Re: At what depth in the earth can you reach a constant, sustained temperature?

The measure of how the temperature in the Earth increases with depth is called the geothermal gradient. The Earth’s temperature always increases with depth, but the amount of increase with temperature depends on which layer of the Earth you are at (how deep). The Earth has three main layers (like an onion): the crust, the mantle and the core. The thickness of the crust depends on whether you are on a continent, where the crust can get up to 200 km (124 miles) thick, or on the oceanic crust, where the crust may only be 100 km (62 miles) thick at the most. The mantle extends beneath the crust to about 2900 km (1800 miles) deep. At this depth, you reach the iron core of the Earth that exists to the very center of the Earth (6371 km, or 4000 miles, below the surface).

The temperature is always increasing with depth, but it increases at a different rate depending on the layer of the Earth. So, in the upper part of the Earth’s crust, the temperature is increasing at about 25 degrees Celsius per kilometer (or 72 degrees Fahrenheit per mile). Within the Earth’s mantle, the geothermal gradient is pretty much constant. This means that, as you increase the depth, the mantle temperature remains fairly constant. This is because the Earth’s mantle is convecting, or moving and mixing with itself, such that the temperature of the rock remains constant throughout a large range in depth. You can think of the mantle as similar to boiling water; as the water boils, the temperature of the water at the top is the same as the temperature of the water throughout the pot. The thermal gradient of the boiling water is constant. (Of course, the mantle of the Earth is not moving nearly as fast as boiling water. It "boils" at a rate of a few centimeters per year. But the idea is the same.) Then, as you get near to the core of the Earth, the temperature increases again to around 5000 degrees Celsius (9000 degree Fahrenheit). For a great plot of what the increase looks like from the surface of the Earth to the core- mantle boundary, check out the first link below, a lecture slide online from Portland State University.

Therefore, the answer to your question of how deep you have to go within the Earth to get a constant temperature with depth is pretty deep! You’d have to get into the mantle before the rocks held a constant temperature over a great depth (about 100 miles deep or so). However, because the geothermal gradient in the crust represents a small temperature change over a large distance (recall, 25 degrees Celsius per kilometer, or 72 degrees Fahrenheit per mile), any given depth will maintain a fairly constant temperature. The only problem is that you would have to dig down about 100 feet to get a temperature increase of 1 degree Fahrenheit. So, in order to get any real increase in temperature, you still have to dig quite a bit down into the Earth.

The good news is that the temperature of the Earth is fairly constant over the year for almost any depth, no matter the weather outside. This is why buildings that are built down into the ground (in Minnesota, for example) save on heating and cooling costs throughout the year. The ground keeps the building warm in the winter and cool in the summer. Typically, buildings that use geothermal heat as an energy source will dig between 200 and 1500 feet down into the Earth – a temperature difference of 2-15 degrees Fahrenheit. For more information about using geothermal energy in homes and other buildings, check out the Department of Energy’s Geothermal Energy Program web page, listed below.

A slide from a geology lecture at Portland State University that shows a nice plot of the temperature increasing with depth inside the Earth, from the surface to the core-mantle boundary: http://www.geol.pdx.edu/Courses/G201/Lecture_3/sld016.htm

U.S. Department of Energy’s Geothermal Energy Program homepage – more information about how we can tap into the Earth’s energy for a clean, renewable energy resource: http://www.eren.doe.gov/geothermal/