTRUSIVE IGNEOUS ROCK BODIES. Extrusive rocks solidified above ground, and intrusive rocks solidified below ground. All features shown occur in Yellowstone National Park. Extrusive rocks are the predominant rock type seen along the Park roads, and the table lists the three principal kinds that are present. (Fig. 15)] Rock name Principal rock-forming Color minerals Rhyolite Quartz,[a] feldspar[b] Light to medium shades of gray (sanidine). and brown. Andesite Feldspar[b] Medium to fairly dark shades of (plagioclase), brown, red, purple, and gray. pyroxene[c] (augite). Basalt Feldspar (plagioclase), Nearly black. pyroxene, olivine,[c] magnetite.[d] [a]Clear to light-colored silicon dioxide. [b]Light-colored aluminum silicate minerals. [c]Dark-colored iron and magnesium silicate minerals. [d]Very dark colored iron oxide mineral.