Читать книгу Exploring the Solar System - Peter Bond - Страница 86

Earth's extremely varied surface ranges from high mountains to deep valleys. This varied topography is largely an expression of crustal processes associated with plate tectonics.

About 70% of the planet is covered by water, i.e. below sea level. Although the continental shelves bordering the major land masses are very shallow, they rapidly drop away to a deep abyssal plain. Much of the ocean floor lies at depths of about 5 km, but it is much deeper in narrow ocean trenches. The record holder is the Marianas Trench in the western Pacific, with a maximum depth of 10,911 m.

Although it is often blanketed by thick sediment, the ocean floor is not completely flat and low‐lying. Running along the spine of the Atlantic Ocean is a ridge of submarine, volcanic mountains, 2–4 km in height and hundreds of kilometers wide. In places, the ridge rises above sea level, most notably in Iceland. Similar mountain ranges, where crustal plates are moving apart, are found beneath the other oceans.

High mountain ranges are also found on the continents, often near the coasts. These fold mountains, largely made up of highly compressed sediments, are usually associated with collision zones where two plates are inexorably pushing into each other. The highest of Earth's mountains are the Himalayas, between India and China, with Mt. Everest towering 8,848 m above sea level.

Large areas of the continents are fairly flat and close to sea level. The higher plains have been lowered and smoothed by weathering and erosion over millions or billions of years. Lowland plains are typically covered in thick layers of sediment. These may have been deposited by glaciers (either as unsorted debris or meltwater deposits), rivers, wind, or ocean waves.