The planet Earth is shaped like an oblong ellipsoid composed of different layers, extending from its center to its outer surface. The outermost layer, which is thin and brittle, is called the Earth's crust. With a thickness of 8 to 40 km (5 to 25 miles), it is made up of fragments of varying sizes that fit together like pieces of a jigsaw puzzle. These fragments are tectonic plates. They often move, causing earthquakes, and when they do so abruptly, they can have catastrophic consequences for human populations.
The Earth's crust forms the continents and the ocean floor. It is composed of ocean basins, cliffs, mountain ranges, hilly regions, and valleys that we humans are familiar with.
The varied shapes that the Earth's crust can take are known as relief, and their appearance is partly due to the rocks that compose them. Rocks are classified by experts according to their formation process or the minerals they contain.
New Earth's crust, and consequently new rocks, are formed each time a volcano erupts or in the deepest regions of the oceans where new ocean floor is constantly being formed. Lava is very hot, molten rock that rises to the surface from deep within the planet. When this occurs, the rocks are called extrusive (or volcanic) igneous rocks. Basalt is a prime example. Conversely, when molten rock seeks its way to the Earth's surface but fails to reach the surface and instead solidifies (or lithifies) in fissures in the Earth's crust, it is called intrusive igneous rock. Granite, which is sometimes used in construction, is a type of intrusive igneous rock.
When tectonic plates move, they can compress, fold, and subject the rocks that compose them to great pressures or temperatures. This often causes changes in the crystals that form them or their chemical composition, creating metamorphic rocks.
Rocks exposed on the Earth's surface disintegrate, break down, crumble, and dissolve when exposed to rain, wind, plant roots, and animals, among other factors. When this occurs, small fragments known as sediments are formed. Sediments are generally transported by wind, snow, or water and are deposited in flat, gently sloping areas, such as valleys and ocean floors. Over time, sediments or the remains of other animals (such as shells, carapaces, or plant remains) harden (or lithify), forming new rocks known as sedimentary rocks. Limestone is a type of sedimentary rock (Strahler, 2006).
The map presented in this section shows the different types of rocks and sediments that make up the Estuary area. This region includes the foothills of the interior mountains, marshlands, wetlands, and coastal areas. This explains the diversity of rock types and sedimentary deposits observed. Alluvial materials predominate (31%), which are a type of sediment formed and transported by the action of water. Artificial fill (14%), sedimentary rocks (15%), and other sedimentary deposits (8%) are also prevalent.
It is important to note that this is a predominantly urban region that has undergone significant changes due to human activity. The construction of houses, buildings, and roads involves the removal of sediment, the channeling and diversion of river courses, the creation of terraces on hills and mountains, and the deposition of artificial fill, among many other actions. This leads to changes in the topography, the original sediment deposition areas, and the configuration of the natural landscape.
This region encompasses a diverse landscape. In its northernmost reaches, it features rocky coastal areas and sandy plains, wetlands, alluvial plains, and hills that encompass the foothills of Puerto Rico's central mountain range. This explains the wide variety of geological substrates observed. Alluvial soils predominate (31%), followed by sedimentary rocks (27%), artificial fill (14%) from construction and urban development, and swamp deposits (12%).
