Top: Dennis Gibbs shows a model of the Earth in the Imperial Valley Discovery Zone classroom at Imperial High School. Peggy Dale photo
Above: Mudpots with a geothermal power plant in the background.
Image by U.S. Geological Survey
Left: The Imperial Valley Discovery Zone model of the Earth. Becky Gibbs photo
Geothermal is one of many forms of renewable energy in Imperial Valley
By Dennis Gibbs
What is it? Where does it come from? Why here? How does it become part of my everyday life?
The Imperial Valley is blessed with several forms of renewable energy. While solar and wind resources are readily available in a variety of locations around the world, geothermal energy is much more limited in its scope, and one place that can take ownership of this rare renewable resource is the Imperial Valley.
What is Geothermal Energy and where does it come from?
The word geothermal can be broken down. The term geo means anything to do with the Earth. Thermal just means that heat (a form of energy that is being transferred) is involved. Geothermal energy then is a product of Earth’s interior where high internal temperatures end up moving heat toward the surface of Earth.
The Earth itself is composed of several different layers, if we could see beneath the surface. These layers generally get progressively hotter as you move downward from the crust to the mantle, then into the outer core (liquid) and finally the inner core (solid). The average temperature on Earth’s surface is approximately 57 degrees Fahrenheit (14℃). But go deeper and that temperature will increase to the point that the inner core is approximately 9032℉ (5000℃), very nearly the temperature on the surface of the Sun. This huge temperature difference means that heat energy will always move from hotter to colder or from Earth’s interior (inner core) toward Earth’s exterior (crust).
Where does the energy in Earth’s interior come from?
There are several sources for this internal energy. First, some of that energy has been within Earth since its formation 4.6 billion years ago. Early in Earth’s lifespan, moving space debris was attracted by the fledgling Earth’s gravitational pull, crashing into the planet at high speed. These blocks of rocky material, due to their motion, had a lot of kinetic energy. These early collisions heated up the material that formed the early Earth, actually melting the planet. The core is primarily nickel and iron (both very dense) while the outermost crust is mostly composed of silicate rocks (much lower density). Density matters when objects float within one another. The molten Earth allowed the dense iron and nickel to sink toward the center while the lighter silicate rock floated to the top. The energy causing this differentiation is still with our planet even to this day. Some of this energy from the colliding space debris and some due to the gravitational friction as elements sank and others floated to the top.
Another source for the energy within Earth is the action of some of the elements that were a part of our early accreting planet. Because they are dense, radioactive elements such as uranium would have been pulled in by Earth’s gravity and become a part of the soup that was the early Earth, concentrating in the core. Since that time, more than 4 billion years ago, those radioactive elements have undergone a decay process where their nuclei broke down into simpler products and released ENERGY, which also increases the interior temperature of our planet.
Several studies have suggested that most of the heat powering Earth’s surface processes such as volcanoes, plate tectonics and earthquakes is still that original heat from Earth’s initial formation. It takes a LONG time for heat energy to move from the core and escape into space at the surface. That energy continues to make that journey to this day, more than 4 billions years since it all started. A tiny fraction of that energy is what we capture at our geothermal power plants.
Why isn’t that geothermal heat everywhere? Why the Imperial Valley?
As that geothermal heat in Earth’s interior moves outward it encounters the insulating blanket we know as the crust. In order to make it to the surface where we are, heat either needs to move through the crust or find a path where the resistance to heat flow is lower. This means every place on Earth’s surface will have a particular geothermal gradient. This number describes how much the temperature increases as you move downward into the crust at that location. The average gradient is about 25℃ (77℉) for each kilometer of depth, meaning the temperature doesn’t increase very quickly. In productive geothermal regions, the gradient is much higher. The Salton Sea Geothermal Field (SSGF) contains a measured geothermal gradient greater than 200℃/kilometer or 8 times the normal average.
The Salton Trough, where we live, has a set of geologic features that provide an easy pathway for heat to move upward toward the surface. These are faults and the features created by the movement of those faults. The image on the right shows a map of our region (CA Dept. of Fish & Game image), fault lines are in black, and geothermal facilities are red dots. Geothermallly speaking, the important features are the shaded regions within the yellow Salton Trough. These regions are known as “pull apart basins,” areas where the faulting has created extensional motion where the crust is being literally pulled apart. This extension thins the crust and can actually split apart the crust within those regions. This gives the heat, and sometimes even molten rock material, a pathway along which it can move upward.
The gradients in these areas are especially high as a result of that heat moving to the surface. This heat and the rock that contains the energy is what provides the energy powering our geothermal power plants. This energy also produces features related to heat such as our mudpots, and volcanic features such as the Salton Buttes along the Salton Sea’s southern edge and Cerro Prieto, a volcano in the Mexicali Valley.
The groundwater beneath the surface is heated well above the boiling temperature of 100℃, but that water doesn’t boil and remains a liquid due to pressure at depth, which is exactly how a pressure cooker functions. The hot water will assist in dissolving numerous elements including Zinc and Lithium which become part of the geothermal brine. The brine is brought to the surface and flashed into steam. The steam is used to spin generators in geothermal power plants to produce electricity for our use.
The dissolved minerals in the brine can also be harvested, which leads to our new moniker, “Lithium Valley.” But that story is one for another day!
Dennis Gibbs is a retired Imperial High School science instructor who has a master’s degree in geoscience from Mississippi State University and an indepth knowledge of the geothermal industry.
He regularly posts information about earthquakes, energy, and other geological phenomena on his Facebook page, www.facebook.com/dennis.gibbs.5