Scientists in China have discovered well-preserved fragments of the marine crust 2.5 billion years ago in Zunhua, Hebei Province, which is recently recognized by the international scientific community as the oldest marine crust found so far in the world. What is the significance of this discovery? What does this mean? The reporter interviewed Dr. Li, the project leader and the Department of Geology of Peking University.
The topic begins with plate tectonics. 19 12, 32-year-old german meteorologist Wei Gena miraculously proposed that continents were connected and drifted, and the theory of continental drift opened the way for the later theories of submarine expansion and plate tectonics.
"The theory of plate tectonics follows the method of" from the present to the past ",and through the study of the ongoing geological process, the causes of major geological events in geological history are inferred." Geological research is inseparable from the earth. Every year, he will stay in the wild for three or four months. Li has dark skin and a crew cut, and looks very clean.
According to the theory of plate tectonics, when the plates move relatively, the crust stretches to form rifts and small ocean basins, such as the Red Sea, and evolves into a wide ocean basin similar to the Atlantic Ocean. When two plates move relatively, the ocean basin between them will disappear, and a trench will be formed between the ocean and the continental lithosphere. The marine lithosphere will dive into the asthenosphere and melt, forming volcanoes and island arcs. Based on this, scientists predict that the Red Sea may become an ocean, while the Pacific Ocean is shrinking.
Under modern normal geological conditions, the marine lithosphere, including the oceanic crust, will generally subduct into the asthenosphere and die out because its topography is much lower than that of the mainland and its rock density is higher than that of the mainland. This is also an important reason why all the ancient marine lithosphere is difficult to preserve.
Along the suture lines between ancient plates in different periods, such as the Mediterranean, Alps, Central Asia, Qinghai-Tibet Plateau, Pacific Rim, Northwest Europe and eastern North America, most of the remaining ancient marine lithospheric fragments were formed 600 million years ago.
In the long geological history, the superposition and transformation of various geological processes may make the ancient plate suture "unrecognizable" and difficult to identify. Long-term weathering and denudation make it difficult to preserve the ancient marine lithosphere. Therefore, with the passage of time, it has become the most challenging scientific problem to identify the suture lines between ancient plates and find the fragments of ancient marine lithosphere.
At present, ocean drilling rarely exceeds 5000 meters deep and only penetrates the shallow part of the ocean crust. Rock samples obtained from seabed fishing are scattered and incomplete, and most of them come from the shallow surface of ocean crust. In order to gain an understanding of the material composition, geological structure and even geological evolution of the marine lithosphere, human beings rely more on indirect scientific means, especially the comprehensive research of multiple disciplines.
Ophiolite represents the fragments of ancient marine lithosphere, and this unique genetic relationship makes scientists in different eras pay great attention to ophiolite.
Li pointed out that the scientific significance of studying ancient ophiolite lies in that it is the golden key to understand the material composition, geological structure, heat flow change and even life evolution of ancient oceans; The appearance of ophiolite shows that there was a large-scale collision between plates, which made it rush to the mainland and be preserved. The lithosphere closed the ancient ocean and disappeared through horizontal movement.
The 1 100 million-year-old ophiolite is very rare. It is recognized that the oldest ophiolite is Finnish ophiolite and Canadian ophiolite 2 billion years ago. Although the ancient plate suture has been identified in North America and southern Africa for 2.6 to 2.7 billion years, no geological record of marine lithosphere has been found, which has become the regret of international geoscientists in the past 10 years.
To some extent, this regret was filled by scientists in China. The remaining oceanic crust discovered by scientists in China was 2.5 billion years ago, which is "500 million years older" than the oldest fragment of oceanic crust previously recognized in the world. Its significance is self-evident.
Experts described an ancient picture: 2.5 billion years ago, North China was still an ocean. For hundreds of millions of years, the large-scale crust lying quietly on the seabed rushed to the continental crust in eastern North China with the collision of plates, and the ancient ocean disappeared, and the North China plate was "spliced" to the continental plate.
"It is very lucky to be able to preserve the ocean crust 2.5 billion years ago so completely. These rock fragments record the' childhood' of the earth. " Li said, "The crust of the ancient ocean often died out in plate movements, and even the preserved ocean crust had to be lifted at least about 2 kilometers before it could' climb' to the ground." Compared with the foreign shell fragments found in the past, the northern China shell fragments have the characteristics of complete sequence, huge scale and the oldest age, which has aroused widespread concern in the international geoscience community.
It is reported that researchers will conduct a series of studies on minerals, rock composition and structure, isotopic chronology of the paleo-marine lithosphere while investigating the paleo-marine crust in North China in detail, and further contribute to the plate tectonic theory.