The Origin and Evolution of the Earth — Class 11 Geography

This chapter traces ideas about the origin of the earth and the universe. Early theories include the Nebular Hypothesis of Immanuel Kant (revised by Laplace in 1796), and the modern version by Otto Schmidt and Carl Weizsacker, who held that the sun was surrounded by a solar nebula of hydrogen and helium from which planets formed by accretion. The most popular modern view of the origin of the universe is the Big Bang Theory (expanding universe hypothesis); Edwin Hubble in 1920 provided evidence that the universe is expanding. The Big Bang occurred about 13.7 billion years ago when a "tiny ball" of infinite density exploded; within the first three minutes the first atoms began to form, and after 300,000 years atomic matter arose. Stars formed from clumps within nebulae about 5–6 billion years ago, and planets developed through stages: a rotating disc of gas and dust, the formation of planetesimals by cohesion, and finally accretion into larger planets. The earth began as a barren, hot, rocky body and acquired its layered structure (crust, mantle, outer core, inner core) through the process of differentiation, where heavier materials sank to the centre. The atmosphere evolved in three stages, including degassing from the interior and modification by photosynthesis; oceans formed about 4,000 million years ago and life began to evolve around 3,800 million years ago.