Earth Science: Geology, the Environment, and the UniverseChapter 5: Igneous RocksProblem of the WeekIgneous Rocks From Deep Within Introduction: In Jules Verne’s novel, "Journey to the Center of the Earth," Professor Hardwigg and his nephew, Harry, take a journey down into the interior of Earth. As they traveled deeper and deeper, they encountered strange "worlds," a mid-earth ocean, and odd forms of life, all unlike anything on the surface. Their discoveries on this fictional adventure were far different from what scientists have determined that our Earth is actually like below the surface. Today, scientists describe Earth as being divided into four layers, the solid inner core, the liquid outer core, the mantle and the crust. They tell us that the crust is made of rocks composed primarily of granite and basalt. The mantle is composed of a rock called peridotite, and parts of the upper mantle are partially melted. The core is composed of iron and nickel. The outer core is liquid and the inner core is solid. Have you ever wondered how we know all of this? Well, most of our information comes from studying seismic wave travel times and patterns of seismic waves produced by earthquakes. Using over 100 years of seismic data, Earth’s interior has been mapped pretty precisely, but this evidence is indirect. Geologists, like other scientists, long to put their hands on mantle material and so they seek concrete evidence to support the findings from the indirect methods. Our most direct evidence of what the mantle is made of comes from one of our most precious gems–diamonds, and the type of igneous rock in which they are found – kimberlites. Kimberlites are formed from magma that comes from deep within the mantle in explosive eruptions. The diamonds are not formed in the eruption. They form at great depths, where intense heat and pressure allow the carbon atoms that make up the diamond to form the dense crystalline structure that makes diamonds so hard and dense. The eruption simply acts as the elevator, bringing them rapidly to the surface. Not only do these eruptions bring diamonds up from the mantle but the diamonds often contain inclusions or small bits of rock from the mantle. These little inclusions are more precious to the geologist than the diamond. These inclusions are the hard evidence that geologists are searching for! So, what’s the problem? Well, let’s get back to the kimberlites. Their shapes are quite different from most igneous rock intrusions. Your text describes them as tube shaped, but most sources describe them as carrot shaped, 30-90 m across at the surface, narrowing as you descend. This shape indicates a rapid rise and explosion as the magma reached the surface. Problem: Think About This-What conditions could have created a narrow at depth, wider as it nears the surface, kind of "blow hole." Things to think about as you tackle this problem:
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