Arenal Volcano in Costa Rica.
A recent article on ice volcanism on Enceladus, one of Saturn’s moons, made me think about why Earth is hot and has tectonic activity. As a geologist, I think about plate tectonics often. Most people don’t think about plate tectonics much in their daily lives. After all, unless you live on a fault that happens to move or are affected by a volcanic eruption or a tsunami, plate tectonics doesn’t have much affect on your daily life.
Tectonic plates move extremely slowly on human timescales, so they’re difficult to wrap one’s mind around. Yet, the plates are moving. Moving on the order of centimeters per year, places such as California and East Africa will eventually rift away while South America and Asia will come crashing together as the Pacific Ocean closes over millions of years. Considering that the Earth is approximately 4.5 billion years old, that kind of motion is significant. Over the course of Earth’s history, tectonic plates have moved a fair amount. Geologists believe there have been several supercontinents, such as the most recent one Pangea, and that oceans have opened and closed many times.
The theory that Earth’s lithosphere is broken up into several moving plates was only fully accepted by the scientific community in the 1960s. Quickly, though, plate tectonics became recognized as a paradigm of geology. Today, plate tectonic theory is essential in understanding Earth’s geology, from volcanic deep sea vents at the very bottom of the ocean to weathering processes at the top of Mt. Everest. Scientists also realize that plate tectonics has played an important role in shaping our planet’s atmosphere and hydrosphere and may have played an important role in the origin of life on Earth. For instance, some scientists have proposed that volcanoes at the bottom of the ocean may be the places where life first originated.
In the Earth, there are three main sources of heat that drive plate tectonic activity:
1. Heat of initial planetary accretion
2. Heat of planetary differentiation (the sinking of heavier material to form Earth layers, such as the mantle and core)
3. Heat of radioactive decay
Rock is actually a fairly poor conductor of heat. Lavas flows, for instance, can remain molten for hundreds of years below a thin crust of solidified rock. Remarkably, much of the heat of initial accretion and subsequent differentiation still remain trapped in the Earth. Radioactive decay also contributes to keeping Earth hot. Elements such as potassium (K), uranium (U), and thorium (Th) decay and release energy as a result. Many of the most radioactive elements, such as uranium, are fairly heavy and are thus concentrated in Earth’s core. So, radioactive decay is somewhat concentrated in Earth’s center and the associated heat takes awhile to be released into space as a result.
Plate tectonics and volcanic activity are two ways which Earth releases heat. Heat is conducted to Earth’s surface very slowly, and– more significantly– heat energy is transformed into kinetic energy through physical convection of the mantle, a solid yet slowly-moving (like tar or honey) layer in the Earth on top of which tectonic plates float and are moved around.
The layers of the Earth. These layers formed as denser materials sank to the middle of the Earth while lighter materials floated to the top. This differentiation released energy which still helps heat the Earth.
Eventually, Earth will cool down. Indeed, she has already cooled down significantly. Komatiites, for instance, may represent gigantic volcanic eruptions which only occurred early on in Earth’s history in a time when Earth was much hotter. There is some controversy about the nature of komatiites, but most geologists agree that Earth is cooler now than she was back in the Archean. In millions of years– billions, perhaps– Earth will cool and plate tectonic activity will cease.
On other planetary bodies, such as the moon and Mars, plate tectonic activity has already ceased. The moon has been tectonically dead for billions of years while Mars has no current volcanism, though there may have been volcanic eruptions on Mars as recently as several thousand years ago.
Why are the moon and Mars dead tectonically while Earth is still very active tectonically? The key lies in surface area to volume ratio. Earth has a sort of golden ratio in this regard. The moon and Mars have just too much surface area (relative to their volume) through which heat can be released into cold space. Thus, Earth is hot while they are cold.