28. Great Dike, Zimbabwe, Africa
The Great Dike of Zimbabwe forms the prominent dark band running from top to bottom of this picture. The dike, which can be traced for up to 500 kilometers (300 miles) northward across the Archaean craton of central Africa, is of basic composition and cuts a complex series of gneisses and deformed igneous rocks. Although basaltic dikes are among the most common of igneous intrusions around the world, the Great Dike appears to be unique in its scale. Its thickness (many kilometers) is more exceptional than its length, and it is not clear what the tectonic factors were that led to the formation of such a large dike. Most dikes are members of dike swarms, where large volumes of magma are intruded along many closely spaced, narrow dikes. In the case of the Great Dike, however, a large volume of magma has been channeled into one exceptionally large, complex dike.
The dike is compound; it is formed from more than one single intrusion. Some thinner dikes parallel to it can be seen (particularly on the left), and a number of north-west striking faults that offset the dikes are conspicuous.
STS-8, August-September 1983. Picture #8-33-1000.