Making Earth suitable for life: differentiation

In this video, Dr Morten Andersen explains how Earth began to take the shape we know today.

During the first 800 million years or so of Earth’s existence it was continuously bombarded by debris mostly from within its orbit.

The collisions made the Earth grow in size, and at the same time made it an inhospitable place. Over time, debris within Earth’s orbit cleared and there were less impacts.

Today, the amount of material falling to Earth is estimated to be around 40,000 tons a year, mostly in the form of dust particles. These impacts do not disturb our current ecosystem.

The formation of the Moon

One of the most violent bombardments was a collision with a massive meteor that led to the formation of the Moon. The collision scattered huge amounts of debris into space, which eventually clustered together and formed the Moon.

Artist impression of the Moon-forming event by NASA/JPL-CALTECH/T. PYLE.

Whilst it is unclear what role the Moon played in the emergence of life on our planet, it did have a major and lasting effect on the evolution of life and the nature of our habitat.

From a cosmological viewpoint, the formation of moons is not a rare event and many of the larger planets in our Solar System have multiple moons. For example, Mars has two moons and Jupiter has 79 known moons.

However, from a geological viewpoint the formation of the Moon was an extreme event and it exerted a strong influence on the future of our planet.

Internal layering of the Earth

The early Earth was exposed to violent conditions and was incredibly hot, much hotter than it is today. At this time, the material comprised of the heaviest elements (such as iron) migrated to the deepest layer of the Earth, forming the core.

The middle part of the Earth is the mantle. It surrounds the core and is made up of material comprised of lighter elements. Due to the heat contained within it, melting occurs in the mantle which leads to the production of magma.

This natural material contains liquids and gases that are transported to the Earth’s surface through volcanic eruptions. Erupted magmas form the Earth’s crust, and the liquids and gases from magmas form the oceans and the atmosphere.

This layering or differentiation of the Earth into a structure of core, mantle, crust and atmosphere also kickstarted a key process at the surface of the Earth, the movement of tectonic plates.

Plate tectonics

This is the theory that Earth’s surface is made up of several plates that move over the mantle and is a unique characteristic of Earth in our Solar System. The plates move continuously, driven by temperature differences across the mantle. This enables chemical elements from deep in the Earth to flow out to its surface, which is essential for the creation of life and sustaining it.

When did differentiation occur?

It is difficult to pinpoint when the differentiation of Earth occurred, but it is estimated that it took about 34 million years to separate the mantle from the core and about 82 million years to separate the crust and atmosphere from the mantle.

This is quite fast relative to the age of the Earth (approx. 4.5 billion years) and established pretty stable conditions at the Earth’s surface. This is one of the key extreme events that had to happen on our planet to make it habitable.