Geology of the Solar System 2: Formation and structure of the Earth


earth good one

Earth’s Formation

During cold accretion (which we are assuming to be true whether you like it or not), at first particles just stick together without mixing too much, like sticking a bunch of bits of different colored play dough together. This is referred to as being an undifferentiated body. As heat increased, materials began to melt and, like mixing oil and water in a glass and watching them separate, the denser materials sank closer to the center of gravity, and the less dense material floated on top. This is called a differentiated body, and is how the earth wound up with the structure it has.

earth structure

The heat that caused this melting and differentiation came from a number of relatively uninteresting sources:
Gravitational accretion – as gravitational forces pull things closer together, gravitational potential energy is released as heat.
Accretionary heat – kinetic energy from objects colliding with each other transferred some kinetic energy into heat
Core formation – as dense material sank to the centre of the planet, more gravitational potential energy would have been converted to heat
Radiogenic heat – radioactive decay of atoms creates heat
Solar energy – the sun is hot, etc…
Tidal heating – nothing to do with water, so stop thinking about it. The gravitational pull of the moon creates flexing in the earths structure

Earth’s Structure

The core is dense and iron rich. We know this because the entire density of earth is 5.52 g/cm3, but the density of crust materials is only 2 – 3.5 g/cm3, so there must be something in the center of the planet that is much denser, to make up for the difference. We can tell how big the different sections of the earths internal structure are by seismic wave information, and considering the density of the whole earth, the average density of the crust, the size of the core, and the density of iron, the core must be made mostly of iron (with some other elements). We also have confirmation of a liquid or partially liquid iron core from earths magnetic field. There are a few different ways to create a magnetic field, as we will see with other planets, but molten iron being churned around in the centre of the planet by Earth’s rotation on its axis is the most likely origin of ours.

The mantle is made of silicate material with significant amounts of iron and magnesium. This is called mafic or ultramafic material. On a side note, I have been struggling all semester against my Apple products’ insistence that the word ultramafic be autocorrected to ultra magic.


Apple, do you even science?

The crust was originally formed when a planet-wide covering of molten rock cooled and became a thin layer of solid material covering the planet. There are two types, dense and thin oceanic crust, and thick and less dense continental crust. There is a discontinuity that marks the base of earths crust called the mohorovicic discontinuity or Moho for short. It marks a change not in state but in seismic velocity, with less dense crust above the discontinuity and more dense mantle below.

The Earth also has a lithosphere and asthenosphere. The lithosphere is the solid, brittle crust of Earth and the uppermost section of its mantle.  The asthenosphere is located just below the lithosphere in the upper mantle, and is a section of Earth where rocks approach, but don’t generally reach, their melting point, making them more plastic.  Plastic means they can sort of flow in spite of not being liquid, much like glaciers, or silly putty. This plasticity is caused by the assimilation of water in the rocks (which lowers their melting point), may be unique to Earth (at least in our solar system), and makes Earth’s plate tectonic system possible.

Plate tectonics

Rather than spell this out for you, here’s a picture to give you an idea of how this works. Please ignore the numbers.

One of the consequences of this structure is that rock on earth is constantly being renewed and recycled, so features like craters don’t stick around for very long (on a geologic time scale). This is important when discussing the age of other bodies of the solar system.

Earths spheres

What the heck do atmospheres, hydrospheres, and biospheres have to do with the geology of the solar system. Geology is about rocks. This isn’t meteorology or oceanography or biology. Seriously!

Well… In no other area of study I’ve looked at does the interconnectedness of these spheres become more apparent or important. I know we’re always told that they are connected to each other and influence each other, but looking at the composition of other planets can really drive home what a huge effect the relationship between the spheres can have on a planet’s composition.

Most obviously, the hydrosphere erodes rocks and deposits sediments, changing the structure of rocks, eroding the original rocks, and covering features like fractures in the crust, or craters. Water ice also carves the surface, changing the shape of parts of the geosphere. Another thing the hydrosphere on earth has allowed is the creation of a biosphere. The early biosphere is responsible for the composition of our atmosphere, because it removed carbon from the atmosphere and increased oxygen levels. The biosphere, in this process, is also responsible for the amount of carbonate rock in Earth’s geosphere.  Our incredible abundance of carbonate rocks was created by living things in the ocean. A biosphere creates huge differences between planets that otherwise might be almost identical. For example, Venus’ and Earth’s atmosphere would be very similar without Earth’s biosphere.  Instead, Venus has an atmosphere that is almost entirely carbon dioxide, while Earth now has an atmosphere that is mostly nitrogen and oxygen, with hardly any carbon dioxide at all.

So there are the basics on the geology of Earth, off of which I will explain the geology of subsequent bodies in the solar system.


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