Can you dig all the way through the earth to reach the other side? As a child, I loved to dig in the backyard of my Cincinnati home. When I was a kid, my grandfather joked I’d end up in China if I continued to dig. If I could have dug straight through the earth, I’d end up in the Indian Ocean about 1,800 km (1,100 miles) west of Australia. This is the opposite, or antipode, point of Earth from where I live. The only tool I had to dig the ground was a small garden spade. After hitting a rock less than three feet (one meter) beneath the surface I was unable to go any deeper.
I am a geophysicist now and have a better understanding of Earth’s structure. There are three layers to the Earth:
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Its outer layer, or crust, is made up of a thin, light-colored rock. The thickness of the crust compared with Earth’s circumference is like how thick apple skin is compared with its diameter. As a child, when I scratched at Earth’s top crust to dig holes, I was actually scratching the surface of Earth.
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Like the apple’s flesh, the mantle is thicker and lies below the crust. The mantle is made up of heavy, strong rock which flows at a rate of a few inches per annum. The Earth’s core is surrounded by cooler, drier rock.
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At the Earth’s core is a super-hot liquid metal and super-hot liquid. The temperatures here range from 4,500 degrees to 9,300 Fahrenheit (2,700 to 5,200 degree Celsius). Earth’s outer layer exerts pressure on layers below, which increases with depth. Think of the pressure you feel in your ear as you go deeper into the water. This is relevant when digging into the Earth, because the rock above the surface of the earth exerts tremendous pressure on the side walls. They are also very unstable due to the empty space adjacent. The stronger rocks will be able to withstand greater forces but any rock can break if there is enough pressure.
Volcan26/Wikimedia (CC BY SA): The Earth has layers. The outer, solid part of our planet is called the lithosphere. It includes the upper, brittle portion of mantle, and crust. To prevent walls collapsing under pressure, you can make the slopes less steep. They should slant inward, like cone-shaped sides. As a rule, make sure the hole is three times as wide as its depth.
Unstable walls
Bingham Canyon Mine, Utah is the deepest pit on Earth. It was excavated with explosives and excavators in early 1900s for copper mining. The mine pit is 1.2 km deep (0.7 miles) and 2.4 kilometers wide. The walls of the pit aren’t too unstable or steep because the mine walls are more than three-times wider than they are deep. In 2013, one slope collapsed causing massive landslides and 145,000,000 tons of crushed stone to fall into the pit. The landslides were fortunately not serious, but they caused damage worth hundreds of millions. Imagine you had to dig through Earth if it were solid. This is the easiest scenario, even though we know it’s impossible. Earth’s diameter is the depth of the hole that goes through a planet. This line runs through the middle of the circle. To be stable, your hole needs to be three times the size of Earth’s diameter. This is an impossible job that will completely change the shape of our planet.
Drilling Versus Digging
Drilling is faster and deeper than digging, because it requires less material to be moved. Also, the borehole’s smaller surface can be designed to resist more force. Oil and gas companies drill up to 3 miles (5 km) below the surface in order to locate oil.
In northwestern Russia is located the Kola Superdeep Borehole, which extends to a depth of 7.5 miles (12 kilometers). Scientists can learn a great deal about Earth’s inner workings by drilling deep boreholes. The Kola project, however, was abandoned because of drilling difficulties, including temperatures that were too high for equipment to work, failures in equipment, and expensive costs.
The process of drilling is tedious. The rotary bit on the end of an empty, mud-filled tube grinds rocks, only penetrating a few inches per minute in very hard rock. If the rate of progress is maintained, it will take many hundreds of years for the Earth to be drilled. It takes longer to replace damaged parts as the drill bit gets deeper. The drill pipes can get so heavy they are unable to be pulled or twisted out of the holes. The pressure is another issue. The walls of boreholes are subjected to tremendous pressure, and they can fail. Slow movement in the Earth’s Mantle could eventually bend and cause collapse of a borehole. Under tremendous pressure magma, liquid metal, or gases deep within the Earth could explode through the borehole and reach the surface. The current drilling technology is not fast or durable enough for the Earth’s core and mantle. We can marvel at achievements like the Kola Superdeep Borehole or Bingham Canyon Mine, and still dream about digging rocks up from deeper depths. Andrew Gase, Assistant Professor of Geology at Boise State University (19659021), is a geoscientist. The Conversation has republished this article under Creative Commons. Original article can be read here.
