Felsic magmas have a silicate content of approximately 65 to 70 percent, and because of the high silicate content, tend to be light in color. This type of magma has a high viscosity and a thick, gooey flow. It also has a high gas content and high explosivity during eruption. Felsic magmas tend to form granite and rhyolite rocks.
Intermediate magmas have less silicate content than felsics, about 57 to 63 percent. Their viscosity is less than that of felsics and more than that of mafics. Intermediate magmas are more explosive than mafics, but less than that of felsics. Their eruption temperature is approximately 1,000 degrees Celsius, and these magmas typically form andesite and diorite rocks.
Mafic magmas are often found in ocean basins and are dark in color, because of higher magnesium and iron content. The silicate composition of mafics is typically about 50 percent. Mafics are low in viscosity and explosiveness. Eruption temperatures for mafic magmas are about 1,100 degrees Celsius. Mafic magmas crystallize into basalt rock formations.
Ultramafic magmas are named for their extremely low silicate contents. A typical ultramafic magma has around 50 percent silicate content. Ultramafics are very old magmas and rocks; a few are 90 million years old, but most are 3 billion years old or older. The crystallization pattern indicates extremely high temperatures at the time of formation, and it is estimated that the eruption temperatures exceeded 1,600 degrees Celsius, which is much hotter than eruptions that occur today. Geologists at San Diego State University suggest that these properties would have resulted in a low viscosity and low explosive magma -- lower than that of today's mafic magmas.