Shulou(Shulou.com)11/24 Report--

Too long to look: in the depths of the earth, rocks melt from solid to liquid under the influence of local abnormal conditions such as decompression, heating and adding water, forming "Magma". When magma is ejected to the surface, it is called "Lava".

In human culture, volcanoes and magma often mean disaster. In ancient Rome, people associated volcanic eruptions with fire and Vulcvolcannus, the god of craftsmen. When the volcano erupted, it was Volgan's anger, which gave rise to the root of the word Volcano. His treasure is the largest volcano in the world, hence the name of Vulcano in southern Italy.

The name of this god can be traced back to the myths and legends of Crete. After 1500 BC, a volcano erupted 70 kilometers from Crete, which exceeded the eruption of Mount Tambora in 1815. This led to the rapid decline of Minoan civilization on Crete. In 1420 BC, the island of Crete was conquered by the backward Mycenais in the north, and the myths and legends about volcanoes may be inherited. The price is the destruction of a civilization.

Figure 1 | the magma from the Crete volcano is the melted rock. Where did it come from?

When I was a child, I thought our earth was full of hot magma, which was a thin layer in the earth's crust. In the weak parts of the earth's crust, the magma would erupt, forming volcanoes and bringing disaster.

Figure 2 | the description of the magma scene in the game, and is this really the case, Σ (o magma scene)?

In fact, this kind of imagination is more similar to what the surface of the early Earth's magmatic sea (magma ocean) looked like when it just cooled. In the early days of the formation of the earth, a large amount of interstellar matter constantly collided under the action of gravity, a large amount of gravitational potential energy was converted into heat, and the decay of some radioactive elements also produced a lot of heat. This heat completely melts almost all the rocks on the earth, forming a vast sea of magma [1].

Fig. 3 | imaginary picture of the magmatic sea however, after 4.5 billion years of cooling, the earth is no longer in such a hot state. On Earth today, most of the silicate rocks have been cooled into solids. According to the results of seismological research, the current internal structure of the earth is roughly crust, mantle and core [2].

Fig. 4 | Earth's circle structure according to seismological observations, only the outer core of the earth's core exists in liquid form, and the rest of the earth exists in solid form.

Of course, there is still a lot of heat in the depths of the earth. The temperature of the core is about 6000 ℃ [3]. Near the surface, for every 1 km increase in depth, the temperature will rise by 25-30 ℃. Near 660km, the temperature can reach about 1700 ℃.

Fig. 5 | Geothermal gradient line (map source network [4]) so the underground temperature is so high, why is the rock still not melting? The answer is stress.

We know that the "melting point" of matter is also affected by pressure. Take an example that we are familiar with: boil water. The lower the pressure, the easier it is to boil, while the higher the pressure, the less likely it is to boil. For most rocks, the higher the pressure, the higher the temperature required to start melting. Judging from the average temperature and pressure of the earth, under a certain pressure, the temperature is always lower than the temperature needed for the rock to begin to melt [5]. I use "starting melting temperature" here, not for the number of water words, but because the composition of rocks is very complex, they do not have a fixed "melting point", only the temperature at which they begin to melt, which we call "solidus". As can be seen from the diagram below, the earth's average ground hot wire falls to the left of the rock solid line, indicating that under normal circumstances, rock does not melt into magma.

Figure 6 | ground hot line and rock solid line, although for most cases, the above picture is true. But in some places, the geothermal wire or the solidus of the rock will change, so that part of the geothermal wire enters the right side of the solidus, and the rock melts into magma.

So how do you get a bowl of hot magma? Give you a six-word kit: decompression, heating, adding water. The typical areas that successfully melt rocks into magma under these conditions are mid-ocean ridge, mantle plume (ocean island) and subduction zone (island arc).

1. Decompression

Figure 7 | decompression causes the hot wire to move upward. The mid-ocean ridge is an undersea mountain range with similar characteristics across the four oceans, also known as the ocean ridge or central ridge, where the plate was born. On both sides of the mid-ocean ridge, the plates stretch to both sides, the pressure decreases, the geothermal line moves upward, and the rocks can be partially melted to form magma. These magmas erupt quietly at the bottom of the sea, forming a strange landscape of "black chimneys".

Figure 8 | Ocean ridge (top) and black chimney (bottom) 2. Heating up

Figure 9: warming causes the rock hot wire to move to the right. The mantle plume is a source of rising heat from the depths of the earth. The temperature here is higher than anywhere else, and the geothermal wire moves to the right, causing the rock to partially melt to form magma. This magma can form a series of volcanic island chains. One of the most famous is the Hawaiian volcano.

Figure 10 | schematic diagram of Hawaii volcanic island chain 3. water

Figure 11 | adding water changes the rock solidus subduction zone is the place where the plate dies, where one plate subducts to another plate, bringing a large amount of volatile and water-rich material carried on the plate into the deep, which will change the chemical composition of the rocks in this area, just as adding salt can accelerate the melting of snow and ice, the addition of volatile matter and water will change the solid line of the rock, making it easier for the rock to melt into magma. These magma upwelling will form island arc volcanoes. The more famous one is Mount Fuji in Japan. The recent eruption of Tonga volcano is caused by the subduction of the western Pacific plate under the Australia-India plate.

Figure 12 | schematic diagram of the formation of island arc volcanoes and magmas formed in different regions also have different properties. In general, volcanoes on mid-ocean ridges and islands tend to have lower SiO2 and volatile content, resulting in lower viscosity and milder eruptions. Such magma eruptions flow "quietly" in most cases, affecting only the area where the lava flows, and you can even enjoy the lava flow up close.

Fig. 13 | the magma formed after the lava flow and the subduction zone is different. They tend to have higher SiO2 content and volatile components. This kind of magma has high viscosity and is not easy to flow, usually "if it doesn't erupt, it will be amazing." Will send a large amount of material thousands of meters into the air. This Tonga volcano falls into this category.

Historically, such volcanic eruptions have tended to be extremely destructive. In 79 AD, in the aforementioned Italian region, the eruption of Mount Vesuvius directly buried Pompeii, the second largest city in Italy. At that time, the famous naturalist old Pliny was unfortunately poisoned by the poison gas emitted by the volcano while saving the victims. After recording the scene of the volcanic eruption in the distance, Pliny Jr., compared and studied the types of volcanic eruption. The Pliny type eruption is named after Pliny Jr.

In recent times, with the in-depth study of volcanoes, scientists have found that volcanoes are closely related to the rise and fall of human civilization. in addition to the well-known "year without summer", volcanoes also play an important role in the crisis of the 17th century. An article published in communications earth & environment in November 2021 pointed out that volcanic eruptions systematically affected the downfall of dynasties. Although this news is surprising, the article also points out that extinction is also directly related to social coping and resilience [6]. Therefore, the eruption of volcano can not be avoided, and all we can do is to improve the ability of human society to cope with disasters, which requires the scientific community to reach a consensus with the whole society to promote the ability to resist disasters. This is the humanistic value of science.

References

[1] Elkins-Tanton, L. T. Magma Oceans in the Inner Solar System. Annu Rev Earth Pl Sc 40,113-139 (2012).

[2] Brown, J. M., & Shankland, T. J. (1981). Thermodynamic parameters in the Earth as determined from seismic profiles. Geophysical Journal of the Royal Astronomical Society, 66 (3), 579,596.

[3] https://www.nsfc.gov.cn/publish/portal0/tab446/info66077.htm

[4] https://vibaike.com/125528/

[5] Wyllie, P. J. Solidus curves, mantle plumes, and magma generation beneath Hawaii. J Geophys Res Solid Earth 93, 4171-4181 (1988).

[6] Gao, Chaochao, et al. "Volcanic climate impacts can act as ultimate and proximate causes of Chinese dynastic collapse." Communications earth & environment 2.1 (2021): 1-11.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone). Author: Xiao Zicong, Xingsi, American Editor: Qijia cc

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