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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Land | 2/9 | https://en.wikipedia.org/wiki/Land | reference | science, encyclopedia | 2026-05-05T07:20:45.096604+00:00 | kb-cron |
The earliest material found in the Solar System is dated to 4.5672±0.0006 bya (billion years ago); therefore, Earth itself must have been formed by accretion around this time. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory, a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disc, out of which the planets then grow (in tandem with the star). A nebula contains gas, ice grains and dust (including primordial nuclides). In the nebular hypothesis, planetesimals begin to form as particulate matter accumulates by cohesive clumping and then by gravity. The primordial Earth's assembly took 10–20 myr. By 4.54±0.04 bya, the primordial Earth had formed. Earth's atmosphere and oceans were formed by volcanic activity and outgassing that included water vapour. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, protoplanets, and comets. In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly formed Sun was only at 70% luminosity. By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind. The atmosphere and oceans of the Earth continuously shape the land by eroding and transporting solids on the surface. Earth's crust formed when the molten outer layer of Planet Earth cooled to form a solid mass as the accumulated water vapour began to act in the atmosphere. Once land became capable of supporting life, biodiversity evolved over hundreds of millions of years, expanding continually except when punctuated by mass extinctions. The two models that explain land mass propose either a steady growth to the present-day forms or, more likely, a rapid growth early in Earth history followed by a long-term steady continental area. Continents are formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the Earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken apart three times. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.
=== Landmasses ===
A continuous area of land surrounded by an ocean is called a landmass. Although it is most often written as one word to distinguish it from the usage "land mass"—the measure of land area—it may also be written as two words. There are four major continuous landmasses on Earth: Africa–Eurasia, America (landmass), Antarctica, and Australia (landmass), which are subdivided into continents. Up to seven geographical regions are commonly regarded as continents. Ordered from greatest to least land area, these continents are Asia, Africa, North America, South America, Antarctica, Europe, and Australia.
=== Terrain ===
Terrain refers to an area of land and its features. Terrain affects travel, mapmaking, ecosystems, and surface water flow and distribution. Over a large area, it can influence climate and weather patterns. The terrain of a region largely determines its suitability for human settlement: flatter alluvial plains tend to have better farming soils than steeper, rockier uplands. Elevation is defined as the vertical distance between an object and sea level, while altitude is defined as the vertical distance from an object to Earth's surface. The elevation of Earth's land surface varies from the low point of −418 metres (−1,371 feet) at the Dead Sea, to a maximum altitude of 8,848 metres (29,029 feet) at the top of Mount Everest. The mean height of land above sea level is about 797 metres (2,615 feet), with 98.9% of dry land situated above sea level. Relief refers to the difference in elevation within a landscape; for example, flat terrain would have "low relief", while terrain with a large elevation difference between the highest and lowest points would be deemed "high relief". Most land has relatively low relief. The change in elevation between two points of the terrain is called a slope or gradient. A topographic map is a form of terrain cartography which depicts terrain in terms of its elevation, slope, and the orientation of its landforms. It has prominent contour lines, which connect points of similar elevation, while perpendicular slope lines point in the direction of the steepest slope. Hypsometric tints are colors placed between contour lines to indicate elevation relative to sea level. A difference between uplands, or highlands, and lowlands is drawn in several earth science fields. In river ecology, "upland" rivers are fast-moving and colder than "lowland" rivers, encouraging different species of fish and other aquatic wildlife to live in these habitats. For example, nutrients are more present in slow-moving lowland rivers, encouraging different species of macrophytes to grow there. The term "upland" is also used in wetland ecology, where "upland" plants indicate an area that is not a wetland. In addition, the term moorland refers to upland shrubland biomes with acidic soils, while heathlands are lowland shrublands with acidic soils.
=== Geomorphology ===