Mount Everest

Long ago, two huge pieces of land crashed together very slowly.

They pushed the rock up and up. That made the Himalayas. Everest is still growing, just a tiny bit each year. Some rocks at the top were once at the bottom of a sea!

Earth's outer shell is broken into giant pieces called plates. Between about 40 and 50 million years ago, the plate carrying India moved north and crashed into the plate carrying the rest of Asia. The crash slowly squeezed and folded the rock upward, and the Himalayas began to rise.

Some of the rock near Everest's top formed on the floor of an ancient sea, the Tethys Sea. A band of yellowish limestone just below the summit is made of that old sea-floor rock. Measuring instruments on the mountain show it is still moving a few inches northeast and rising a fraction of an inch every year. Glaciers, rivers of ice, keep wearing it away at the same time.

The Himalayas were thrust upward as the Indian-Australian Plate moved northward and was subducted beneath the Eurasian Plate after the two collided between about 40 and 50 million years ago. Britannica dates the start of the Himalayas' rise to about 25 to 30 million years ago, and the Great Himalayas took their present form during the Pleistocene Epoch (about 2.6 million to 11,700 years ago). Everest sits in a massif that forms a focal point, or knot, of this tectonic action.

The mountain is built of nappes, layers of rock folded back on themselves. Metamorphic schists and gneisses at lower elevations are topped by igneous granites, and above them lie sedimentary rocks of marine origin, remnants of the floor of the Tethys Sea that closed after the collision. The Yellow Band, a limestone formation, is visible just below the summit pyramid. GPS instruments in place since the late 1990s show the mountain moving a few inches to the northeast and rising a fraction of an inch each year, while glacial action is the main force eroding it.

The top of Everest is very, very cold.

The wind blows hard up there. The air is thin, so there is less of it to breathe. Snow covers the top all year round.

The summit of Everest is bitterly cold and windy all year. Even in July, the warmest month, the average daytime temperature at the top is about −19 °C. In January it averages about −36 °C.

The peak is so high that it pokes into the lower edge of the jet stream, a belt of very fast wind high in the sky. Winds can blow at more than 160 kilometers per hour. The air is thin too: at the summit there is only about one-third as much breathable oxygen as at sea level.

Britannica describes Everest's climate as always hostile to living things. Average summit temperatures are about −19 °C in July and −36 °C in January, and they can drop as low as −60 °C. Because the peak reaches the lower limit of the jet stream, it can be battered by sustained winds of more than 160 km/h, and storms can arrive suddenly.

At the summit the amount of breathable oxygen is about one-third of that at sea level. Snow falls mainly during the summer monsoon (late May to mid-September). The summit is hard-packed snow topped by softer snow whose depth varies by about 1.5 to 6 m during the year: it is deepest in September, after the monsoon, and lowest in May, after strong northwesterly winter winds have stripped it.

Most visitors walk on trails far below the top.

They walk up slowly, so their bodies get used to the thin air. Rubbish is a big problem there. Teams carry old rubbish down from the mountain to keep it clean.

Most people who visit the Everest region are trekkers who walk the trails and villages of Sagarmatha National Park, far below the summit. Visitor numbers have grown fast: about 3,600 visitors in 1979 and more than 25,000 in 2010. Tourism has helped local schools and health care, but it has also brought problems such as rubbish, new trails and more demand for firewood.

UNESCO says getting rid of garbage properly is one of the park's biggest challenges, and a local committee works to control pollution. High up, the air is thin. Health experts say the body can get used to it, but only with time, so people who travel up high go gradually and give themselves several days.

Sagarmatha National Park was established on 19 July 1976 and inscribed as a World Heritage Site in 1979. UNESCO reports that visitors rose from 3,600 in 1979 to over 25,000 in 2010, boosting local incomes, health care and education but also degrading fragile ecology and traditions. Illegal trails, resort development, energy supply and firewood demand on mountain forests remain management issues; a buffer-zone program returns part of park revenue to local communities.

UNESCO names proper garbage disposal as one of the principal obstacles, despite the efforts of the Sagarmatha Pollution Control Committee. For the altitude itself, the CDC's Yellow Book explains that the body can adjust to moderate oxygen shortage at altitudes up to about 5,200 m but needs time, with the crucial acute phase taking the first 3 to 5 days. Besides a person's own susceptibility, the risk depends mainly on the altitude reached first and how fast the traveler climbs from there, so the CDC advises gaining no more than 500 m of sleeping altitude per night once above 3,000 m. Britannica adds that expeditions have hauled tons of old litter down from the mountain.