Space and time began about 13.7 billion years ago but it consisted almost exclusively of hydrogen until some stars grew sufficiently massive to create the heavier elements in gigantic explosions called supernovae.
The Sun formed about 4.6 billion...
The Earth formed about 4.5 billion years ago from some of the debris spinning around the Sun as this coalesced into planets. Gravity and the Sun's radiation sorted the disc material so that the four planets nearest the sun (Mercury, Venus, Earth and Mars) were left with most of the heavier elements while those farther out coalesced into gas giants.
Initially, as meteors smashed into the growing mass, the Earth was hot enough to melt the rock (most of it compounds of silicon and oxygen). The heavier molten elements, principally iron, sank to the core along with the most of the heavier radio-active elements.
As the meteor showers became fewer, the Earth radiated heat into space and gradually the surface became cool enough for rocks to solidify around the core kept molten by the natural, spontaneous decay of radio-active elements. Many of the rocks now in the Earth's crust formed less than 100 million years ago but the oldest solid rock particles suggest that Earth had a solid crust 4.4 billion years ago.
After Earth was formed, 100 million years passed before the planet was cool enough for a rocky crust and, when it was cool enough for water to remain liquid , the oceans formed. (The Earth consists mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulphur (2.9%), nickel (1.8%), calcium (1.5%), and aluminum (1.4%), with the remaining 1.2% consisting of traces of the other 86 elements from hydrogen to uranium).
(Over 99% of the crust is composed of 11 oxides, principally silicon dioxide, aluminum oxide, iron oxides, limestone (usually calcium oxide or calcium hydroxide), magnesium oxide, potassium compounds (such as potassium carbonate) and sodium compounds (such as sodium carbonate).
Over time, gases escaped through the rock surface as tectonic activity lifted up the rock and volcanoes broke through. Nitrogen and carbon dioxide, as gases, collected in the atmosphere. Neither combined easily with other elements and Earth had just enough gravity to prevent them from drifting off into space.
Free oxygen, a very reactive gas with a natural affinity for almost everything, was not present in the early atmosphere.
Free hydrogen, floated to the top of the atmosphere and escaped the Earth's gravity. What remained was combined with many compounds, such as water, the most abundant compound on Earth. (Hydrogen is also present in organic compounds and makes up about 61 % of all the atoms in the human body).
Some of the rocky surface crust was pushed up to form mountains or pushed back down into the molten magma and melted. Other parts were heated, cooled and reheated and cooled again. In the process some of the elements were combined and concentrated. The less chemically reactive elements, like gold, ran molten into rock fissures. Many elements and compounds were converted into crystalline form. Some of the carbon was compressed into diamonds and graphite.
In 1953, Clair Patterson, measured the ratio of U-235 and U-238 isotopes found in a meteorite that struck Arizona about 50,000 years ago. Uranium-238 and uranium-235 both have 92 protons and are thus chemically identical but U-238 has 3 more neutrons. Both are slightly radio-active and, over time they naturally decay; losing both neutrons and protons until they become stable isotopes of lead.
U-238 starts with 238 protons and neutrons and loses 10 protons and 22 neutrons as it decays to Pb-206 (a stable isotope of lead). This is a slow process. After 4468 million years, half of the original U-238 becomes lead. (The half life is 4468 million years).
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U-235 starts with 235 protons and neutrons and loses 10 protons and 18 neutrons as it decays to Pb-207 (another stable isotope of lead). This process is a little faster. After only 704 million years half of the original U-235 becomes lead. (The half life is 704 million years).
On the reasonable assumption that the meteorite had been formed at the same time as Earth and that the two isotopes originally existing in equal proportions, Clair Patterson found the current ratio of the two isotopes indicated the Earth's age was about 4.5 billion years.
(Currently, these two isotopes of uranium occur naturally on Earth at 0.7% U-235 and 99.3% U-238).
The heat released by the natural decay of uranium and other radioactive isotopes has kept the centre of the earth a mass of molten rock and iron since the time it was formed. This energy is responsible for the movement of tectonic plates causing volcanoes, mountain building and the concentration of some minerals in the Earth's crust.
The molten iron in the core is responsible for the Earth's magnetic field which benefit life on the Earth's surface by deflecting much harmful radiation from the sun.
The Moon is the Earth's only natural satellite and may have been formed, from impacts between Earth and other large objects, about 4.51 billion years ago, some 60 million years after the origin of the Solar System.
The moon has a mass of only 7.342×1022 kg compared to Earth mass of 5.972 × 1024 kg but the mutual gravitational attraction causes the ocean's tides which have had a significant effect on the evolution of life. Every day there are two high tides, one nearest to the moon and the other on the opposite side of the Earth. These follow the moon as it moves around the Earth once every 27 days 7 hours, 43 min and 11.5 seconds.
This means high tides occur about every 12 hours and 25 minutes since the Moon orbits the Earth in the same direction of the Earth's rotation. If the Earth was entirely covered with water the average tide would be about 1 metre but land modifies tidal movement and in some places the tidal flow into estuaries can produce a tidal range of 16.3 metres (53.5 feet) (in the Bay of Fundy, Canada).The narrow straits of Saltstraumen in Norway has one of the strongest tidal currents in the world with a maximum speed exceeding 9 mph.
