From far out in the Solar System, the Earth appears as a pale blue dot. Carl Sagan elaborates: “On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives.”
Not only has every human being lived out their lives here, they did so on the very surface of the Earth. Beneath our feet is something many people take for granite (at least judging by counter-tops given that name), as Randall Munroe pointed out above.
The conditions far beneath our feet are vastly different than they are here at the surface. In general, as you go deeper into the Earth, the temperature rises 20-30 °C/km. Not only that, but unlike air, rocks are quite dense, so the pressure rises rapidly. One atmosphere of pressure (or the roughly-equivalent metric unit, bar), from 100 km of air pushing down on us, is equivalent to the downward pressure exerted by 3.3 meters of rock.* Going down into the Earth, pressure increases about 300 bar/km. It’s not a particularly hospitable place for fragile creatures like humans.
In short, we can’t just go down there and get a sample. We have to wait for it to come here. The thing is, sometimes things which are stable at one temperature and pressure are not stable at another. This makes it very difficult to see things as they are, and scientists have to wait for rocks from the mantle to be transported upward to the surface. Consider the following:
You are confined, for whatever reason, to the interior of the United States Senate, where the climate is controlled to be about 21 °C and there is no precipitation. If you wish to study snow, you will have to wait until there is snow in the Senate. Fortunately sometimes the chair of the Committee on the Environment and Public Works will bring in a snowball [which proves
the world is not warming upsome Senators are unfit for the committee responsibilities given them].
In any event, we can’t study the mantle directly, except when bits of mantle-rock are ripped up and moved along by magma and transported to the surface. Such rocks are called xenoliths [etymology: xeno- foreign, and -lithos rock]. When mantle xenoliths are brought to the surface, the minerals within them don’t last very long [geologically] before changing phase or reacting to form new minerals, just like the snowball changes from solid to a liquid. This is what a mantle xenolith looks like on Earth’s surface.
For reasons mentioned previously, I am unable to provide a picture of a mantle rock looks like in the mantle. However, here is an artist’s impression:
In defense of that perhaps-shocking picture, I will point out that the mantle gets pretty hot. By 800-900 °C, objects start to glow red-orange from blackbody radiation. Also, I quibble with the XKCD cartoon shown at top; the mantle should be red and the core light yellow, not the other way around.
Volcanoes are among the places where scientists can gain insight into the mantle. Here, melted rock makes its way to the surface, and by studying the chemistry of that rock, we can understand the chemistry of the mantle.
However, the rock at a volcano is not necessarily all from the mantle. Continental crust is less dense than mantle rocks, and tends to float. Continents and the mountains upon them can be weathered into sand and silt, transported down streams—or even by the wind—and settle into the ocean. In areas of subduction, such as around the Pacific rim and in Indonesia, those little bits of continent can get pushed down into the mantle, where they melt and rise back to the surface.
Volcanoes can also form on top of a mantle hotspot. Kilauea is an example of this, as are all the Hawaiian volcanoes.
What about Heard Island? Is it a hotspot volcano, or something else? And where does the magma come from? An excellent set of questions! Before we dive into the papers by Jane Barling and others on the subject [1, 2], we will need to cover a few more topics to understand the work being done!
 Barling, J.; Goldstein, S. L. (1990) Extreme isotopic variations in Heard Island lavas and the nature of mantle reservoirs. Nature 348:59-62, doi 10.1038/348059a0.
 Barling, J.; Goldstein, S. L.; Nicholls, I. A. (1994) Geochemistry of Heard Island (Southern Indian Ocean): Characterization of an Enriched Mantle Component and Implications for Enrichment of the Sub-Indian Ocean Mantle. Journal of Petrology 35:1017-1053, doi 10.1093/petrology/35.4.1017.
* The math:
The density of rock is ~3 g/cm3, or 3*103 kg/m3. One bar is 105 kg/s2/m in SI base units.
Pressure = density * height * g [ed: little-g, 9.8 m/s2], and we’ll round g up to 10.
Height = pressure / (density * g), which works out to about 3.3 m.