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The rock slab on display comes from a quarry in the state of Espírito Santo, on the east coast of Brazil. It is an example of a metamorphic rock – a rock that has been altered in mineralogy and texture by elevated temperatures and pressures within the Earth. In particular, this rock is referred to as a gneiss (pronounced ‘nice’), characterized by segregation of the minerals into light and dark bands. There are several types of gneisses, this one being a migmatite, a gneiss in which temperatures were high enough to melt parts of the rock.
Many of the interesting textures in this rock were produced by deep burial and compressional forces that occurred when the continents of South America and Africa collided. The continental collision occurred over a long period of time, mostly between about 800 million to 400 million years ago, when tectonic activity slowly closed the Atlantic Ocean and sutured South America and Africa together, along with other land masses, to form a supercontinent called Gondwana. Note that on the diagram below, the areas shaded in red identify rock involved in collisions that formed Gondwana, including in the present location of the state of Espírito Santo. The gneiss in the slab is approximately half a billion years old, and so was impacted by the force of the continents colliding. Since that time, of course, tectonic activity has reopened the Atlantic Ocean basin and again separated South America from Africa.