Astronomers estimate that it was about 4.6 billion years ago that Earth solidified as a full fledged planet and began to evolve towards its present-day form. The next step in this evolution was its differentiation into three regions known as the core, mantle and crust. This occurred more than 4 billion years ago.
The core, at Earth's center, is iron-rich and very dense. Surrounding the core is a broad layer called the mantle, which is less dense. Earth's outermost layer, the crust, is thin and considerably lighter, made primarily of oxygen, silicon, aluminum, and iron.
There are 2 competing theories to explain how Earth became differentiated. The first propose that during its first "moments" of life, Earth was an evenly mixed hodgepodge of materials melted into a liquid. This allowed iron and nickel to sink to the core, while the lighter elements rose to form the crust.
Many scientist believe that necessary heat required to turn that early "hodgepodge" molten came from three sources. First, the abundant radioactive elements contained in the young planet were only just beginning to break down, and this process release tremendous energy. Second, each time a stray meteorite or other bit of debris struck the young planet, tremendous heat was generated. Finally, the mass of the young planet would itself generate heat as it pressed inward on its own core. According to scientist calculation, these three factors together could have raised Earth's temperature by as much as 2,000f (1,100c).
Some scientist argue that even this huge temperature rise would not have been enough to have melted the early Earth. Instead, they say, differentiation had already occurred before the planet had been born. When Earth first began to form, they explain, the densest materials accreted next, and the elements that make up the core were added as "icing" on the cake. Calculation of such a scenario show that in a cooling cloud of gas and dust, it would indeed be the heavy elements that would first condense into a more solid form, followed by lighter and lighter elements.
Whatever the actual mechanism, the differentiation of the early Earth is responsible for the subsequent formation of the continents, oceans, and atmosphere.
HOW OLD IS EARTH?
In the 16th century, scientists estimated Earth's age in thousands of years, based in part on the fossils they had found and in part on biblical interpretation. Today scientist estimate the planet's age in billions of years, using radiometric dating as the universal timepiece. How can they be sure?
THE FOSSIL RECORDS : The antiquity of our planet first became apparent when people recognized fossils as the remains of ancient organisms, rather than fanciful rocks oddities. When digging into a rock bed, they also noticed that different types of fossils could be found at succeeding depths.
By the beginning of the 17th century, geologists had established that the rock formation in which fossils were found had been deposited in more or less distinct layers, or strata. They further accepted that the oldest layers that is, those deposited first-generally lay at the bottom of the sequence. Each subsequent, or younger, layer was then deposited on top. Therefore, in places where earth movements had not toppled and mixed up the rock beds, the distinct layers could be read as a time scale from top to bottom, youngest to oldest.
By the mid 1800s, geologist had laboriously identified nearly all the types of fossils that could be found in the exposed rock layers of Europe. They then classified the fossils in a definite order, oldest to youngest. Thus, a geologist or collector could classify newly unearthed rocks and rock layer as younger or older than other based on types of fossils contained within them. But no one could say by how many years.
EARTH'S CRUST
A patch of bare rock that interrupts the more pleasing pattern of a lush meadow or green forest reveals the true nature of Earth's Crust, the solid outer layer of the planet. The soil of the land, upon which life flourishes, is an exceedingly thin film. So, too, is the layer of ooze that carpets the ocean floor. Beneath the blanket of these sediments is bedrock, the layer of solid rocks that makes up Earth's crust. When compared to the planet's actual diameter, Earth's crust is itself relatively thin, extending downward for some 22 miles (35kilometer) under the continents and about 4 miles (6kilometers) beneath the ocean floor.
There are 2 main types of crust: the CONTINENTAL CRUST, which forms the landmasses and consist primarily of low-density rock such as granite; and the OCEAN CRUST, which is a thin layer of volcanic rock called basalt that lies under the sea.
The ocean crust is younger than the crust that underlies the continents. This is because new crust is constantly formed along a ridge that runs through the middle of the world's oceans. The new crust material wells out of Earth's interior and cools as it emerges from the mid-oceanic ridge to spread outward. It is this creation of new oceanic crust that is believed to power the slow movement of the continents across the face of Earth.
The Rocky crust of Earth both the oceanic and the continental crust - is made up of three kinds of rock: IGNEOUS, SEDIMENTARY, and METAMORPHIC.
IGNEOUS rocks forms when molten rocks cools either within Earth's crust or on its surface. The crust of the primitive Earth is believed to have once consisted entirely of igneous rock.
SEDIMENTARY rock is made from small fragments of eroded rock and the remains of tiny sea animals and plants that have become compacted and cemented together over thousands of years. Most sedimentary rocks are formed on the ocean floor.
METAMORPHIC rock is derived from pre-existing rock that sinks deep into Earth and is subjected to intense heat or pressure or both, causing the characteristics of the rock to change.
