Komatiite

Komatiite is a very rare rock.

It comes from very hot lava. This lava was much hotter than most lava. It was as runny as water. Because it was so runny, it flowed very fast.

The rock has a lot of magnesium in it. Some parts look like blades of grass. This is called a spinifex texture. These rocks are very old. Most were made a long time ago. They tell us about the early Earth.

Komatiite is a rare type of volcanic rock.

It is named after the Komati River in South Africa. This rock has a lot of magnesium in it. It also has very little silicon and potassium.

Most komatiite formed a very long time ago. These rocks come from the Archaean Eon. That was between 4.03 and 2.5 billion years ago. The early Earth was much hotter than it is today. This heat helped make komatiite. The lava was extremely hot. It could reach 1600 °C. Because it was so hot, the lava was very runny. It was as fluid as water. It could flow very fast across the ground.

Many komatiites have a special look called spinifex texture. This name comes from a type of Australian grass. It happens because crystals grow in long, blade-like shapes. These shapes are made of minerals like olivine and pyroxene. Most komatiite rocks have changed over time. This change is called metamorphism. It happens when heat and fluids touch the rock. This can turn the rock into new minerals like talc or serpentine.

Komatiite is a very rare type of volcanic rock.

It is special because it comes from deep inside the Earth's mantle. This rock contains a huge amount of magnesium. It has very little silicon, potassium, or aluminium. Because of its high magnesium, scientists call it a picritic rock. It is a window into how our planet used to work.

This rock forms in a very specific way. First, the mantle must melt a great deal. This is called partial melting. In komatiite, more than 50% of the rock melts. This process creates a liquid that is extremely hot. The temperature can reach over 1600 °C. Because it is so hot, the lava is very runny. It flows as easily as water. It can create very thin layers only 10 mm thick.

Most komatiite formed a long time ago. These rocks come from the Archaean Eon. That was between 4.03 and 2.5 billion years ago. The early Earth was much hotter than it is today. The mantle was hotter due to extra heat from the planet's birth. It also had more radioactive isotopes like uranium 235. These isotopes produce decay heat. This heat made komatiite possible. Today, cooler mantle melts like basalt have mostly replaced it.

Scientists find these rocks in special places. They are mostly in Archaean shield areas. These are often found in greenstone belts. The name comes from the Komati River in South Africa. One rare example is in Manitoba, Canada. Another is on the island of Gorgona near Colombia. Some komatiites show a spinifex texture. This means they have large, blade-like crystals. These crystals are made of olivine and pyroxene.

Many komatiites have changed over time. This change is called metamorphism. Heat and fluids touch the old rock. This can turn the rock into new minerals. It might create talc or magnesite. This is called a carbonated reaction. If water is used, it creates serpentine. This is called a hydrated reaction. You can think of this like a cookie changing in the oven. The heat and moisture change the original shape and parts.

Komatiite is a rare and distinctive type of ultramafic volcanic rock. It is defined by its origin in the Earth's mantle and its extremely high magnesium content. Specifically, a rock is classified as komatiite if its lava contains at least 18 wt% magnesium oxide (MgO). Because of this high magnesium, scientists also classify it as a picritic rock. These rocks are unique because they have very low levels of silicon, potassium, and aluminium.

Studying komatiites helps geologists understand the thermal history of our planet.

The formation of komatiite requires a specific and intense melting process. It is created through high degrees of partial melting, which means more than 50% of the source rock melts. This process produces a magma with a very high melting point. Calculated eruption temperatures can reach or even exceed 1600 °C. For comparison, common basaltic lavas usually erupt at temperatures between 1100 and 1250 °C. This intense heat makes the resulting lava extremely fluid. While basaltic lava flows like honey or treacle, komatiitic lava has a viscosity close to that of water. This allows it to flow swiftly and create very thin flows, sometimes only 10 mm thick.

Komatiites are categorized into two geochemical classes based on their aluminium and titanium levels. The first class is aluminium undepleted komatiite (AUDK), also known as Group I. The second class is aluminium depleted komatiite (ADK), or Group II. Scientists define these groups using their Al2O3/TiO2 ratios. It was once thought that these groups represented different depths of melting. In ADK, high pressure might prevent garnet from melting in the source. In AUDK, melting likely occurs at a lesser depth. However, recent studies of fluid inclusions in chrome spinels suggest a single flow might actually mix different parental magmas.

Most komatiites are ancient relics from the Archaean Eon, occurring between 4.03 and 2.5 billion years ago. They are rarely found in younger Proterozoic or Phanerozoic rocks. This age restriction is likely due to the cooling of the Earth's mantle over time. The early Earth had much higher heat production than it does today. This heat came from residual energy left over from planetary accretion. Additionally, the early mantle had a greater abundance of radioactive isotopes. Specifically, shorter-lived isotopes like uranium 235 produced significant decay heat.

Geographically, these rocks are mostly found in Archaean shield areas and greenstone belts. The rock was named after its type locality along the Komati River in South Africa. Other notable locations include the Abitibi greenstone belt in Ontario, Canada. Rare examples also exist in the Winnipegosis komatiite belt in Manitoba, Canada, and on the island of Gorgona near Colombia.

These sites provide essential data on how mantle plumes or subduction zones functioned billions of years ago.

A defining feature of many komatiites is their unique spinifex texture. This texture consists of large, dendritic, or blade-like crystals of olivine and pyroxene. The name comes from the Australian grass, Triodia, which grows in similar clumps. This texture forms due to the rapid crystallization of olivine within a thermal gradient in the upper part of a lava flow.

In a typical flow, the mineralogy changes from the bottom to the top. The base often contains olivine cumulates, while the upper zones show the spinifex texture. Some samples may even show a volcanic glass chill zone at the very top.

Because they are so old, almost all known komatiites have undergone metamorphism. This means they have been changed by heat and fluids, and are technically called metakomatiites. These changes are often categorized as either hydration or carbonation. Carbonated komatiites form when the partial pressure of carbon dioxide (XCO2) is above 0.5. This process creates minerals like talc, magnesite, and tremolite. Hydrated komatiites form when XCO2 is below 0.5 and water is present. This results in minerals such as serpentine, chlorite, and brucite. These metamorphic processes can significantly alter the original texture and mineral makeup of the rock.