| Popis: |
Throughout the long course of earth history, the continents have continually rearranged their positions, size, and shapes, and the familiar outlines observed today serve merely as fleeting snapshots in the long cavalcade of time. We may think of these shapes and arrangements as permanent fixtures on Earth’s surface but this is only an illusion revealing more about our own temporal and spatial limitations as tiny human beings. The continents move about and drift across Earth’s surface and their motion is derived from the slow but inexorable churning of the mantle below, the layer that immediately underlies the crust. When the mantle moves, it pulls on the undersides of the continents, dragging them across the surface at about the rate fingernails grow, roughly 2–5 cm per year. The mantle is in motion, at about the same rate, because of radioactive decay near the core-mantle boundary. This generates heat and as it escapes upward it churns the mantle. Stripped to this essential fact, it is radioactive decay in the Earth that creates heat, setting in motion the mantle and the plates of crust that overlie it. This simple but important starting point helps in understanding how the western Cordillera evolved. |
