A Hole to China? Going Down to Understand What’s Up
Recently we were planting trees. Little ones. We needed to dig holes about a foot deep. Our spade hit hardpan three inches down, so it took us at least half an hour to dig one hole. This reminded me of when I was a child, and tried to dig to China. I do not need to describe my mother’s face when she saw her garden. But If I had dug a foot six days a week for a whole year I would have had a hole about 310 feet (80 meters or so) deep.
I might have found gold: I was digging in the foothills of the Colorado Rockies. But let’s forget that and consider instead what I would have found if I had managed to dig all the way through the Earth. What does this have to do with astronomy? Well, almost everything we know about the stars comes from what we have learned from what we can touch and see here on Earth.
Suppose my hole had gone straight down and come out on the other side. What would I have found? People walking upside down? No, I would have missed China by a long shot. Straight down from Colorado (and also from Bainbridge Island) I would have come out in the middle of the Indian Ocean southwest of Australia. No people, but the cold water might have been welcome after the heat of what I had dug through.
The figures concerning such a hole are daunting, but they’re easier to face than the actual dig. To start, the surface of our Earth, or its crust, is on average about 40 kilometers deep. It is churned up by tectonic activity—the constant recycling along the edges of the continental plates of the next layer down. Besides the surface soil —some odd inches of dust and manure and tangled roots (oops—sorry: I should have said some odd centimeters)—the crust is mostly crystalline rocks like quartz and feldspar.
About every 35 m down the temperature goes up one degree Celsius. The thinnest crust is generally under the oceans; the thickest is under the Himalayas —75 km deep. The deepest hole ever dug into the Earth was 12,262 m deep (a bit more than 12 km) off the Kola Peninsula near the Arctic Circle in Russia. Its purpose has been to study the Earth’s crust. At this depth the heat is a problem for the equipment. The second deepest is the Bertha gas well in Oklahoma. The diggers of that one stopped working at 12,191 meters when they hit molten sulfur.
From these figures, it should be obvious that, except for the outcroppings, no one has managed to explore down the 40 km to the next layer of the Earth. So what we know about what is underneath our feet below the crust comes from a variety of sources, of which earthquake measurements are the most important. Earthquakes and large man-made explosions create shock waves that ripple around the Earth. Those ripple patterns show where the earthquake center was, when it occurred, how big it was, and what kinds of densities the waves traveled through.
Meteorites bring evidences of the kinds and relative quantities of elements that are present outside the Earth and that are the stuff the Earth was made of originally. The eruptions of volcanoes give us a chance to study the deeper levels of the Earth: The chemical composition of what is inside the Earth shows up in the volcanic lava, gas, and dust.
The chemical and physical changes in an element can be studied in a laboratory when it is heated, or under pressure, or combined with other elements. Thus an element that is found in a rock can be tested it to see if it has properties similar to the same element that has been studied in the lab. From comparing the spectra of elements on Earth with the spectra of the Sun’s light we have learned what elements are present in the Sun, and their relative quantities. Of course this is true for the light of all the other stars also.
Much of the Earth’s heat is in the mantle, the area below the crust. The mantle appears to be in two distinct layers, separated by a relatively narrow transition: it is the region where volcanic activity originates. The upper mantle goes down to about 400 km; it is exposed in places like eroded mountain belts and volcanic eruptions. Where it is under the crust it is solid rock, but it behaves like a viscous liquid. The heat and pressure there, for instance, have turned the element carbon into crystalline diamonds, about 150 to 300 km below the surface. Beneath the upper mantle the transition zone is about 150 km deep; it is mostly basalt, calcium, aluminum and garnet. It is dense when it is cold, but when it is hot it becomes buoyant and rises up as magma in volcanoes. The lower mantle is rich in silicon, magnesium and oxygen and goes down to 2700 km; this is about half-way into the center of the Earth. There is another transition layer called the “D” layer about 190 km thick. While it is part of the lower mantle, earthquake records show that it has a different density.
The very dense core of the Earth is also in two distinct layers. The outer core is made up largely of liquid iron and nickel; the inner core is mostly solid iron. The theory is that when the Earth was first formed 4.5 billion years ago it was all molten. As it cooled over time the denser elements—like iron—sank into the center, while the lighter ones—carbon, calcium—rose.
Some of the original heat is trapped in the outer core which is thought to be between 5000° C and 7500° C. (hotter than the Sun’s photosphere!). The distance down from the “D” layer to the bottom of the outer core is 2670 km; from the inner core to the center is another 628 km. The outer core is separated from the mantle; as it combines with the Earth’s rotation, it controls the Earth’s magnetic field.
So far, the elements scientists have identified on Earth have the same characteristics as what they have observed in outer space. In other words, the Earth appears to be made of the same stuff as everything else, if in varying amounts. Therefore, what we can observe here should be true of what is happening on the Moon, or on Jupiter, or in any one of the stars in a distant galaxy.
Adding the depths of the crust, the mantles, and the cores gives a figure of 6378 km as the average radius of the globe of the Earth. So, to come back to the original question, to dig a hole through the Earth, one would have to make it 12,756 km deep. That is over 1,000 times deeper than the hole in Russia.
And it reminds me of a comment by the political wit Will Rogers: “If you’ve dug yourself into a hole, stop digging.”
Will Rogers also reminded his listeners that “good judgment comes from experience, and a lot of that comes from bad judgment.” Also, “one should never miss a good chance to shut up.”