Big Ideas in Volcanology #08:


#08.1 _ Earth’s primordial atmosphere has different origin than volcanic degassing

In the beginning, a Proto-Earth grew by accretion of a solar nebula (i.e. gravitational attraction of typically gaseous matter in a disc) until the inner part of the protoplanet was hot enough to melt siderophile metals. These liquified metals gained a higher density and began to sink to the planet center of mass. This so-called “iron catastrophe” resulted in the separation of a primitive mantle and a metallic core, producing the layered structure of the Earth and setting up the formation of a magnetic field. When a star’s magnetic field passes through a disk gets bogged down like a spoon in molasses. This locks the star’s rotation to the slower-turning disk. Consequently the star, while continuing to shrink, does not spin faster.


Three dimensional simulation of an accretion disk

During the accretion which formed the first Proto-Earth from the solar nebula, a cloud of gaseous silica must have surrounded the protoplanet. Most of these gases condensed then as solid rocks, whereas what was left constituted an early atmosphere made of light elements, mainly H2 and He. Later on, this light atmosphere had been progressively driven off by solar winds and Earth’s heat. This changed when Earth accreted to about 40% its present radius, and gravitational attraction retained an early atmosphere. This would have contained 60% hydrogen, 20% oxygen mostly in the form of water vapor, 10% carbon dioxide, 5-7% hydrogen sulfide, and smaller amounts of nitrogen, carbon monoxide, free hydrogen, methane and inert gases.

Earth’s accretion and core formation, 4.5 Ga (modified from Kleine, 2011)

Part of the ancient planet is theorized to have been disrupted by the impact with a Mars-sized body called Theia which created the Moon, which should have caused melting of one or two large areas. Present composition does not match complete melting and it is hard to completely melt and mix huge rock masses. However, a fair fraction of material should have been vaporized by this impact, creating a rock vapor atmosphere around the young planet.

View of a Mars-sized object hitting the Proto-Earth, resulting in a collision that would have formed our Moon from the debris produced in the impact


#08.1                    #08.2                    #08.3                    #08.4                    #08.5                    #08.6

Earth’s hydrosphere comes from Volcanic Degassing