Dry land covers under a third of Earth, yet nearly all human history happened there
Only 29.2 percent of Earth's surface is dry land, and the fresh water that land-dwellers depend on makes up just three percent of the planet's water. Still, almost everything people have ever done has unfolded on this thin slice, and some scientists think the very first cells may have appeared there too.
The English word traces back through Old English to a Proto-Germanic term for untilled ground. Its suffix form runs through place names like England, Greenland and New Zealand. A parallel ending from Indo-Iranian, -stan, meaning place, appears in Pakistan and Afghanistan and also in ordinary Persian words, such as golestân, a place of flowers, or garden.
Height varies enormously. The lowest dry ground lies 418 metres below sea level at the Dead Sea, while the summit of Mount Everest rises 8,848 metres above it. Averaged out, land stands about 797 metres high, and 98.9 percent sits above sea level. Geographers group it into four major continuous landmasses: Africa–Eurasia, the Americas, Antarctica and Australia, which are further divided into as many as seven continents.
None of this is permanent. In 1912 Alfred Wegener proposed that continents drift, an idea later developed into plate tectonics, the theory that Earth's rigid outer shell is broken into moving plates. Over hundreds of millions of years, supercontinents have assembled and split three times. Rodinia began breaking up roughly 750 million years ago, the pieces later joined as Pannotia, and finally Pangaea formed before splitting apart 180 million years ago. Plate edges are where the ground shakes most: the Ring of Fire around the Pacific holds two-thirds of the world's volcanoes and over 70 percent of its earthquake activity.
Surface processes work more slowly but relentlessly. The Colorado River has been carving the Grand Canyon for about 70 million years and still deepens it by roughly 0.3 metres every 200 years. People have sped such change up dramatically, making erosion 10 to 40 times faster than normal. As a result, half of the topsoil on Earth's land has been lost in the past 150 years.
Source: Land
