A landmass is a geographic and geologic term describing any large, continuous area of land not significantly broken up by oceans. Geologically, it represents a defined section of continental crust that extends above sea level, forming the foundation of all terrestrial environments. A landmass includes not only the visible dry ground but also the submerged extensions of the Earth’s crust. This concept allows scientists to categorize and study the planet’s surface structure, from continents to islands.
Defining Landmasses by Scale and Composition
The material composition beneath the surface determines if a structure qualifies as a landmass. The Earth’s crust is divided into two types: continental and oceanic. Continental crust is considerably thicker, typically ranging from 30 to 50 kilometers in depth, and is primarily composed of less dense, felsic rocks such as granite, which are rich in silicon and oxygen. This lower density allows the continental crust to float higher on the underlying mantle.
Oceanic crust is much thinner, generally between 5 and 10 kilometers thick, and is made of denser, mafic rocks like basalt, which contain more iron and magnesium. Because of its higher density, the oceanic crust sits lower, forming the deep ocean basins. A landmass requires this thick, buoyant continental crust to rise above the ocean surface, distinguishing it from the submerged oceanic floor.
The Largest Landmasses: Continents
Continents represent the most extensive examples of landmasses, encompassing both the dry land and the shallow, submerged terrain that surrounds them. There are seven recognized continents: Asia, Africa, North America, South America, Antarctica, Europe, and Australia. The geological boundary of a continent extends far beyond the visible coastline, incorporating an area called the continental shelf.
The continental shelf is the gently sloping, underwater extension of the continental crust that can stretch for hundreds of kilometers from the shore. This shelf shares the same rock composition and structure as the land above sea level. This submerged region ends at the continental slope, where the seabed rapidly descends to the deep ocean floor. The actual edge of a continent is the point where the continental crust transitions into the oceanic crust below the waves.
Smaller Structures: Islands and Archipelagos
Islands also meet the basic definition of a landmass, being pieces of land surrounded by water, but they are significantly smaller than continents and often have a different origin. Islands that sit on a continental shelf, like Great Britain, are known as continental islands, as they are extensions of the nearby large landmass. These islands were often connected to the mainland during periods of lower sea level, such as the Ice Ages.
Other islands are formed independently of the continental crust and are known as oceanic islands. Their creation is highly varied, often resulting from tectonic activity or mantle hotspots. A collection or chain of these smaller landmasses is referred to as an archipelago.
Volcanic Islands
Volcanic islands, such as the Hawaiian chain, emerge from the ocean floor due to mantle hotspots or tectonic activity. They build up a mass of igneous rock until it breaks the surface.
Coral Islands
Coral islands, or atolls, form when coral reefs grow around a subsiding volcanic island. This process leaves a ring of coral rock encircling a lagoon.
Geological Processes of Landmass Formation
The Earth’s landmasses are constantly being created, moved, and reshaped by plate tectonics. The lithosphere is broken into several large, rigid tectonic plates that float on the semi-fluid asthenosphere beneath them. The slow, continuous movement of these plates drives the formation of continents and mountain ranges.
Landmasses grow and collide at convergent boundaries, where two plates are pushed together, forcing continental crust to crumple upward into mountain belts, such as the Himalayas. Conversely, landmasses are fragmented at divergent boundaries, where plates pull apart, a process known as rifting. This spreading creates new oceanic crust and can split a single landmass into two separate continents, as happened with the supercontinent Pangaea about 150 million years ago. This cycle of assembly and dispersal, sometimes called the supercontinent cycle, demonstrates that the configuration of the planet’s landmasses continues to evolve over geologic time.