In addition to impact and volcanic craters, nearly 40% of Mercury’s surface is covered by plains. They are divided into two types: intercrater and smooth. Intercrater Plains are some of the oldest geological features visible on Mercury's surface. They are located between large ancient craters. They are believed to have formed early in Mercury’s history as a result of lava flows from many fissures. Much of the surface has experienced high levels of volcanic activity. Some depressions are potential sites for explosive volcanic vents. And the double ring basins have a fairly smooth interior surface; this may be the result of so-called effusive volcanism. They are thought to be the result of previous episodes of volcanic activity. Smooth Plains are younger geological features. They almost always fill the bottom of large impact basins. A striking example of this type of plain is the Caloris Basin and some other large craters. It is believed that the vast majority of smooth plains are of volcanic origin. Characteristic tectonic features on the surface of Mercury are the presence of a global network of so-called bladed ledges, or scarps. They are asymmetrical, cliff-like structures, with a length of hundreds, and in some places over a thousand kilometers. They were first discovered by the “Mariner 10” spacecraft. Their formation is the result of the global compression of Mercury due to the gradual cooling of its mantle and large core. As it cooled, the volume of the planet's inner layers gradually decreased, and this caused its radius to decrease. The harder outer crust was forced to adapt to such a decrease, which led to its crumpling and cracking. According to estimates, the total decrease in the radius of Mercury over the time of its geological history was up to 7 km. The most convincing evidence that Mercury has undergone a significant reduction in size during its evolution is the formation of a global network of scarps. An important discovery of the “MESSENGER” space mission was the discovery of very small, bladed fault ledges; they are only a few tens of meters high and a few kilometers long. They turned out to be very young, since they have a relatively fresh morphology. This increases the possibility that some of these features may be active even today.
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