Rocks constitute a great part of our Earth. Hills and mountains are made of rock, valleys and seas rest over rocks; rocks are the best chronicle of Earth's past. By studying our native indian's stone utensils we may determine facts about their culture. Even the traces of animals that no longer exist, carved in rock in different places around the world, help us reach conclusions about our Planet's prehistoric era. Therefore, great part of what we know about our past has been learned from rocks.

     Eventhough some may state the contrary, it is mistaken to believe that rocks never change. Day to day variations are imperceptible, but the truth is they are constantly changing their shape and size. These variations are generally very slow, but incessant. Cracks are commonly visible on rocks, fragments often loosen around them, causing the rock mass to diminish. Wind and water, little by little, also wear them out. The day inevitably comes when after decades, centuries, or milleniums only a few grains of sand or diminute particles of mud remain in place of the rock. What was once stone, slowly converted into soils and, some of these, dragged by rivers, filled out interior valleys.

     Other soils, sands, clays, or muds are dragged to the bottom of the sea, where they settle and create layers. The weight of the top layers cause the consolidation of particles of soil in the lower layers to the point where, with time, they turn into rocks. In other words, worn out rocks make up soils that evolve into rocks which, in turn, are worn out and converted into soils that later consolidate to make up rocks.

     Many other changes occur at the Earth's surface. Powerful natural forces cause layers of rock to rise and form mountain ranges. This may happen as well on the bottom of the seas, causing volcanic cones to emerge from their depths and form islands like Martinique and Saba, on the Caribbean, and some islands on the Pacific. In other places, segments of Earth's crust have sunken, and the surrounding water has precipitated to the spaces left unoccupied, forming new seas.

     Many millions of years ago, turbulent seas covered what we know today as the Antilles including Puerto Rico and parts of North America. The incesant changes to which our Earth is subjected force rocks to emerge over the ocean's surface in some cases, and in others, to submerge under it. In some places these risings and sinkings or submersions have appened many times, as they have in the little piece of Earth we call Puerto Rico. This is why we need to be familiar with a country's geology to be able to appreciate it more and know it better.

     Geology is the scientific study of what occurred and what is occurring on the surface of the Earth; it studies the characteristics of the terrestrial cortex , the variations occurring on it, and the natural forces causing them. Some of the related subjects that geologists tend to are the accumulation of minerals, the formation of rocks and soils, the changes in elevation (sinkings and risings), earthquakes, volcanic erruptions, and erosion.

     Geologists have determined that rocks can be classified in three groups according to their origin: igneous rocks, like granite and andesite, of volcanic origin; sedimentary rocks, like limestone and sandstone, formed by the action of wind and water; and metamorphic rocks, like marble and serpentine, which were originally igneous or sedimentary, but suffered changes over time mainly due to the effects of heat and pressure.

     In order to know Earth better, geologists make explorations, magnetic studies and chemical analisis. In their studies, they make use of X rays and Geiger counters, which are radioactivity detectors. During the last ten years, the use of new scientific methods and technology in geological investigations carried out in Puerto Rico has permitted a better evaluation of the existence, extraction, and exploitation of rocks and their minerals.One of geology's most important contributions to human understanding is its concept of the long time elapsed since the history of the Earth began.

     Man's history makes up a very insignificant part of this extension. A long, extremely long, time had to pass for the surface of the Earth to reach its current state. A political history student speaks of decades and centuries, a geology student refers to periods of 20 to 40 million years and to eras, which are even longer periods. A long time has passed since valleys, mountains, rivers, beaches, rocks and soils in Puerto Rico began to take shape.

     Trough their studies of rock composition and structure, and of their superposition anddisintegration, geologists have amplified our concept of time. They have also studied fossils, which contain many rocks and are evidence of life in the past.   

     With these studies they try to reach an understanding of how terrestrial materials are formed and changed as a result of the action of natural forces operating through time; and they try to reconstruct the geological history of continents, countries and regions. Then, they investigate how to apply the knowledge they have acquired by studying Earth's changing surface.

     Geologists search for scientific answers to the following questions: What happened?  How did it take place and how will what happened affect us?  What might happen in the future? In what way can we use our knowledge to better our environment, our culture, and our economy? Each of us may have also asked ourselves questions like these many times in an effort to know more about our world and our Island.

Author: Rosa Navarro Haydon                                                                                

Translated by: Karen Mojica Franceschi