Understanding Geothermal Gradient and Three factors that affect it.

The Earth is warm, and it is not just because of the sun. Many kilometers underneath the ground you’re standing on is molten rock and fiery hot temperatures that could almost rival the surface of the sun. In this article, let’s dig deep underground and explore what a geothermal gradient is.

What is the Geothermal Gradient?

You are probably aware of the fact that the Earth is quite warm as you dig deeper underground. A geothermal gradient is defined a difference in thermal energy between two different points underground, vertically. The general change in temperature as we dig deeper into the soil is 25 °C (equal to 77° F) per kilometer (which will be 0.62 in miles). The geothermal gradient is caused by the distribution of heat from the core and all the way to the surface. It is what powers geothermal systems. What determines the Geothermal gradient is the rate at which heat is transferred from the hottest spot to the coldest spot.

Three main factors affecting the geothermal gradient.

The thermal gradient at any given point in the Earth’s mantle is caused by three main factors: temperature, pressure, and composition. Temperature governs how quickly heat is transferred from one location to another. Pressure influences how easily heat can flow through rock and influences the amount of heat that can be generated at a particular location. Composition affects how well rocks conduct heat. Cooler materials cool more quickly than warmer materials, so they have greater thermal resistance. This means that they take longer to conduct enough heat to overcome their resistance and pass it on to layers close to the surface.

How does the Earth Heat up?

There are temperature variations as you move through different layers of the planet. The geothermal gradient exists due to the contrast in temperature between the surface and the mantle. The Earth’s internal heat, which we often refer to, is actually how hot the Earth’s mantle is. While the core is technically the hottest part of the Earth, the mantle is the layer closest to the crust.

The Earth’s mantle is a layer right below the crust that is roughly 2,900 kilometers thick. It makes up about 85% of the planet’s volume and is amongst the hottest places on the planet, only second to the core. The temperature of the mantle can be as high as 3700° Celsius near its boundary with the outer core. The mantle rises and falls with the temperature changes in the core, and this affects how much heat we get from the Earth’s interior.

The upper part of the mantle and the lower part of the crust (the lithosphere) is generally cooler and relatively solid, but as you reach the aesthenosphere, you will encounter much higher temperatures and semi-molten rock. As you go even further below, you will reach the widest layer of the mantle, the outer core, and finally the inner core at 3963 miles, or 6378 kilometers.

As our planet ages, our core cools down. The heat released from the core as it cools is transferred through the mantle, through the crust, and onto the surface. This is called convection and is ultimately the source of all the heat that causes the geothermal gradient.

How the geothermal gradient helps geothermal power plants

The Geothermal Gradient is the name given to the temperature differential between the Earth’s surface and its interior. This temperature differential drives the convection of heat in the Earth’s crust and inner core. Convection in turn drives plate tectonics and the formation of mountains and valleys on our planet.

The geothermal gradient plays an important role in how geothermal power plants work. In areas where the heat near the surface of the Earth is the greatest, or in areas where the geothermal gradient encourages volcanic activity, we are provided with a unique opportunity to generate power. Building geothermal power plants will allow us to reap great benefits from the geothermal gradient by generating electricity without using fossil fuels. Burning fossil fuels harm the environment, which causes global warming. Since geothermal power plants do not release harmful gasses, it is considered a green energy option.

Another way we can benefit from the geothermal gradient is by building geothermal heat pump systems. Geothermal heat pumps rely on the temperature difference between the surface and the fluid within a pipe buried several feet underground to heat and cool properties throughout the year. This system requires the Earth’s underground temperature to remain stable, as opposed to the temperature above ground.

Oceanic Geothermal Gradient

The ocean affects the temperature gradient found between the Earth’s surface, and wise versa. The ocean has a profound effect on the rate at which heat flows from the Earth’s mantle to the surface. The oceanic geothermal gradient is what drives geothermal activity which in turn affects the ocean.

The ocean largely affects the speed at which heat is circulated by affecting both the temperature and pressure of the mantle. The colder waters of the deep ocean are generally much cooler than the surface temperatures on land. The colder waters contrast greatly with the heat of the mantle which causes a much greater geothermal gradient. This is because the geothermal gradient relies on the temperature difference to operating.

The oceans cover 71% of our planet’s surface. This means that a large portion of the surface has cooler temperatures for the geothermal gradient to exist in a much steeper form. The deepest parts of the ocean (the ocean bed) are generally warmer than the water found a few meters above. This is due to the geothermal gradient.

Conclusion

The geothermal gradient is simply the temperature difference between two points in an Earth system. The higher up on the planet’s surface you are, the colder it is — this is due to your greater distance from the hot core of the planet. In areas where the geothermal gradient is a lot steeper, we may find just the perfect spot to build a geothermal power plant. Understanding the geothermal gradient is the first step toward generating renewable, cleaner energy by using the Earth’s own heat.