Earth's surface is shaped by the theory of plate tectonics, which holds that the lithosphere is broken into rigid plates that move and interact over geologic time. Where plates move apart, divergent boundaries form mid-ocean ridges or continental rifts. Where plates collide, convergent boundaries build mountains, trigger volcanoes, or create subduction zones where one plate dives beneath another. At transform boundaries, plates slide horizontally past one another, producing faults such as California's San Andreas Fault. Surrounding the Pacific Ocean, the Ring of Fire is a belt of intense earthquake and volcanic activity produced largely by these converging plate motions.
Volcanoes are dramatic expressions of tectonic activity. Mount Vesuvius, the active volcano near Naples, famously destroyed Pompeii and remains closely monitored. Mauna Loa in Hawaii is the largest active volcano on Earth, part of a volcanic archipelago formed over a Pacific hotspot. In Greek mythology, Mount Olympus was revered as the home of the gods, while Sicily's Mount Etna and Iceland's many volcanoes continue to shape their surroundings. Hawaii itself is a chain of volcanic islands rising from the seafloor in the central Pacific.
Bodies of water and narrow land bridges define many of the planet's most important political and geographic boundaries. The Strait of Gibraltar, a narrow passage between Europe and Africa, separates the two continents at their closest point, while the Bering Strait divides Asia from North America. Rivers also serve as borders: the Rio Grande, called the Río Bravo in Mexico, forms much of the boundary between the United States and Mexico, and the Oder River traces part of the border between Germany and Poland. The Isthmus of Panama, a narrow strip of land, joins North and South America and is sliced by the Panama Canal, which links the Atlantic and Pacific oceans. In Africa, the Suez Canal separates the continent from Asia and provides a vital shipping route between the Mediterranean and the Red Sea. A strait, more broadly, is any narrow body of water connecting two larger ones, while a fjord is a long, deep, narrow inlet carved by glacial erosion between high cliffs. An isthmus is a narrow strip of land connecting two larger landmasses, and a peninsula is land surrounded by water on three sides.