“Brief History of Earth”
Course: ENV 107
Brief History of Earth
The history of the Earth concerns the development of the planet Earth from its formation to the present day. Nearly all branches of natural science have contributed to the understanding of the main events of the Earth's past. The age of Earth is approximately one-third of the age of the universe. An immense amount of biological and geological change has occurred in that time span.
Earth formed around 4.54 billion (4.54×109) years ago by accretion from the solar nebula. Volcanic outgassing probably created the primordial atmosphere, but it contained almost no oxygen and would have been toxic to humans and most modern life. Much of the Earth was molten because of extreme volcanism and frequent collisions with other bodies. One very large collision is thought to have been responsible for tilting the Earth at an angle and forming the Moon. Over time, the planet cooled and formed a solid crust, allowing liquid water to exist on the surface.
Figure: Evolution of earth from a fireball
How the Earth formed
Scientists think that after the sun formed, about 4.55 billion years ago, it was surrounded by a disk-shaped cloud of dust and gases. As these particles of dust and gas orbited the sun, they collided with one another and stuck together. As more and more particles stuck together, they began to form bodies called planetesimals. In time, these planetesimals grew in size and formed the Earth and other planets.
During its early history, the Earth was struck many times by nearby planetesimals. These collisions, together with radioactivity and other processes like crystallization of the inner core, heated the primitive planet. These energy sources drive the dynamic Earth we know today, but, compared to the modern Earth, the energy available for heating Earth’s interior was orders of magnitude higher shortly after Earth
formation. The expectation, thus, is the heat caused
most of the material in it to melt. As this material
melted, the elements in it began to separate and flow to
different parts of the planet. The heaviest element, iron,
flowed to the center. Depending upon their weight and
density, other elements floated to the surface or to other
levels. As these materials separated and moved, they
formed the Earth's three basic layers. In the center is an
iron and nickel core. Surrounding the core is a thick
layer of intermediate-density rock called the mantle. Around that there is a thin surface layer of lighter rock called the crust. Scientists have detected these three zones by studying waves that pass through the Earth's interior during earthquakes.
We know surprisingly little, however, about the consequences of this early activity. Recent discoveries suggest that important features of the current Earth owe their origin not to the long-term differentiation of the planet, but to processes occurring within tens to a hundred million years of Earth formation.
The Earth's Early Environment
The Earth's early environment was very different from what it is today. There were no plants or animals. Landscapes consisted of barren plains, eroded valleys and hillsides, snowfields, and deserts. Fossil evidence shows that no substantial plants or animals existed before about 3.5 billion years ago.
Evidence suggests that the Earth's early atmosphere was also very different from what it is today. Then the earth was surrounded by an atmosphere of hydrogen and helium. Because the Earth didn’t have a magnetic field to protect it yet, the intense solar wind from the young Sun blew this early atmosphere away.
As the Earth cooled enough to form a solid crust, it was covered with active volcanoes. These volcanoes spewed out gasses, like water vapor, carbon dioxide and ammonia. This early toxic atmosphere was nothing like the atmosphere we have today.
Over millions of years, forces deep...
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