The Earth was formed (as were all of the planets in our solar system) from the aggregates that orbited our sun approximately 4.6 billion y.a.—less than half a billion years after the sun itself formed (and about 9 billion years after the Big Bang). As it was forming, the Earth’s accumulating mass was constantly bombarded by matter-adding meteorites, comets and huge planetesimals (mini-planets), and this, together with the energy released by radioactive decay of the heavy elements that settled to its core, kept our planet in a molten state for its first two or three hundred million years. The Earth’s solid surface crust formed about four and a quarter billion y.a.
Today, our planet consists chiefly of a liquid iron alloy core (at a temperature of approximately 5,000°C), covered by a mantle of hardened oxides that float on the core’s surface. The core is slowly cooling as convection currents and eddies in its upper layers transfer heat into the mantle. These currents also cause the Earth’s continents to drift, almost imperceptibly, but constantly. In several places, subterranean tectonic plates bump into each other, inevitably pushing one under the other. The uppermost plate (usually the lightest one) lifts to form mountains, with rifts, or valleys, forming in between. The plate pushed underneath remelts to form magma, some of which may find its way to the surface again through volcanoes.
Most of the water that covers three-quarters of the Earth’s surface arrived in the form of comets early in our planet’s life, but another 500 tons or so is added daily as the Earth sweeps up space-dust (much of which is water). Water is ubiquitous throughout the universe simply because its constituents (two atoms of hydrogen, and one atom of oxygen) are elemental and universal—hydrogen being the first element formed following the Big Bang, and oxygen being one of the elements produced as stars evolve, and later thrown into space as they nova.
As we doubtless remember from our school days, the sun’s radiation evaporates water (that then condenses to form the clouds that produce rain, lightning and thunder). The sun’s energy also heats the land and sea; temperature differences between these create winds and drive the water cycle that sustains life on land.
More than three billion years of continuously varying weather has eroded the Earth’s ever-changing mountains, turning their rock into the sand, dust and silt that have become major constituents of our planet’s soil and the bottoms of its lakes, rivers and oceans. Silt and material on the ocean floors, compressed by the weight of water and accumulated matter above, has formed layers of sedimentary rock that trap and hold evidence of the conditions and life forms that existed from the recent past to many hundreds of millions of years ago. Ice locked at depths within glaciers provides ancient liquids and gases that scientists have collected and analyzed, further adding to what is known about how our planet aged and changed. These sources have also told us much of what is known about how life evolved on our planet.
- The same process (i.e., radioactive decay of elements such as plutonium) is used to provide heat (subsequently converted into electrical energy) in satellites sent to inspect planets that are too remote from the sun to allow effective use of solar panels.
It is conjectured that radioactive elements in the Earth’s core created an atomic reactor that still operates, keeping the core molten even though heat is continually being lost through the Earth’s mantle. See Brad Lemley, “Nuclear Planet,” Discover, August 2002, 36-42. For information on naturally occurring nuclear reactors, see http://nuclearplanet.com.
- Calculations involving the rate of radioactive decay, as well as the amount and kind of decay products, give scientists one method of dating the Earth’s beginning. For instance, analysis of the decay products of uranium isotopes found locked within zircon crystals from the Jack Hills section of north-western Australia, shows that these particular crystals are between 4.3 and 4.4 billion years old.
- Just a little cooler than the temperature at the sun’s surface. (The temperature of the sun’s core is very much hotter—about 16,000,000°C.)
- See “The Sound of One Rock Falling,” Discover, February 2002, 18.