How does Lost City Form in the Absence of Volcanoes?

Unlike almost all other known seafloor hot spring systems, Lost City does not sit on top of an active volcano, and the chemistry of the vent fluids indicate that they have not reacted with either a cooling or fossilized magma chamber. So a very important question arises “Where does the heat come from to drive circulation of fluids within this 1-2 million year old mountain?”

On all active planets, movement of heat is important to a diverse array of natural processes. One major way of exchanging heat can be by placing an extremely hot material, such as lava, within a much colder substance, such as water. If hot, molten lava is placed within cold water, the water becomes heated and it increases in temperature as heat is removed from the hot lava. This removal of heat results in a corresponding decrease in the temperature of the molten lava.

Another way to move heat around within nature is through chemical reactions. All chemical reactions have a property where they will exchange heat with their surrounding environment by either taking heat and cooling the surroundings or giving off heat and warming the surroundings. For example, to change liquid water to steam during boiling, one must add heat to the water. During conversion of the steam back to water the same amount of heat will be given off as was originally added.

This process of giving heat off through chemical reactions, which is a property referred to as exothermic, is very important to the Lost City Hydrothermal Field. We believe that the heat required to drive hydrothermal circulation at Lost City comes from chemical reactions between the rocks that comprise the mountain and deeply circulating seawater. The mantle or peridotite rocks that comprise the Atlantis massif were initially formed much deeper in the Earth. Because of faulting they are now exposed at or near the seafloor and they are out of equilibrium with their surroundings. Because they are out of equilibrium, when these rocks fracture and seawater-derived fluids migrate into the cracks, abundant chemical reactions occur between the minerals that comprise the mantle rocks and the fluids. This reaction gives off an enormous amount of heat-- enough to heat the fluids within the rocks to a few hundred degrees Fahrenheit. Like a boiling pot of water on your stove that is heated from below, these heated fluids rise through the fractures in the mountain and eventually vent on the seafloor. Mixing of the chemically changed fluids and seawater causes calcium carbonate to precipitate and form chimneys.