Earth Science: Geology, the Environment, and the UniverseChapter 5: Igneous RocksProblem of the WeekIgneous Rocks From Deep Within Kimberlites are found in the "hearts" of the continents and are believed to have formed throughout Earth’s history. Kimberlite intrusions range in age from 50 million years old to 1.6 billion years old. However, the diamonds are much older in most cases–from slightly less than 1 billion years old to over 3.3 billion years old. The magma that formed these intrusions is believed to have come from depths of over 300 km. Only at these depths are the temperatures and pressures high enough to form diamonds. The rise of the magma is believed to have been rapid, only taking a day or two. As they near the surface the intense heat of the magma vaporizes groundwater. On the rise to the surface as pressure decreases, gases in the magma begin to come out of solution. This explosive pressure change increases the diameter of the "blow hole" as it nears the surface. This results in the carrot-shaped structure. The diamonds remain unchanged as they make this rapid rise. So, in addition to the diamonds, the inclusions they may contain, and the kimberlite intrusion, other mantle rocks is also carried up with the magma. A higher melting point allows pieces of mantle rock to come up unchanged by the carrying magma. This problem is a good opportunity to have students brainstorm and make connections between the composition of the magma and how it would or could react as it ascended. They should also think about the types of materials that magma might encounter as it ascends, how the magma would affect these materials and how these interactions might create the unusually shaped intrusions. Conditions necessary for diamond formation can be discussed and how those conditions help us to know that they could only form naturally in the mantle. Information Resources:
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