The Paraná Basin, in southern Brazil, contains about 1.2 million square km of flood basalt, one of the largest flood-basalt provinces in the world. The basin also has two other distinctions. First, it is host to the largest waterfall in the world, the Iguaçu Falls, where immense volumes of water spill over a cliff in the basalt sheets. Second, its basalts contain some of the world's largest known vesicles (preserved gas bubbles). These bubbles appear when magma rises and undergoes decompression; they're like the bubbles you see in a carbonated drink when you open it. In most basalts, vesicles range in diameter from millimeters to a few centimeters, but the Paraná's reached a meter in length. Through time, water percolating through the basalt precipitated spectacular crystals of amethyst (purple quartz) along the surface of these vesicles. After millions of years, in some places tropical chemical weathering transformed the basalt into weak, clay-rich soil. This weathering, however, does not affect the amethyst-lined vesicles because amethyst (quartz) is very resistant to weathering. Thus, the vesicles, well known to gem collectors worldwide, remain as intact spherical-to-ellipsoidal geodes. Miners quarry these geodes out of the weak matrix. When cracked open, they look like giant eggs lined with gems.

Other Feature Articles in this chapter:  1

top of page