Part 1.8 Archean Eon, Metamorphism, Earthquakes, Volcanoes

Fundamentals of metamorphism, earthquakes and volcanoes a primer for Plate Tectonics

Komatities

Komatites are a type of ultramafic, volcanic rock that are predominantly composed of a magnesium-rich silicate mineral called olivine. They are named after the Komati River in South Africa, where they were first discovered. Komatiites are significant because they provide insights into the early Earth's geological history and the conditions that existed during the formation of the planet.

Komatites are formed from extremely hot, magnesium-rich magma that originated from the Earth's mantle. The high temperature of the magma, which could have been around 2,900 to 3,500 degrees Fahrenheit (1,600 to 1,925 degrees Celsius), allowed it to be highly fluid and mobile. As a result, komatiites often exhibit a distinctive "spinifex" texture, named after the Australian spinifex grass, which is characterized by long, needle-like crystals of olivine that formed as the magma cooled rapidly upon reaching the Earth's surface.

The presence of komatiites indicates that the Earth's mantle was much hotter in the past than it is today. Komatiites are primarily found in Archean-age rocks, which are over 2.5 billion years old, with the youngest komatiite occurrences dating back to around 1.8 billion years. As the Earth's interior gradually cooled over time, the formation of such high-temperature magmas became less common, and komatiites essentially ceased to form.

Komatiites are of interest to geologists not only because they provide information about the Earth's early thermal history, but also because they are often associated with valuable mineral deposits. For instance, komatiites can host significant deposits of nickel, copper, and platinum-group elements, which form as a result of the concentration of these elements in the magma and their subsequent precipitation during cooling and solidification.

Komatiites are mainly found in Archean greenstone belts, which are sequences of ancient, metamorphosed volcanic and sedimentary rocks that date back to the Archean Eon, more than 2.5 billion years ago. Some notable locations where komatiites can be found include:

  1. Barberton Greenstone Belt, South Africa: This greenstone belt, where the type locality of komatiites (Komati River) is situated, contains well-preserved komatiite flows and spinifex-textured rocks.

  2. Eastern Goldfields Province, Western Australia: The Yilgarn Craton contains the Kalgoorlie Terrane, where komatiites are found in the Norseman-Wiluna greenstone belt. This region is known for its nickel sulfide deposits associated with komatiites.

  3. Abitibi Greenstone Belt, Canada: This belt extends through Quebec and Ontario and contains komatiite flows, some of which are associated with significant nickel and copper deposits.

  4. Kostomuksha Greenstone Belt, Russia: This greenstone belt in the Karelian Craton contains komatiite flows and is known for its iron deposits.

  5. Zimbabwe Craton, Zimbabwe: The Belingwe (now Mberengwa) Greenstone Belt within the Zimbabwe Craton contains komatiites and associated nickel deposits.

These are just a few examples of regions where komatiites can be found. Due to their age and the metamorphic processes that affected many ancient rocks, well-preserved komatiite exposures are relatively rare. However, their presence in Archean greenstone belts worldwide provides valuable information about the early Earth and its geodynamic evolution.

 

ARCHEAN EON Earth’s heat flow was 3x the current rate, but had cooled to the point that a solid crust covered the surface, covered by a thick ocean of water which escaped from deep within the mantle via convection and plate tectonics.

  • 4.0:

  • 3.5: Crystallization of the inner core

  • 3.8: PILLOW LAVA - Greenland volcanic rocks - evidence of submarine igneous rocks

  • 3.5-3.8 Stromatolites which will turn out to be remnant fossils from cyanobacteria (but not enough to impact free oxygen)

  • Banded Iron Formations start, peak around 2.5 Bya

  • Nitrogen and water dominate the atmosphere along with nitrogen

COLLISION OF EARLY CRATONS RESULTS IN METAMORPHISM AND MOUNTAINS

Metamorphism results in a new crop of mineral and rock formations

PreCambrian rocks of the US with the banded iron formations in dark red. One significant iron mine is highlighted, note the number of railway lines connecting the iron mines with Lake Superior. The iron from this region was very important during the industrial revolution, used to build steel for ships, tanks and aircraft during the First and Second World Wars. For 17 years, Edmund Fitzgerald carried taconite iron ore from mines near Duluth, Minnesota, to iron works in Detroit, Michigan; Toledo, Ohio; and other Great Lakes ports. She sank in a storm in 1975.

Early landmasses, location of early dates and stramatolite fossils