January 15, 2011
|Komati River, South Africa. Image from Wikipedia here.|
1. An ultramafic, volcanic rock that is primarily composed of the minerals pyroxene and olivine.
2. A very unusual and rare volcanic rock type that is not produced today. Most komatiite lavas were produced in the Archean (approximately 2.5 to 3.8 billion years ago).
3. A rock type whose hotly– and wetly– debated origin sometimes galvanizes geologists to shouting matches, fist fights, and drinking contests.
Komatiites are ultramafic volcanic rocks. An ultramafic rock is a rock that has very low silicon (SiO2), sodium (NaO), and potassium (K2O) and very high and iron (FeO) and magnesium (MgO) content, generally greater than 18 weight percent MgO*. For comparison, mafic basalts have about 7 weight percent MgO and felsic rhyolites have less than 1 weight percent MgO.
For those of you who are not used to thinking about mafic verses felsic rocks, let me try to explain the terms simply. A mafic rock is basically a dense, dark-colored rock that is more “primitive” or closer to the composition of the Earth’s mantle. A felsic rock is a less dense, lighter-colored rock that is “more evolved” or less close to the composition of Earth’s mantle. Mafic lavas, such as basalt, are generally produced through fairly high degree melting (about 10-20%) of the mantle. Felsic lavas, such as rhyolite, are produced through lower degrees of mantle melting and/or melting of continental and oceanic crust. Mafic rocks are enriched in heavier elements, such as magnesium and iron, and are depleted in lighter elements, such as silicon (SiO2) and sodium (NaO).
|Table of igneous rock types, ranging from ultramafic to felsic. Taken from Wikipedia here. Click on the table to view a larger version.|
An ultramafic rock is ultra depleted in silicon and other “felsic” elements and is also ultra enriched in magnesium and other “mafic” elements. Ultramafic rocks are primarily found in Earth’s mantle. Since the mantle represents about 84% of the Earth’s volume, most of the Earth is actually ultramafic. Although not quite correct, you can think about the mafic, intermediate, and felsic rocks that primarily comprise Earth’s crust as the light froth that floated to the Earth’s surface. Most of the Earth is comprised of denser ultramafic rocks– primarily my favorite rock peridotite. Most of the Earth is made of peridotite, which is primarily comprised of the iron and magnesium-rich minerals olivine and pyroxene.
|Structure of the Earth. Image taken from here.|
Although ultramafic rock makes up most of the Earth, geologists rarely find ultramafic rocks on Earth’s thin crust. This is because when the Earth’s mantle melts to produce magmas, it does not melt 100%. Rather, it generally melts between about 5% and 20%. This partial melting fractionates elements and has the effect of making the melt more felsic in composition than the ultramafic mantle. For instance, at mid-ocean ridges, 10% to 20% melting of ultramafic mantle produces mafic basaltic magmas. Some felsic lavas are actually melts of mafic or intermediate rocks. Whenever you partially melt a rock, it generally becomes more felsic in composition.
Understand all this so far? I know that I am putting a large amount of geology terminology in this week’s word of the week. In addition to komatiite, I am also trying to explain the geology words felsic, mafic, and ultramafic and the concept of partial melting. I realize this may be a bit much for my non-geology readers. I promise to return in future weeks to these words and concepts, but for now just try to understand the basics. The reason that I want you to have a basic understanding of these words and concepts is because this will help you to understand why komatiites are so remarkable and rare.
So, we’ve established that when the ultramfic mantle melts today, it generally melts no more than about 20%. This degree of melting produces mafic melts. These magmas make their way to the surface and, if they make it all the way to the surface, they erupt as lavas that eventually cool into mafic volcanic rocks. Today, the most primitive lavas that erupt are mafic basalts. No one has ever observed ultramafic lavas erupt; there are no places on the Earth today where ultramafic lavas are produced.
Before I continue, let me define another pair of geology words: volcanic verses plutonic. A volcanic rock is a rock that forms from the quick cooling of subaerial lavas. Since volcanic rocks cool quickly, they have textures that reflect this quick cooling. A plutonic rock, on the other hand, is a rock that forms from slower, deeper cooling in a place where magmas accumulate, such as a magma chamber. One final set of geology words: magma verses lava. A magma is what you call a melt before it reaches Earth’s surface. When magmas erupt subaerially, they are called lavas. Hopefully, I am done defining geology words now. For all the geologists who read my blog, I apologize for the review. I just want to make sure all my readers are up-to-speed with the terminology I’m using.
Back to the komatiites. Komatiites are ultramafic lavas. Now that you are familiar with the terms ultramafic and lava, you hopefully appreciate how amazing and befuddling komatiites are for geologists who, again, have never observed ultramafic lavas erupting. Komatiites are fairly rare rocks, but they are found throughout the world in places such as Canada and South Africa. Most komatiites were formed billions of years ago in the Archean (approximately 2.5 to 3.8 billion years ago). The youngest komatiites on Gorgona Island, Columbia were formed 90 million years ago, but these very young komatiites are anomalous. Almost all komatiites are billions of years old.
Komatiites were first described in the early 20th century in publications by the geological surveys of Zimbabwe, Canada, and Australia (Middlemost, pg. 101). The first chemical analysis of a komatiite was presented in 1928 in the Southern Rhodesia (now Zimbabwe) Geological Survey Bulletin (Macgregor, 1928). Early studies of komatiites noticed their unusual compositions but did not understand the full implications of these rocks. Remember that plate tectonics and the understanding that Earth’s mantle is composed of ultramafic peridotite were not fully realized until later in the 1960s and 1970s. In the late 1960s the Viljoen brothers described an incredible exposure of komatiite near the Komati River in South Africa (Viljoen and Viljoen, 1969). As you might guess, the Viljoen brothers named komatiites after the Komati River**.
After the Viljoen brothers study, geologists began to realize the implications of these unusual rocks. They realized that komatiites formed from ultramafic lavas, and they realized that komatiite lavas are not produced today and, furthermore, could not be produced today. Geologists also quickly realized that most komatiite lavas are incredibly ancient, billions of years old. The anomalously young komatiite lavas on Gorgona Island were not discovered by geologists until 1979 (Echeverria, 1980). Komatiites were loosely defined as a rock type until the early 1980s when Arndt and Nisbet (1982) defined komatiites as rocks that:
1. Have a mineral assemblage or chemical composition that indicates an ultramafic composition
2. Have structures and/or textures that indicate a volcanic (extrusive) origin.
That is, they are ultramafic rocks that erupted as lavas. But how is this possible? And why aren’t komatiites produced today? Well, geologists have argued about the origin of komatiite lavas since komatiites were first discovered. Geologists agree that ultramafic lavas can only be produced when the ultramafic mantle is able to melt to a greater extent. The degree of mantle melting required to produce komatiite melts is about 50% to 60%, which is far greater than the maximum of about 20% that the mantle melts today. What geologists do not agree on is what conditions led to this much higher degree of mantle melting. There are two hotly– and wetly– debated possibilities.
The first possibility is that the mantle was hotter back in the Archean, when most komatiites were produced. If the mantle used to be hotter back in the Archean, much higher degrees of mantle melting would be possible. The mantle– indeed the entire Earth– was much hotter back in the Archean because of higher amounts of radioactive elements (which have now decayed) and other sources of heat that have now dissipated. However, to form the many komatiites the mantle needed to be hotter by about 500 degrees Celsius, which is a big difference. Also, not all lavas that were produced in the Archean were ultramafic– there were also plenty of mafic basalts produced. So, if the mantle were much hotter, why would komatiite lavas only be produced in certain places? This leads many geologists to challenge the hot mantle theory for the origin of komatiites.
The second possibility is that the mantle was wet and less hot. Wet rocks– that is, rocks with a large amount of water and other volatiles in them– melt at lower temperatures than dry rocks. So, if the mantle were wetter back in the Archean, it would produce higher degrees of melt even if mantle temperatures were not much higher than today. For instance, some authors propose that mantle temperatures higher by only 100 degrees Celsius or so could produce komatiites in wet subduction zone environments (e.g. Grove and Parman, 2004).
The origin of komatiite lavas is a passionately debated topic in geology. Back when I was a first year graduate student, an older student told me that he purposely didn’t work on komatiites (which his advisor famously studied) because he didn’t want to work on such a controversial rock. I laughed at the comment and wondered how a rock could be controversial. Then, he told me stories about geologists passionately debating their opinions on komatiites. At one conference, scientists started yelling at each other and nearly broke out into a fist fight. Certainly, the passion of komatiite researchers shines through in their papers, which makes them entertaining to read. When studying komatiites, I guess you have to join “team wet” or “team dry.” If you want to read a review of the komatiite debate, an excellent paper (though I have to warn you that this paper comes from “team wet”) is the Grove and Parman (2004) reference below.
You might be wondering how geologists know that komatiites were erupted as subaerial lavas. How do geologists know that these rocks aren’t just plutonic ultramafic rocks that were exposed through uplift or erosion? The answer is that komatiites have textural and structural characteristics that indicate that they formed from lava. For instance, komatiite lavas are particularly known for their beautiful spinifex textures. Spinifex*** is a texture created by elongated olivine crystals that form when olivine cools extremely quickly, a sure sign that komatiites formed from subaerial lavas. I leave you with these beautiful pictures of spinifex texture in komatiite.
|Spinifex texture in komatiite. Image taken from here.|
|Spinifex texture in komatiite. Image taken from here.|
*For those of you who are not familiar with the convention, major elements in rocks are always reported as oxides (combinations of an element with oxygen). Oxygen is actually the most common element in rocks and binds with the other elements, so this convention makes sense for rocks. However, those who are not used to thinking about geochemistry and petrology may find this convention a little confusing at first. Don’t worry– after awhile you become accustomed to it. So, if you continue reading my blog, you’ll be an oxide expert in no time!
**As a quick etymological aside, komati comes from the Swati word “inkomati,” which means cow. So, komatiite means “cow rock.”
***Here’s a list of the geology terms I’ve introduced in this post: komatiite, mafic, felsic, ultramafic, mantle melting, volcanic, plutonic, magma, lava, mantle, crust, peridotite, spinifex.
Arndt, N. T. and Nisbet, E. G. (editors) 1982. Komatiites. London: George Allen & Unwin.
Echeverria, L.M., 1980. Tertiary or Mesozoic komatiites from Gorgona island, Colombia: Field relations and geochemistry. Contributions to Mineralogy and Petrology, vol. 73: 253–266.
Francis, P. and Oppenheimer, C. 2004. Volcanoes. New York: Oxford University Press, 35-36.
Grove, T. L. and Parman, S.W. 2004. Thermal evolution of the Earth as recorded by komatiites. Earth and Planetary Science Letters, vol. 219: 173-187.
Hall, Anthony. 1987. Igneous Petrology. Essex: Longman Scientific & Technical, 341-342.
Kamenetsky, V. S., Gurenko, A. A., and Kerr, A. C. 2010. Composition and temperature of komatiite melts from Gorgona Island, Colombia, constrained from olivine-hosted melt inclusions. Geology, vol. 38, no. 11: 1003-1006.
Macgregor, A. M. 1928. The geology of the country around the Lonely Mine, Bubi District. Southern Rhodesia Geological Survey Bulletin, vol. 11.
Middlemost, Eric. 1985. Magmas and Magmatic Rocks. Essex: Longman Scientific & Technical, 100-101, 183-185.
Viljoen, M. J. and Viljoen, R. P. 1969. Evidence for the existence of a mobile extrusive peridotitic magma from the Komati Formation of the Onverwacht Group. Geological Society of South Africa, Special Publication, no. 2: 87-112.