For people with deep-sea phobia, the trench is definitely a terrible place. In this seabed terrain, there is no light, the visibility is zero, the pressure brought by seawater is extremely great, and the environment with low temperature and low oxygen content makes only a few extreme creatures survive here.
Mariana Trench, located in the Pacific Ocean in the northeast of the Philippines, with a depth of 6,543.8+0,000 meters, is the deepest submarine crack detected on the earth at present. The total length is 2500 kilometers and the average width is 7 1 kilometer.
Mariana Trench
On June, 2020165438+1October 10, China's self-developed deep-sea submersible Endeavour successfully dived to the bottom of the Mariana Trench, and the accurate depth of this trench was 10909 meters.
This kind of submarine crack is widely distributed in all kinds of oceans, and its formation is closely related to plate movement. Generally speaking, trench belongs to plate subduction zone in geological structure, and its geological activities are frequent and intense. These trenches often become the export of magma eruption during volcanic movement and play an important role in shaping the seabed topography.
Nowadays, a "strange behavior" of the trench has attracted the attention of geologists. It turns out that the earth is sucking a lot of seawater into the earth through these submarine cracks. Through the in-depth study of marine geological scientists, the mystery of the deep earth is being revealed.
Mariana Trench is devouring seawater.
2018165438+10/4, Nature magazine published a paper from Washington University in St. Louis, USA. In the report, the research team from the university exposed the strange behavior of the Mariana Trench, which is a big crack in the sea floor.
Team member Professor Jennings Loew said:
Papers published by scientists at Washington University in St. Louis.
Professor Jennings' team started scientific research in Mariana Trench from 20 15. Their main job is to test the speed at which seawater enters the earth.
In order to obtain the relevant data of the earth's seawater swallowing speed, the researchers installed monitoring points at different depths in the Mariana Trench. These sonar instruments can capture the propagation speed of seismic waves in this big crack, and these values will become the original data for studying the speed of seawater entering the earth.
The researchers installed 19 seismic wave detectors in the Mariana Trench and 7 seismographs in the nearby islands.
After summarizing the data, scientists found that the propagation speed of seismic waves dropped significantly about 30 kilometers below the surface, which convinced scientists that seawater had been completely inhaled by the earth when it reached 30 kilometers below the surface, and it was stored in the earth.
Trench detection experiment
By accurately recording the propagation speed of seismic waves, scientists can calculate the distribution of geological structures on the deep seabed by combining the characteristics of different media on the seabed. Finally, the research team built a three-dimensional image of the deep strata in the Mariana Trench through mathematical modeling, and calculated the propagation speed of seismic waves in different strata.
Surprisingly, the evaluation results of scientific research directly subvert the cognition of the scientific research team. In the depth of 30 kilometers, there are still a lot of water resources, and the Mariana Trench is now devouring water at a rate of more than 3 billion tons per year!
Prior to this, scientists predicted that the Mariana Trench swallowed seawater at a rate of about 700 million tons per year, which was based on the movement process of the Oceania plate where the Mariana Trench was located. Now, scientists conservatively estimate that the seawater intake has reached 4.3 times of the previous results.
Earth plate movement
In addition, the Greek Ionian sea area is also absorbing a lot of seawater, which is located near Cayes Fallea Island in the Mediterranean Sea. Scientists say that the seawater in this sea area is being absorbed by a bottomless hole at the bottom of the water. According to the observation results, this bottomless pit can absorb more than 32,000 tons of seawater every day, which has been going on for nearly 1000 years.
Scientists have come to the conclusion through observation that there are still a large number of rock formations that can preserve water under the surface of 30 kilometers, which is one of the reasons for the slow propagation speed of seismic waves under the Mariana Trench. There are a lot of hydrated rocks 30 kilometers below the sea floor, which is also the way for the earth to store the inhaled seawater.
Hydrated rock
Hydration in a broad sense refers to the formation process of water molecule compounds. Water on the earth combines with molecules or ions in other substances through water molecules to form compounds. This chemical change is hydration. Today, this hydrated rock has penetrated into Mariana Trench and other places, tens to hundreds of kilometers under underground cracks.
Under the chemical action of hydration, mantle rocks deep underground will be transformed into compounds containing a large number of water molecules.
Scientists have discovered a kind of hydrated rock which is deformed by high pressure in the deep mantle. This compound, called "Linwood Stone", is considered to be the main component of the transition zone of deep strata in the earth, that is, it is a rock-forming ore widely distributed in the mantle.
Linmushi
Wood stone is a solid compound, which is composed of mineral elements and water molecules under hydration.
Geologists believe that strong external impact is the main reason for the formation of hydrated rocks such as Linwood stone. The high temperature and high pressure produced by plate movement deform the ore, and under the high-speed operation of water molecules, the ore is finally transformed into a new rock with high water molecule content.
Take wood stone as an example, it is the product of strong impact of magnesium and fayalite. This special ore is widely distributed in the strata below the surface 150 km to 600 km. Geologically, Linwood stone is located under the mantle and above the crust.
Its water molecule content is extremely high. The latest research results show that the recent Linwood stone contains about 2,500 water molecules per 6,543.8+0,000 ore crystal molecules, and the water content will increase greatly with the increase of pressure and temperature. In other words, the closer to the earth's crust, the exponential growth of water molecules in Linwood stone.
Rock distribution
In March, 20 14, the research team from Canada first measured the water molecule content of wood stone in different depths.
This result was finally published in the British magazine Nature. According to the report, researchers found a piece of Linwood stone between the upper mantle and the lower mantle of the earth for the first time, and there are still a lot of Linwood stones in this area.
This transitional zone is between 420 km and 640 km below the surface, mixed with a large number of hydrated rocks. The water content of this wood stone is about 1.5%. In addition, there are many kinds of hydrated rocks in this transitional layer between mantle and crust.
According to the prediction, there are 1 0.5 million water molecules in every110,000 ore crystal molecules in the Linwood stone 400 kilometers underground.
Mantle structure, composition and water reserves
By calculating the volume of these hydrated rock formations, scientists believe that the total amount of water resources below the surface 100 km is at least three times that calculated before.
According to the geological structure, the earth can be divided into three layers, namely crust, mantle and core.
It is very close to the mantle layer where seawater begins to transform into hydrated rocks. The mantle is the middle layer of the earth's crust and a solid rock layer wrapped by magma. It starts from 30 kilometers below the surface and has a thickness of 2,800 kilometers.
The bottom of the mantle is a high-frequency place for geological activities such as earthquakes and volcanic movements, and it is also a special channel for the exchange of high-temperature materials and surface materials in the earth.
The position of the mantle
The process of drinking water on the earth is consistent with the way trenches are formed. The bottom of the ocean plate is mostly sedimentary rocks of hundreds of millions of years, with high density and quality. When the ocean plate and the continental plate move with each other, the ocean plate with low geographical position will dive downward and melt into the deep mantle after collision. A trench was formed where the two plates collided.
Before the research team of Washington University in St. Louis measured the speed at which the Mariana Trench swallowed seawater, they were tracking a mysterious sound. In addition to seismic waves, there is noise in the depths of the seabed, which scientists preliminarily speculate is the sound produced by plate movement.
After nearly a year, I finally figured out the source of this low noise. It turned out to be the sound of seawater being brought into the mantle with the plate movement.
Plate movements form trenches, which promote the inflow of seawater.
During the subduction of the Pacific plate to the Eurasian plate, seawater played the role of "lubricant" and was brought into the deep mantle, forming an "aquifer" in the deep mantle.
The discovery of a large number of hydrates on the earth confirmed the existence of "hidden oceans", and some seawater remained in the earth through hydration and transformation.
Of course, the seawater swallowed inside the earth will not be sealed under the crust forever, but will reappear on the earth in various ways.
The subduction zone produced by plate collision has been transporting seawater to the interior of the earth, and it also makes seawater return through geological movement.
Seawater inflow in subduction zone
"Plate renewal activity" and "volcanic activity" are two main ways of material exchange between the inside and outside of the earth. In addition, the movement of submarine hot springs is closely related to hydrated rocks, and seawater that has not yet become hydrate will move back to the ocean through hot springs.
Submarine hot springs are the direct manifestation of crustal movement on the seabed, which mostly occur in the fragile or cracked crust on the seabed. 1979, bischoff, a scientist from the United States, saw the undersea hot springs for the first time at a depth of 2000 meters under the Pacific Ocean.
Its principle is the same as that of a volcanic fountain. The water stored under the submarine cracks is heated by the lava layer in the mantle, and finally returns to the ocean from the crust through the submarine outlet.
Seawater circulation flow diagram
In order to keep the total water balance, it is not enough to rely solely on the return of hot springs. The plate movement of the seabed makes the ocean plate and the continental plate collide constantly, and the submarine crust melts after diving into the mantle, renewing the magma inside the earth, melting more hydrated rocks and releasing more water.
In a sense, volcanic eruption is a supplement to water above the surface. Magma leaves the mantle through the volcano and releases other substances at the same time.
The data show that water vapor, carbon dioxide and sulfur dioxide account for a large proportion in volcanic gas, and the content of water vapor reaches more than 60%.
Volcanic eruption
Generally speaking, the earth has always maintained the dynamic balance of water through the circular movement of matter, and the water on the earth will not be reduced due to internal swallowing, nor will it be increased due to various backflow methods.
Large trenches such as Mariana Trench are still widely distributed on the earth. At present, the research results show that more than 20 large trenches are still accelerating the swallowing of seawater. These submarine cracks span the Atlantic Ocean and the Pacific Ocean, and the sediments and seawater inhaled every year far exceed human imagination.
Interplate sulcus
The ocean with an area of 7 1% has existed on the earth for billions of years. Now people's exploration of the seabed is still very low. For humans, more than 95% of the underwater world is unknown.
I believe that in the near future, with the progress of science and technology, human beings will explore the depths of the ocean more deeply and eventually have a more comprehensive understanding of the interior of the earth.