Evolution of the Atmosphere
- as the Earth formed, in addition to rocky silicates and metallic iron, impacting bodies emplaced volatile materials such as carbon dioxide, hydrogen and hydrogen compounds (methane and ammonia), and nitrogen in the mantle.
- the Earth's atmosphere was formed by outgassing; the volatile gases were released from the mantle via volcanic activity
- the early atmosphere was probably rich in carbon dioxide, nitrogen, methane, ammonia, hydrogen, and water vapor; this reducing atmosphere was devoid of free oxygen (O2)
- the oceans condensed from the water vapor that outgased from the mantle
- photosynthesis produced large quantities of oxygen once life evolved by 3.5 b.y.; this eventually led to the oxidizing atmosphere with free oxygen that we have today
- hydrogen escaped into space; carbon dioxide
was used by photosynthetic organisms; oxygen was added to the
atmosphere by photosynthesis; nitrogen gradually increased by steady
outgassing to eventually become the most abundant gas in the
atmosphere
Archean Geology:
- Earth formed about 4.6 b.y. (most meteorites and oldest moon rocks are of this age, though there are no surviving rocks from Earth of this age)
- early Earth was very hot from energy from gravitational compression, from meteor bombardment, and from 5 times as much radioactive decay as there is today
- no remnants of early Earth surface because of constant resurfacing through volcanism, subduction, and meteor bombardment as Earth continued to grow by collisions
- early crust was probably ultramafic
(komatiite) because early Earth was so hot that the ultramafic
mantle could melt completely or to a high degree, thereby yielding an
ultramafic melt which rose to the surface forming an ultramafic
crust; today the Earth is much cooler and the ultramafic mantle can
only melt to a small degree (1-10%) yielding mafic and felsic
magmas
- oldest rocks found on Earth are 4 b.y. old
- normal plate tectonics probably didn't exist because of high heat flow and volcanism rapidly resurfacing the Earth in small-scale, random patterns, compared with today's generally broad plates and widely separated midocean ridges and subduction zones
- as the Earth cooled, mafic and felsic igneous rocks could be differentiated from the ultramafic mantle by partial melting; in areas of the upper mantle where temperature and pressure were such that the ultramafic minerals of the mantle were on the verge of melting, a low melting point fraction, rich in silica and poor in iron and magnesium could form a melt that rose up to form mafic to felsic crustal rocks
- greenstone belts (metamorphosed komatiite, basalt, and sediments) and granite/tonalite (most metamorphosed to gneiss) terranes characterize Archean geology; the felsic granite/tonalite gneiss formed by partial melting of the ultramafic mantle, probably over subduction zones like the Andes Mountains or Aleutian Islands today; the greenstones formed either from ocean crust or as back-arc basins; continental crust grew by island arcs (granite/tonalite gneiss) colliding and suturing together, often trapping slices of ocean crust or back-arc basins (greenstones)
- the end of the Archean Eon, between about 2.6
and 2.5 b.y., was a period of major crustal formation
(granitization), crustal thickening, and aggregation of mini
continents from smaller pieces of Archean crust (called the
Algoman/Kenoran orogeny in the Great Lakes region and
Canada)