The Commonwealth of Puerto Rico has an area of 13,790 square kilometers (5,320 sq mi), of which 8,870 km2 (3,420 sq mi) is land and 4,921 km2 (1,900 sq mi) is water. The maximum length of the main island from east to west is 180 km (110 mi), and the maximum width from north to south is 65 km (40 mi). Puerto Rico is the smallest of the Greater Antilles. It is 80% of the size of Jamaica, just over 18% of the size of Hispaniola and 8% of the size of Cuba, the largest of the Greater Antilles.
Puerto Rico is mostly mountainous with large coastal areas in the north and south. The main mountain range is called "La Cordillera Central" (The Central Range). The highest elevation in Puerto Rico, Cerro de Punta 1,339 meters (4,393 ft), is located in this range. Another important peak is El Yunque, one of the highest in the Sierra de Luquillo at the El Yunque National Forest, with an elevation of 1,065 m (3,494 ft).
Puerto Rico is composed of Cretaceous to Eocene volcanic and plutonic rocks, overlain by younger Oligocene and more recent carbonates and other sedimentary rocks. Most of the caverns and karst topography on the island occurs in the northern region in the carbonates. The oldest rocks are approximately 190 million years old (Jurassic) and are located at Sierra Bermeja in the southwest part of the island. They may represent part of the oceanic crust and are believed to come from the Pacific Ocean realm.
Puerto Rico lies at the boundary between the Caribbean and North American plates and is being deformed by the tectonic stresses caused by their interaction. These stresses may cause earthquakes and tsunamis. These seismic events, along with landslides, represent some of the most dangerous geologic hazards in the island and in the northeastern Caribbean. The most recent major earthquake occurred on October 11, 1918, and had an estimated magnitude of 7.5 on the Richter scale. It originated off the coast of Aguadilla and was accompanied by a tsunami.
The Puerto Rico Trench, the largest and deepest trench in the Atlantic, is located about 115 km (71 mi) north of Puerto Rico at the boundary between the Caribbean and North American plates. It is 280 km (170 mi) long. At its deepest point, named the Milwaukee Deep, it is almost 8,400 m (27,600 ft) deep, or about 5.2 miles. The island experiences frequent tremors and is an area of concern for major earthquakes.