| Bags Of Time - more
than enough time has existed for biological evolution to work its magic and create the
astounding variety we see in the living world today. In fact, seeing as life on
earth has already suffered five major extinctions - evolution
has actually worked its magic more than once! Our minds of course struggle to grasp the magnitude of time that has actually passed. So to help us see the big picture, we divide geological time into smaller and more manageable pieces:
|
Click on each of the geological Periods on the left to find out more about them.
A nice way to memorise the 12 major geological periods is through this mnemonic (starting with the Pre-cambrian): Pregnant Camels Often
Sit Down
Carefully, Perhaps
Their Joints
Creak, Perhaps
Not |
 |
| ON THE ORIGIN OF THE
EARTH "The Earth was not constructed with a delicate hand. It was hammered into shape slowly, by the brute force of a meteor bombardment that lasted hundreds of millions of years. The soils, the seas, and our primitive microbial ancestors emerged in the midst of apparent chaos and catastrophe. The process began billions of years ago as our entire solar system was congealing from a swirling cloud of hot gases and nuclear ashes left behind by exploded stars. Some of the objects colliding with the Earth at this time were planetesimals -- objects as big as small planets. The kinetic energy released by these impacts literally shook the Earth to its core and melted much of the rocky crust and interior. Some chunks of the planetesimals and meteors became permanently embedded in the Earth, while other pieces were sent hurtling off into space like giant shrapnel. The mass of the primordial Earth accumulated slowly, like a globe that grows as a sculptor slaps on clay, one handful at a time. With greater size, Earth increased in its gravitational force, attracting even more of the wandering debris of space. It is hard to come up with a specific date of birth for our planet, given its gradual development. Basing their calculations on the "radioactive clock" -- measurements of the level of radioactive decay of certain elements found within the Earth's crust, such as uranium and lead -- most geologists place the Earth's age at about four and a half billion years. The Earth went through horrendous growing pains during its first billion years. Just as the frequency of meteor impacts began to decline, violent volcanic eruptions began to spring up around the globe as the planet's hot interior "degassed." When the Earth's surface temperature finally began to cool, the massive volume of water vapor in the atmosphere condensed and poured down from the heavens in fierce rainstorms of truly biblical proportions. The torrential rains lasted millions of years, creating our oceans -- the hydrosphere as we know it -- in the process. The original igneous and metamorphic rocks on the Earth's surface, left behind by volcanic eruptions and upliftings from the mantle layer below, were washed by the relentless rains, and their minerals flowed into the oceans. This was an essential first step in the formation of primitive soils that would eventually support a vibrant plant and animal life. These primitive soils lacked organic matter but contained sand, silt, and clay minerals in various proportions. Clays are unique among the mineral components of soil. They are chemically reactive, microscopic, crystal-like structures that form out of saturated solutions of silicate and metal oxides. Sand and silt, in contrast, are large, chemically inert particles formed by the simple weathering and pulverization of rock. Some clays are crystallized deep within the Earth's mantle layer, at high temperature and pressure, and then brought to the surface by the churning motions of the Earth. This process is driven by radioactive heating deep within Earth's mantle and is part of the same plate tectonic geological cycle that gradually moves the continental crusts." David W. Wolfe: Tales from the Underground: A Natural History of Subterranean Life Perseus Publishing, Cambridge (MA) 2001, p.17. Web Links: origin of the Earth ScienceWeek http://www.scienceweek.com |
||
