Earthly Beginnings

The early history of the earth is more theory than fact, and those theories are taken more from astrophysics than other disciplines. Those with other interests tend to pick through the available information in hopes of arriving at some early history for their subject.

A great many look for the thread that explains the origin of life. Others want to know how the moon was made.

My focus has been the creation of the conditions that made possible the emergence of that part of the North American plate where New Mexico sits between 1710 and 1600 million years ago, with a certain inclination to pay some attention to the formation of Michigan, the state where I was raised.

For me, this means envisioning how a 4600 million year old cloud of gas and dust evolved into a stratified ball whose layers have been defined by the workings of heat and cold.

The most important element has been iron. According to Wikipedia, iron begins in stars hot enough to burn silicon and initiate a chain of reactions with helium that transform matter from silicon to calcium to titanium to chromium to an unstable iron. That iron fuses with a helium nucleus to create ⁵⁶nickel, after which the star collapses and the ⁵⁶nickle transforms into ⁵⁶iron via ⁵⁶cobalt.

During the earliest millennia of the planet, radioactive decay continued in particles of nickle in the cloud which created conditions warm enough to heat the iron dust whose melting point is 1535 degrees centigrade. Following one basic law of physics, liquids are heavier than gases, the molten iron would have begun to isolate itself from the heat generating gases.

As it moved away from the source of heat, the molten iron would have gone through several structural phases. When the temperature fell below 1540c degrees, it would have begun to solidify and at 770c degrees it became magnetic.

Following another simple rule of physics, heat rises and cold falls, the cooler materials would have drifted towards the center of the ball, and the gases remained on the surface. Once enough iron had fallen below 770c degrees, the magnetic core could form. This process took about 50 million years and was complete around 4535 million years ago.

Once the nickle and iron coalesced into a ball divided into two parts, the magnetic, solid inner core and the molten outer core where temperatures today range from 5000c to 2200c degrees, silicon, magnesium and similar elements were segregated into an outer layer. Steve Kershaw suggests the mantle emerged around 4000 million years ago, and that it took about 2000 million years for the separation to be completed sometime around 2000 million years ago.

While the interior was still evolving, the outermost layer cooled into a skin that became the precursor of the crust. The oldest rocks found so far on the North American plate are from the Nuvvuagittuq greenstone belt near Hudson Bay. They are estimated to be between 3800 and 4280 years old.

The skin created a barrier between the warmer interior and the lighter gases which allowed water to condense without turning into steam. Oceans existed more than 3900 years ago, according to Kershaw. Scientists argue whether all the water was native or was increased by collisions with meteors and comets which could still easily penetrate the surface gases.

At this point, the history of the earth diverges into four separate narratives - the mantle and its skin, the oceans, the gases, and the crust and its plates - which rejoin sometime before New Mexico makes its appearance on the stage.

Notes:
Chandler, Harry. Metallurgy for the Non-Metallurgist, 1998, on properties of iron.

Kershaw, Steve. “Precambrian Ocean Change” in Oceanography: an Earth Science Perspective, 2000, with contributions from Andy Cundy.

Wikipedia entries on Earth’s history and iron.