Introduction

Abstract

Approximately 200 million years ago, the continental nuclei of Europe, Africa, and the Americas began to separate as a result of dynamic processes taking place within the earth (Figure 1). As the land masses moved apart, a large crack or rift zone developed in the Earth’s rigid outer layer, the lithosphere. In response to separation of the lithosphere along this zone of rifting, plastic material from the earth’s interior rose toward the surface filling the void. Unlike the lighter granitic rock which forms the core of most continents, the rising material from the upper mantle crystallized to form basaltic and related denser rock. Since the lithosphere of the earth is floating on denser, more plastic deeper layers, this newly created basaltic rock initially equilibrated at a level lower than the lighter granitic continents nearby. The result was a depression, the Atlantic Ocean, situated between the separating continental masses. This depression was filled by the invading waters of the Tethys Sea (now Mediterranean) and Pacific Ocean which existed at that time.