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We all know water can extinguish fire, and there's no place with more water than the ocean. Yet, why can't the water there extinguish underwater volcanoes?
When picturing a volcano, many may imagine a tall, conical mountain emitting thick columns of smoke, resembling an erupting volcano. Moreover, after an eruption, a large amount of white volcanic ash will fall over a vast area. Hence, many subconsciously think volcanoes can indeed erupt with fire.
But upon close observation, that's not the case at all. Because the magma erupted by volcanoes is essentially a high-temperature liquid, fundamentally different from fire.
Close-up photos of magma clearly show it as a liquid. When we study physics back in school, we learn about the phase transitions of matter: gas, liquid, and solid, related to melting and boiling points.
Taking water as an example, its melting point is 0 degrees Celsius and boiling point is 100 degrees Celsius. When the temperature is below 0 degrees Celsius, we see water as a solid - ice; when the temperature is between 0 degrees Celsius and 100 degrees Celsius, we see water in liquid form; when the temperature is above 100 degrees Celsius, water exists as a gas - water vapor.
Fundamentally, all substances behave similarly, exhibiting melting and boiling points, and undergo phase changes according to temperature. Magma, a molten rock composed of complex minerals, experiences varying melting and boiling points due to its heterogeneous composition, resulting in most magmas being a mixture of solids, liquids, and gases. Conversely, volcanic eruptions give rise to flames, a process where combustible materials emit light and heat. In these flames, the primary material components are carbon dioxide, water vapor, oxygen, nitrogen, and other gases.
Understanding that volcanic eruptions involve magma, which is molten material at high temperatures rather than actual fire, leads to the realization that when underwater volcanoes erupt, magma intrudes into the sea akin to continuously spraying hot water into a pool of cold water—where the cold water cools the hot water but doesn't extinguish it as water does to fire. Thus, seawater cannot extinguish underwater volcanoes.
Because volcanoes result from planetary heat and material circulation, they remain perpetually active. Earth as a whole operates based on fundamental principles of physics and chemistry, concepts we've learned since high school.
For instance, the formation and activity of volcanoes can be explained by the second law of thermodynamics—a concept that sounds sophisticated but is intimately connected to our lives: Heat always spontaneously flows from a hotter to a cooler body.
If we were to rewind Earth's evolutionary history over 4.6 billion years, we'd witness the effects of this law: Approximately 4.6 billion years ago, Earth gradually formed through countless planetary collisions, with the energy from these collisions transforming into heat, turning the early Earth into a massive magma sphere (with most or much of its surface being magma), with surface temperatures reaching thousands of degrees Celsius.
Subsequently, due to magma's ability to flow, heavier materials sink while lighter ones rise. As heavy materials sink, gravitational potential energy converts into thermal energy; simultaneously, initial radioactive elements scattered throughout the planets aggregate, continuously decay, and release energy.
These energies maintain the Earth's interior magma in a perpetual state of heat. However, due to the universe's low background temperature, averaging -270 degrees Celsius, the Earth continually radiates heat outward in the form of thermal radiation (there are three heat transfer methods: conduction, convection, and thermal radiation, but in space, being a vacuum, Earth can only radiate heat outward). As heat is lost, the Earth will cool down, with the Earth's surface cooling first, leading to the solidification of magma into rock, forming the initial crust.
To date, Earth has developed a fundamentally structured into three layers: the crust, mantle, and core, with temperatures increasing progressively from the crust to the core. Furthermore, due to the continuous sinking of heavy matter, its density increases— the average density of the crust is 2.8g/cm³, the average density of the mantle is 4.59g/cm³, and the average density of the core is 11g/cm³. Therefore, we can also conceive of the crust 'floating' atop the mantle—like a plank floating on water.
Because the crust is much thinner compared to the mantle and core, the average thickness of the crust is only about 17 km (33 km for continental crust and 10 km for oceanic crust). In contrast, the mantle's thickness extends to 2,850 km. Consequently, the movement of the mantle layer causes these thin solid rock layers to be torn and moved along with the mantle.
Segments of the Earth's crust are torn apart into plates, and as the Earth's crust moves, some collide while others separate. In general, it can be understood that the boundaries of these separating plates are very thin and fragile, the material of the underlying mantle can easily rupture the release of the rock layer and be pushed away from the surface—forming a long belt of volcanoes along plate boundaries.
And as the plates drift further apart, cooled magma after volcanic eruptions at plate boundaries forms a thin layer, namely the oceanic crust—because it is much thinner than the inner layer of the plate, it will sink lower than other areas, over time, water will accumulate to form oceans.
And as the plates drift further apart, cooled magma after volcanic eruptions at plate boundaries forms a thin layer, namely the oceanic crust—because it is much thinner than the inner layer of the plate, it will sink lower than other areas, over time, water will accumulate to form oceans.
In reality, oceans are formed in this manner, and the formation of oceans is closely related to the movement of plates.
Therefore, underwater volcanoes are essentially the result of plate movements, and most of them are plate boundary phenomena. Since World War II, as humans delve deeper into ocean exploration, we have discovered long chains of underwater volcanoes, mostly situated in the middle of oceans, known as mid-ocean ridges. They are the longest mountain ranges on Earth, with a total length of about 80,000 km.
Sources: Earthlymission; Nature; NASA
