The concept of plate tectonics emerged from the observation of strong similarities between different continents on Earth’s surface. Alfred Wegener, known for his theory of continental drift, identified, for instance, common fossils in South America and West Africa in the early 20th century. By demonstrating that the shapes of these two continents could fit together, he hypothesized that they were once connected. Over time, evidence in favor of plate tectonics accumulated, and the model was eventually accepted in the 1960s.
A Complex Model Unique to Our Planet:
According to this model, Earth’s lithosphere (the rigid outer layer of our planet, consisting of the crust and a portion of the upper mantle) is divided into several distinct plates that move relative to each other by “sliding” on the asthenosphere (the ductile portion of the upper mantle capable of deformation without breaking). Currently, there are fifteen major tectonic plates, and their movements create geologically active zones at their boundaries, such as when two plates converge, diverge, or slide past each other. These movements give rise to various geological phenomena, including volcanism, earthquakes, the formation of mountain ranges, and oceanic trenches.
These tectonic movements facilitate chemical exchanges between Earth’s interior and its surface, leading to the regular renewal of surface rocks. According to commonly accepted hypotheses, Earth didn’t always have such active tectonics but likely had much more limited tectonic activity in its early stages. The emergence of life on Earth is believed to be closely tied to the shift to the current tectonic regime, stabilizing the planet’s surface temperature and creating a favorable environment for life.
Do Other Planets Exhibit Tectonic Activity?
While there are no other planets in our solar system known to exhibit tectonic activity similar to Earth’s, some scientists speculate that this isn’t always the case. Venus, our closest neighbor, is a case in point. Despite significant differences from Earth (Venus has surface temperatures hot enough to melt lead, and its atmospheric pressure is 90 times that of Earth), the two planets share strong similarities, such as mass, size, and density. According to widely accepted models, Venus is thought to have only a single, stagnant tectonic plate today, severely limiting exchanges between its interior and exterior.
However, a team of scientists from Brown University in the United States suggests that the current atmosphere of Venus doesn’t align with the idea of a single tectonic plate. By simulating the possible origins of Venus’s atmosphere, which is primarily composed of carbon dioxide and nitrogen and is 90 times denser than Earth’s, they propose that Venus may have had a more active tectonic history in the past. The high levels of nitrogen and carbon dioxide in Venus’s atmosphere would only be possible if Venus had experienced tectonic activity similar to early Earth, with a few plates moving relative to each other. These findings are published in the journal Nature. According to the researchers, this tectonic activity would have led to significant volcanic outgassing, resulting in Venus’s current atmosphere.
Challenging Planetary Evolution Models:
According to their hypothesis, Venus may have had tectonic plates for approximately one billion years, between -4.5 and -3.5 billion years ago. The processes that led to the cessation of plate tectonic movements, resulting in extreme temperatures and Venus’s thick atmosphere, remain unknown. This new conjecture about Venus’s geological history suggests that Venus and Earth actually had very similar early histories, with both planets experiencing tectonic activity in their early stages. However, Earth’s tectonic activity evolved to form numerous tectonic plates, whereas Venus’s tectonic activity gradually declined to its current single stagnant shell. According to scientists, early tectonic activity on Venus may have been conducive to the emergence of life as we know it, but these conditions no longer exist.
This new theory challenges our understanding of the evolution of terrestrial planets. Previous hypotheses regarded a planet’s tectonic activity as a binary issue: either a planet exhibits tectonic activity or it doesn’t. The evidence of past tectonic activity on Venus suggests that a planet can transition from one tectonic phase to another. Thus, if the emergence of life is linked to tectonic activity, some planets may intermittently offer conditions favorable for life rather than being continuously habitable. In light of these hypotheses, Earth’s ongoing tectonic activity may be an exception.
Without going back as far as the dynasty of the Angevin kings of Naples, we can say that the Italian wars involving France began with Charles VIII, himself a short-lived king of Naples for a few months in 1495. It was with Louis XII that the kingdom of France really began to gain a foothold in Italy. As early as 1499, the king took Milan, then Genoa, and attacked the Regno in 1501. But while he quickly managed to oust Ludovico the More, he had to rely on important princes in the peninsula, such as Caesar Borgia, as well as the Pope and the Aragonese. For several years, Louis XII managed to play off his alliances, relying on Florence and the Duchy of Milan and entering the battle against Venice. However, his influence began to bother the Italian princes, starting with Pope Julius II, who turned the alliances against France.