Because the earth is a rocky planet, with the existing science and technology, it is not possible to accurately present and judge the internal structure and movement mode, even though our human aerospace technology has developed rapidly, and many deep space probes can be launched to explore the universe, but we still know very little about the earth under our feet.
In the early 20th century, seismologists used seismic wave detection to indirectly infer that the Earth's interior is a layered structure, that is, composed of a solid core, a molten mantle, and a thin rocky crust based on the time difference between pressure waves (P waves) and shear waves (S waves) during transmission. In the 30s of the 20th century, scientists used this method to estimate that the diameter of the Earth's inner core is about 2440 kilometers, and the material density in it is very large.
We know that about 4.5 billion years ago, the Earth was born in a violent collision of many similar asteroids. The dense iron-nickel elements of the asteroid sink into the molten primordial Earth's core, and the lighter silicate rocks gradually rise to form the mantle. With temperatures of thousands of degrees and pressures of millions of atmospheres, the inner core remains molten, even though the Earth's mantle and crust later cool and harden. Above the inner core, a layer of highly dense liquid matter is encased, and as the earth rotates, this part of the molten metal "shell" located outside the inner core is constantly stirred, so that a magnetic field is formed.
A recent study showed that 560 million years ago, the strength of the Earth's magnetic field declined rapidly, to only about 10% of the current magnetic field strength. However, studies have found that in the tens of millions of years since, the Earth's magnetic field miraculously and rapidly recovered, reaching almost 70% of the current level, and this period was the period of the Cambrian explosion of life on Earth, when complex multicellular life suddenly emerged in large numbers.
The rapid recovery of the Earth's magnetic field has effectively intercepted countless high-energy radiation from the universe. It can be said that without the core "evolution" of that period, there would have been no explosion of life at that time, and there would have been no colorful biological world on Earth now, and the emergence of human ancestors would have to be postponed indefinitely.
Many previous studies have pointed to a conclusion, that is, the earth's inner core regenerated the earth's magnetic field during an extraordinary period in the "evolution" process, so what causes the earth's inner core to change? This is a difficult problem that has long puzzled scientists. As some scientists say: the core of the earth is a planet among planets, it may have its own topography, its own rotation period, its own special structure, it is under the feet of us human beings, and we still don't understand many problems.
More than 30 years ago, a research team from Columbia University in the United States used historical data from seismic wave monitoring to discover that the P wave of seismic waves accelerates slightly when it passes through the earth's core. As a result, the rotation speed of the Earth's inner core is about 1 degree more than that of other structures in the Earth, and the transmission path of seismic waves gradually realigns with the north-south axis of the Earth's inner core, and it is a cycle every 400 years. In other words, the inner core is inside the earth, and every 400 years it undergoes a "self-renewal".
Later, seismologists at the University of Southern California in the United States took a different approach and chose the nuclear explosion experiment carried out underground as the epicenter to observe the "seismic waves" passing through the earth's core. In the time between two U.S. test experiments, the rotation of the Earth's inner core was observed to be somewhat slower than the rest of the Earth's interior, about 0.05° slower per year. However, by the time of the underground nuclear explosion tests in the former Soviet Union, the Earth's inner core somehow changed direction and accelerated.
Some research teams were very excited about this phenomenon, after all, the previous view that the Earth's inner core rotates in sync with the mantle, and they used the relevant data analysis to show that the Earth's rotation gradually increased from the 1970s and then stabilized after 2000. The preliminary view is that the gravitational drag of the mantle may drive the movement of the inner core, but there will be a lag process.
For decades, some astronomers have also used equipment to closely track small changes in the rotation of the Earth's surface, which "shakes" in its rotation rate each year relative to the "reference position" of the quasar. When the impact of annual "jitter" on events such as hurricanes and earthquakes is excluded, a fluctuation pattern that basically follows a six-year cycle is found. As for the reason for the formation of this six-year cycle, some scientists believe that it is closely related to the movement of the earth's core, that is, the result of the mutual gravitational interaction between the earth's core and the mantle, and this mutual tension makes the earth's core oscillate back and forth between "super-rotation" and "sub-rotation". This may also be the root cause of the Earth's magnetic poles, which also drift or even flip from time to time.
Up to now, scientists have not yet figured out the real situation and movement laws of the earth's interior, especially the innermost "core", which is a tightly wrapped mystery. Through the overall rotation of the earth and the use of seismic wave detection, we have discovered that it has a regular characteristics similar to "pulsation", and this "pulsation" from slow to fast, from fast to slow, seems to follow a certain inherent rhythm, just like a living body is "breathing", but we still do not have a comprehensive understanding of the "evolution" of this core, what is the fundamental cause and mechanism of the drive.
We just know that it is precisely because of this "evolution" of the core that the conditions for the existence of life on earth are guaranteed and the prosperity of life on earth is promoted. If it weren't for the scorching core 5,000 kilometers underfoot, none of us would be living on Earth as we are now, and perhaps there would be no human beings at all.