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Formation of the Earth

A short history of how Earth matured as a planet

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Earth wasn't always as it is today. I don't mean as a result of human activities like deforestation, city building and strip mining that have dramatically altered its surface and environment. 

From its birth, it took billions of years before Earth began to take on this shape and appearance. It would be millions of years more before familiar structures, such as the defining continents, rivers and large mountain chains, such as the Himalayas and the Andes, would take on their now familiar shapes.

For example, in the proverbial "beginning", there were no Himalayas, Hawaii didn't exist and neither did the Atlantic Ocean. In fact, nothing of the present form of Earth is the same as it was when Earth was "born".

Birth of Earth

Earth was born and began its evolution about 4.54 billion years ago, most scientists agree. This age has been determined by radiometric dating, whereby known half-lives of radioactive isotopes in rock samples are studied and their decay measured.

The sun is not much older than Earth (in relative terms) as the sun and all of the solar system formed more or less simultaneously from a cloud of interstellar "space dust". This was composed of hydrogen, the most abundant element in the universe, and helium, produced by the nuclear fusion of hydrogen, and crucially, heavier elements that are the remnants of explosions of giant stars called supernova.

Without these heavier elements, life and the complexity of everything on Earth wouldn't exist, which essentially means that we are only here as the product of an exploded star. Kinda cool, I think!

The centre of this protoplanetary nebula, which is essentially space dust, rotated and coalesced, pushing 98 percent of the mass towards its centre. The centre mass eventually formed our sun. The sun fuses hydrogen into helium, which emits heat and light as a result of the nuclear fusion.

In the beginning, what was eventually to become Earth probably looked like this collection of gases and protoplanetary nebula, as taken by the Nasa’s Spitzer Space Telescope of a region of space called W5 where astronomers believe new planets are now forming. NASA/JPL-CALTECH/HARVARD

The rest of the matter rotated around the sun, similar to the rings of Saturn, until the chunks of rocks and ice collided with each other in a ferocious fashion. The more "debris" that collided, the more mass coalesced, and this is how Earth and other planets formed.

Due to this smashing and colliding, a large, spherical Earth-like object was eventually formed, but another massive object about the size of Mars slammed into the young Earth, throwing huge amounts of rock into space. This debris was about a sixth the size of that Earth, and it grouped together, remaining in orbit as a natural satellite to our Earth. We call this object the moon.

Other planets took shape, gathering all of the debris on their orbit, giving them a clear orbital path - a characteristic of a planet. The asteroid belt remains as a collection of rocks, essentially due to Jupiter's gravity preventing a planet from forming.

Early Earth

During the formation of Earth, the collision that led to the creation of the moon left behind a molten, red-hot Earth, barren and certainly inhospitable. The heavier elements of Earth sank to the centre. At the centre of Earth, a solid inner core comprising iron and nickel formed, with a molten outer core, again consisting of mainly iron and some nickel.

This outer core is responsible for Earth's magnetic field and the magnetosphere layer of our atmosphere that protects us from potentially lethal solar winds. The inner and outer cores control the temperature of Earth, where the inner core is at a similar temperature to that of the surface of the sun. The temperature is the remnant heat from early collisions along with the heat generated as a result of the radioactive decay of radioactive elements.

The mantle, the molten, constantly yet slowly moving liquid rock, is the largest layer of Earth. Surrounding the outer core, the uppermost layer cooled and solidified, forming Earth's crust, a solid layer of rock floating on an ever-moving mantle sea.

Atmosphere

The red-hot Earth would have had multitudes of volcanoes spewing forth lava and hot gases. In the atmosphere, there would have been no free oxygen, but lots of methane, ammonia and water vapour, among other gases.

As the volcanic activity slowed and Earth cooled, with much of the heat radiating into space, water vapour condensed into liquid water, allowing the formation of the oceans. The atmosphere, now in its second phase, would still be full of volcanic gases including methane and ammonia, but also carbon dioxide, chlorine, nitrogen and many others.

Oxygen in its free O2 form didn't exist in sufficient amounts for billions of years after Earth's formation. Life arose about 3.7 billion years ago in the form of photosynthetic bacteria called cyanobacteria. Although very primitive, this early life utilised the rich carbon dioxide and water atmosphere and underwent photosynthesis, a waste product of which is oxygen.

About 1.7 billion years ago, the oxygen levels were sufficiently high to allow the evolution of organisms that utilise oxygen and the organic life that created it in respiration. From this simple beginning, we now have the world's incredible biodiversity and a perfect balance between oxygen and carbon dioxide.

Our current atmosphere is now about 21 percent oxygen, 0.04 percent carbon dioxide, about 1 percent argon, and the remaining 78 percent or so is nitrogen, with water vapour as well. Very different from the situation when Earth was just a toddler.

Mountains and rivers

During this cooling period and during the changes in the atmosphere, sedimentation due to weathering and erosion was also taking place. This led to the accretion (a build-up) of land masses that would eventually lead to tectonic plates. Metamorphosis of rocks was also taking place, leading to the vast rock diversity we find today.

Tectonic land masses shifted in various directions, based on convection currents in Earth's mantle. Although continents are still in motion, recently in Earth's history, about 250 million years ago, the continental land (above sea level) was compacted into a giant, super-continent known as Pangaea.

This broke up into two separate masses known as Gondwana and Laurasia, and the plates kept moving, pulling apart, subducting under or colliding against one another, all dramatically changing the face of Earth.

When plates collided, land masses in between were plunged upwards, and momentous mountain ranges formed, such as the Andes and the Himalayas. The Himalayas, which formed only about 70 million years ago, now dramatically affect life on Earth.

Many major rivers begin in the Himalayas. They create the monsoon and essentially affect all weather and climate on Earth. The Himalayas are still rising, despite being constantly weathered and eroded, as the plates are still colliding into each other.

Glaciers, a result of a great ice age spawned by the Himalayas, also served to carve landscapes, with rivers eroding and forming canyons, and weather breaking down mountains.

What will the future of Earth look like? Well, Australia will be in the northern hemisphere, the Himalayas will be bigger, and the Atlantic Ocean will be bigger than the Pacific as South America drifts away from Africa. Let's just hope humans will still be around to see the results of our ever-evolving, dynamic Earth.

Dave Canavan has an MSc in Behavioural Ecology and is the Head of Secondary at Garden International School.


Dave is fascinated by science and loves animals, especially the dangerous kind! You can contact Dave at davidc@gardenbangkok.com

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