Exploring the Earth's Deep Interior and Understanding China's Underground

"Going up to the sky, going down to the earth, and going down to the sea" are the three great feats of human exploration of nature. Among them, "going down to the earth" is no less difficult than "going up to the sky". Deep earth exploration has become one of the last frontiers of earth science development.

Before the 20th century, there was a theory of cold contraction in geology textbooks. It said that the earth we live on is like a shriveled apple. The evaporation of internal water has caused wrinkles on the surface. The earth contracts by cooling, forming folded mountains on its surface. Like the heliocentric theory, this theory is vivid, intuitive, and "easy to understand", and therefore has a wide influence.

However, in the early 20th century, with the accumulation of various geological data and the progress of research results, solid theories of ocean and continent, such as the theory of cold contraction and the theory of eternal ocean, began to become less credible. Geophysicists such as Pratt and Dutton, based on the results of gravity measurements, inferred that the composition of ocean and continent materials was different: from the thickest orogenic belt to the thinnest oceanic areas, the thickness of the crust can vary by more than 70 kilometers. They believed that there must be an interface that makes the pressure of rock on the mantle equal everywhere, and based on this, they established the crust equilibrium theory.

Image source: Tuchong Creative

Like other sciences, geology is also advancing step by step in the face of new evidence, so as to get closer to the original truth. Based on the work of Pratt and others, German meteorologist Wegener proposed the theory of continental drift in 1912: he noticed that the eastern coastline of the current South American continent and the western coastline of the African continent fit together very well. If it is assumed that there was originally a huge "supercontinent", and later this "supercontinent" was torn apart like paper and scattered in the middle of the ocean, the fit of the east and west continental coastlines can perfectly explain that they were once together. Wegener cited a lot of evidence from many different fields such as geology, paleontology and paleoclimatology. In "The Origin of Continents and Seas", he cited a kind of earthworm that lives widely from Japan to Spain, but only exists in the eastern United States west of the Atlantic Ocean. It is impossible for earthworms to cross the ocean, which means that the two regions may have been connected and belonged to the same land.

Nevertheless, as a meteorologist, Wegener's view still attracted a lot of opposition from geologists. In addition, the continental drift theory did have defects in explaining the mechanism of continental drift: it was difficult for people to imagine how the rigid granite that constituted the continental crust could drift on the rigid basalt that constituted the oceanic crust.

Drilling proves drift theory victorious

It was not until the 1960s that the geological theory of "plate tectonics" developed rapidly, which revived the theory of continental drift. This new theory expanded the original drift part to a layer of bedrock with a thickness of about 100 kilometers, including the crust and upper mantle. The lithosphere floats on the mantle asthenosphere due to its low density and can drift; due to uneven temperature, there is a density difference in the asthenosphere. To fill this difference, the asthenosphere flows slowly. More than a dozen plates of different sizes distributed on the surface of the earth are moving slowly under the impetus of the mantle asthenosphere, and the continents are like "icebergs" that have emerged from the water, drifting with the plates. Almost at the same time, a theory called "ocean floor spreading theory" began to prevail. This theory believes that the mid-ocean ridges of the oceans are forming new seabeds and are constantly expanding to both sides. These two have developed the "continental drift theory" very well.

After the mid-20th century, more and more evidence gave the "drift theory" new life. But its authenticity still depends on "seeing is believing". So scientists began to look for more intuitive evidence. In June 1966, the United States began to implement a long-term drilling plan aimed at revealing the mysteries of the upper crust of the ocean floor - the "Deep Sea Drilling Program" (DSDP). This plan used the "Glomar Challenger" scientific drilling ship to drill 1,092 shallow boreholes in the world's oceans. In the end, the plan completed drilling in the world's oceans and obtained a total length of more than 9,500 meters of cores.

In 1968, DSDP arranged 39 boreholes to drill cores across the 64,000-kilometer-long Mid-Atlantic Ridge. Drilling sampling tests showed that the age of the oceanic crust is inversely proportional to the distance from the mid-ocean ridge. As the distance from the axis of the mid-ocean ridge increases, the age of the oceanic crust increases regularly. Based on the vertical distance from the borehole to the axis of the mid-ocean ridge and the age of the seafloor at this point, the expansion rate of this point can be calculated: since 80 million years ago, the seafloor in the borehole has been expanding uniformly at a rate of two centimeters per year.

The oldest sediments obtained by deep-sea drilling in the world's oceans are no more than 170 million years old. Compared with the 3.8 billion years of the oldest known continental rocks, the ocean floor crust is quite young, indicating that the ocean floor is indeed constantly growing and renewing. By analyzing the cores taken out, scientists also found that the thickness of the ocean floor sediments also shows a regular distribution along the ridge axis: at the top of the young ridge, the thickness of the sediment layer is thinner; while on both sides, as the age of the ocean floor gets older, the sediment layer gradually thickens.

In addition, DSDP verifies the relative horizontal movement between oceanic and continental plates. DSDP sampling in the Indian Ocean shows that the Indian Ocean plate drifted northward at a speed of 10 cm per year in the late Cretaceous and late Eocene (36.5 to 65 million years ago). After the collision between the Indian plate and the Eurasian plate, the northward movement speed of the Indian Ocean plate slowed to 5 cm per year. In the past 65 million years, it has moved 4,500 kilometers northward. When the plates collided, the Indian Ocean plate inserted under the Eurasian plate and pushed strongly to the north. The powerful force caused the front edge of the Eurasian plate to rise into mountains, and the Himalayas began to form 25 million years ago. Deep-sea drilling not only verifies the seafloor spreading theory and plate tectonics, but also depicts the dynamic balance of matter on a global scale. This conclusion is enough to comfort Wegener, the "father of continental drift theory", who died in Greenland during a scientific expedition in 1930.

What is the scientific significance of drilling?

The world's first scientific drilling program was the United States' "Moho Drilling Program," which began in the 1950s with the goal of drilling through the Moho surface (the interface between the crust and the mantle) to achieve a major breakthrough in geoscience research. However, soon after the start of the program, due to technical and financial problems, the program ended hastily after drilling only 315 meters into the seabed. In June 1966, the United States began to implement the famous DSDP, which avenged the previous failure and brought waves of major breakthroughs to earth science.

Compared with ocean drilling, continental scientific drilling started a little later. In the 1970s, the former Soviet Union and other countries began to conduct continental scientific drilling. The Kola Superdeep Borehole on the Kola Peninsula is 12,262 meters deep. To date, this is still the deepest borehole in the world and has become the world's first deep laboratory (observatory).

The data from the Kola Superdrill cast doubt on previously widely recognized theories, such as the crustal equilibrium theory. Seismic data from the Kola Peninsula showed that the Conrad surface in the area should be located 7 kilometers below the surface, and the basalt layer should be encountered there. But the actual results were unexpected. The borehole has been passing through a single metamorphic granite gneiss layer and amphibolite layer, and the Conrad surface has never been seen. In addition to a series of geological exploration results, the Kola Superdrill has also brought many considerable resources. For example, when the drilling depth exceeds 9,500 meters, the gold content of the obtained stratum core is as high as 80 grams per ton, while at that time, it was rare to find mineral layers exceeding 10 grams per ton on the earth's surface.

Scientific drilling can help us understand geological structures and find resources in a larger area. It is the most direct and effective way to obtain information about the interior of the earth. In my country, continental scientific drilling began in the 1990s. In 1996, my country joined the International Commission on Continental Scientific Drilling (ICDP) and carried out scientific drilling for different purposes in the Songliao Basin, the East China Sea in Jiangsu, Lijiang in Yunnan, and Qinghai Lake.

It's hard to get into the ground

Deep scientific drilling aims to find out the underground geological conditions. It is different from oil and gas drilling, which achieves its goal by finding the oil and gas layer. The drilling holes are generally selected in crystalline rock areas where the crust is as exposed as possible, and complete core samples are required.

It is not easy to bring up rocks from several thousand meters deep intact. Since crystalline rocks are relatively hard, the temperature of the drill bit will rise by about 1°C for every 100 meters it goes underground. The drill bit must work under high temperature (150-400°C) and high pressure (100-150MPa). The material of the drill bit is usually diamond. In addition, the drill bit and detection equipment that reach the bottom of the hole must be resistant to high temperature and high pressure. On the surface, the drilling tools that look hard to us are like noodles when they are several thousand meters underground. In order to control this "noodle", the staff must measure the depth and inclination of the well every time they drill. After drilling every three or four meters, the drill rod needs to be lifted up to take out the core. Before drilling again, it is necessary to measure whether the borehole is deviated. If it is deviated, it is necessary to find a way to correct it. In 2018, the 10,000-meter drilling rig "Crust No. 1" independently developed by my country reached a depth of 7,018 meters, which is a new breakthrough in the independent research and development of major equipment technology in my country. "Crust No. 1" makes our country the third country in the world to have the special equipment and related technologies to implement the 10,000-meter continental drilling plan.

Development of Deep Earth Exploration in my country

In 2008, my country launched a four-year project called "Deep Exploration Technology and Experimental Research" (Sinoprobe2008-2012), which is the largest deep earth exploration program in China's history. It has made a lot of scientific research progress, such as completing about 6,000 km of deep seismic reflection profiles, independently developing key instruments and equipment, and establishing a detection technology system that adapts to the complex lithosphere and crust of the Chinese mainland, making my country one of the world's major deep exploration powers. Based on the "Deep Exploration Technology and Experimental Research" project, the former Ministry of Land and Resources established the China Deep Earth Exploration Center (Sinoprobe Center) in 2015, which became the base for China's deep earth exploration and research. In recent years, the center has undertaken a number of deep exploration and survey projects.

On May 30, 2023, my country's first 10,000-meter deep scientific exploration well was officially drilled in the heart of the Taklimakan Desert in the Tarim Basin in Xinjiang. The well was named "Deep Earth Takko 1 Well" with an estimated drilling depth of 11,100 meters. "Deep Earth Takko 1 Well" is expected to penetrate more than 10 formations and will become our "telescope" for exploring the deep earth.

my country started its deep earth exploration work relatively late, but in recent years, the intensity of deep earth exploration work has been continuously increasing, and it has gradually formed its own advantages in the fields of geochemistry, magnetotelluric observation, and scientific drilling.

The article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.

Author: Zhu Xinna, popular science author, independent book planner, and excellent reading promoter in Beijing

Reviewer: Zhang Yuxiu, Associate Professor of Geology, School of Earth and Planetary Sciences, Chinese Academy of Sciences