Geological History of the Earth
- Approximately 4.54 billion years ago, a Mars-sized body slammed into the newly formed Earth, partially liquifying the surface and ejecting molten debris into space.
- This ejecta remained as a ring around our planet for a few months, before coalescing and forming the Moon.
- Residual gases were still swirling slowly around the Sun, causing streams and waves in space.
- Elephantine Jupiter got caught up in these currents and started moving inward toward the Sun.
- The movement of this giant, with its powerful gravity wreaking havoc as it danced around, dislodged asteroids and sent them flying inwards into the planets.
- In the next few million years, the Earth and other terrestrial planets went through a period of constant battering by asteroids and other smaller bodies.
- This period in the solar system’s history is called the Late Heavy Bombardment.
- Fortunately, Saturn soon started pulling Jupiter back, toward where it is today, even as the Solar wind stripped away all of the residual gas in the solar system into interstellar space.
- At this point, Earth was still cooling from the formation of the Moon, and the period of bombardment kept it agitated and volcanically active.
- At some point, asteroids or comets containing water ice slammed into the Earth, thereby bringing a lot of water vapor to the Earth.
- Once the Earth cooled, this vapour condensed and fell as rain on the planet.
- Volcanic activity still continued and even under the newly forming oceans, super-volcanoes persisted.
- Lava constantly flowed on the surface for nearly 700 million years.
- Rocks hold records of all kinds of transitions that they have undergone.
- They record their own formation and grow over millions of years, keeping evidence of life and planet activity within.
- The field of geology that studies and dates rock layers is called Stratigraphy.
- This helps scientists figure out the age of a lot of geological processes, and has enabled them to put together a geological time scale for our Earth.
- The early years of the Precambrian saw the formation of the Moon, a molten Earth slowly cooling down, and the planet getting battered by small runaway bodies.
- Water vapour in the atmosphere from asteroid and comet impacts started to condense and rain down on the planet as liquid water.
- Oceans formed amid heavy volcanic activity.
- Portions of the surface periodically cooled off to form occasional landmasses, but they would immediately be swallowed up by lava.
- Then, approximately 100 million years after the Earth formed, the temperatures had become stable enough for a crust to form and survive.
- The atmosphere was heavy and toxic, with almost no oxygen but with large amounts of carbon dioxide, nitrogen and sulphur due to volcanic activity.
- Within another half a million years, multiple tiny landmasses had been born. These went on to become the centre around which present-day continents formed.
- The oldest known rocks on Earth are from this period, now in Australia, dating back to 4.4 billion years ago.
- Towards the middle of the Precambrian, the earth had cooled sufficiently.
- In the atmosphere, there was still no oxygen.
- The oxygen on our planet today is produced and sustained solely by plant life.
- This lack of oxygen implied a lack of ozone to protect the earth, which exposed the Earth to UV rays from the sun.
- However, the earth’s atmosphere could be preserved because its magnetic field had begun to form.
- This protected the atmosphere from being stripped away by the solar wind (as the atmosphere of Mars was).
- Around 3.5 billion years ago (bya), two supercontinents, called Vaalbara and Ur formed within half a billion years of each other.
- These landmasses were actually quite small, probably about the size of India.
- But since they were the only landmasses around, they are called “supercontinents”.
- The lack of oxygen in the atmosphere did not mean a lack of life, though. Life began on Earth in the early Precambrian, 4.1 bya, when earth had just started cooling.
- Gems from this time period, called zircons, have very specific carbon ratios, and possibly show evidence of biological activity combined with water.
- It is commonly assumed and accepted that one of the main causes of the creation of life is the presence of large oceans.
- Liquid water is considered to be a universal solvent, which means that it can transport all kinds of nutrients to all corners of the planet, enabling even the remotest locations to support life.
- Thanks to its almost magical properties, the very presence of liquid water on a body is a giant attraction for space exploration today.
- Banded iron formations – layers of rock from the ocean showing pulses of iron oxide deposits due to reaction with oxygen – dating back to 3.7 bya exist today.
- These show evidence that large quantities of oxygen were pumped into water at intervals; a phenomenon that is explicable only as a biological process.
- More biochemical rocks, called stromatolites, that were formed due to microorganisms trapping sand grains to build colonies, date to 3.5 bya.
- The most solid evidence of photosynthesis, however, dates back to 2.4 bya when cyanobacteria flourished, infusing massive quantities of oxygen into the air.
- So, two billion years after the earth formed, there was finally a constant supply of oxygen in the air for the first time.
- At around the same time, a new supercontinent called Kenorland was formed, while Vaalbara broke up, with parts of it ending up in today’s Australia and Africa. Kenorland was much larger than either Vaalbara or Ur.
- It was as big as Africa and existed somewhere near the equator for a hundred million years before breaking up.
- The Great Oxygenation Event occurred 2.3 bya.
- The rise in levels of this new gas in earth’s ecosystem led to two major events on Earth: the first extinction event and the first ice age.
- An Extinction Event, more commonly known as a mass extinction, is the extinction of a large number of species within a short period of geological time.
- There have been 24 extinction events in all of Earth’s history – before humans came around 200,000 years ago.
- Five of these were particularly destructive, with detailed, well documented evidence of their occurrence and repercussions. These major extinction events are called the Big Five.
- Mass extinctions always occur after a sudden, rapid, and uncontrollable change in global climate – which is obvious because only such widespread changes can kill off diverse species spread out over land and water in a short period of time.
- Conversely, mass extinctions could also affect the global climate as disappearance of a majority of life on Earth could upset the oxygen balance.
- The other effect the oxygen catastrophe had was the formation of glaciers.
- The rise of oxygen naturally removed a lot of greenhouse gases from the atmosphere, most notably methane.
- Oxygen lowers temperatures, which is why wooded areas are so much cooler than cities today.
- The saturation of oxygen in the atmosphere lowered the overall temperature to 5°C lower than today and removed the ability of the atmosphere to keep the planet warm.
- Temperatures started falling steeply, heralding an ice age.
- An ice age is a period, extending to millions of years, of lowered temperature on the Earth.
- A characteristic feature of an ice age is the presence of continental glaciers and polar ice caps.
- An ice age is composed of periods of extreme cold, called glaciation periods, marked by the appearance of large ice sheets and glaciers over continents.
- These alternate within the same ice age with periods of warmth, called inter-glaciation periods, where the ice sheets are confined to the poles.
