| Popis: |
The paper provides a review of early stages of development the Solar System and the geological history of Earth with reference to the latest data on the origin of the Solar System and the formation of the first continental rocks and results of studies of zircon, the oldest mineral so far dated on Earth. The formation of the Solar System from a gas-and-dust nebula is estimated to have begun 4.568 billion years ago. Ice was formed 1.5 million years later; it concentrated at the periphery of the system and served as the material for the largest planets, Jupiter and Saturn. In the central areas of the system, asteroids with diameters of about 10 km were formed. Their small bodies were composed of the basic material of the solar nebula, as evidenced by carbonaceous chondrite, CI, which composition is similar to the composition of the Sun, with the exception of hydrogen, helium, and volatile components that served as the main material for peripheral planets of the Solar System. Due to collision and partial merger of such small bodies, the formation of embryos of the terrestrial planets was initiated. Gravity made such embryos to cluster into larger bodies. After 7 million years, large asteroids and planet Mars were formed. It took 11 million years to form Planet Earth with a mass of 63 %, and 30 million years to form 93 % of its mass. Almost from the beginning of the formation of the Earth, short-lived radionuclides, 26Al and 60Fe, caused warming up of the small planetary bodies which led to the formation of their cores. During the initial stages, small magma reservoirs were formed, and molten iron particles gathered in the centres of the planetary bodies. As suggested by the ratio of 182W/184W, the major part of the core was formed within 20 million years, while its full mass accumulated completely within the next 50 million years. In 30–40 million years after the creation of the Solar System, the Earth collided with a cosmic body which mass was close to the mass of Mars, and it was the beginning of the formation of its satellite, Moon. When the Earth – Moon system was subject to the major meteorite bombardment 4.5–4.1 billion years ago, zircons formed due to impact activity on the Moon, and on Earth it caused major eruption of basaltic magma, the differentiation of which led to the formation of acid magmas in small amounts which caused the formation of zircons, the only remnants of the first continental rocks of Earth. Later on, meteorite bombardment continued and contributed to the burial of the first continental acid and basic rocks in the mantle, wherein such rocks eventually became part of the mantle which became the progenitor of granite-greenstone rock associations. Grey gneisses contain traces of the first continental formations of Earth as evidenced by ancient zircons that are ubiquitous in the crust of Earth. The above is confirmed by results of detailed studies of regional zircons with the use of modern analytical equipment providing for high accuracy of local analyses. |
