[English]In the history of the earth, from the end of Jurassic to the Paleogene, a wide range of crustal movement occurred in China, known as the Yanshan movement. Because the earth's crust during strong extrusion, fold uplift of the granite to become mountains。
Granite etymology is the Latin ‘granum’, it is a kind of magmatite, deep earth magma during ascent, and the future is reaching the surface of the cooling and the formation of rocks. In the cooling process, the occurrence of rock contraction, internal development of joints, these joints is the secret of magnificent mountain.
Granite is very hard and difficult to weathering and dissolve, so it is impossible to form karst caves. But the erosion of the rainwater along the fracture zone of granite will aggravate the erosion of the relatively weak surface of the rock.
Miyun mountains experienced a long geological history, formed in 190 million to 80 million years ago in the carbonate level, there are a variety of complex geological mark: various rocks formed in different stage of the Yanshan movement, the piedmont fault of the large, nearly vertical fault, huge dome mountain, and Yanshan movement the unique formation of anticline and monocline. And dense faults and joints cut rocks into chunks.
In Miyun area, as a result of the formation of the Yanshan mountain building geological landscape everywhere。
Tianmen cave is located at the top of Dongshan mountain in Miyun Tianmenshan, 660 meters above sea level. The peaks of the Tianmenshan scenic area are dominated by granite, but granite is a rock that is not eroded by water, so it does not form the Karst landform as the limestone region. So, how did the Tianmen cave come into being in the granite area?
We explore this process in different ways.
First，Influenced by the Yanshan movement in Beijing area, the granite in Miyun area was uplifted to the earth's surface and eroded by external forces such as sunlight, wind, rain, animals and plants. Beijing is located in the warm temperate semi humid monsoon climate. It is cold and dry in winter, hot and rainy in summer, large in daily temperature difference and annual temperature difference. Therefore, granite is prone to collapse because of strong weathering.
Second，according to geologists, since 120 million years ago, caused by the Yanshan movement of crustal uplift and strength greatly, resulting in a variety of structural layers, the peaks of the sub tectonic layer eroded, and later experienced a long-term continuous uplift and new tectonic layer superposition caused mountain rock loose. Rock joints develop continuously. Under the action of flowing water and wind, the joints and fissures expand constantly, rock collapses, while the top rocks are relatively stable and retain, and gradually form caves. Caves gradually expand, deepen, and finally through, forming a very rare "Tianmen cave" landscape.
Standing in the cave of Tianmen, carefully observing the rocks on both sides and top, it will be found that some rock joints and fissures developed very clearly, just like to collapse immediately. However, there has been no rock collapse in recent decades. As you can see, some of the granite stones come in different colors because of the different minerals that are formed during the process.
Every morning the sun from Tianmen injection, the seasons of the year in different locations, forming a spectacular "sunrise Tianmen" landscape; 2 days before and after the full moon, the moon like a silver mirror hanging in Tianmen cave, known as the "Tianmen month suspension", the two national rare spectacle.
Bus routes: Dongzhimen (980, 987) - Miyun Gulou station, ride to the direction of the car to a rock arch to 500 meters waterfall in Tianmenshan Scenic Area
By car: Sanyuanqiao Jingshunlu along - Beijing Road - Miyun reservoir to the westbound direction -- a waterfall 500 meters inside the archway Tianmenshan scenic area.
In order to register this geographical treasure, you have to go into the scenic spot and go on a mountain trip, but it's not difficult. There is an entrance fee of around $30 CNY to access this cache. You can first log, and then send the answers to the following questions to my mailbox, to ensure your log is true and effective. If you don't receive your email within two weeks, I'll delete your log.
1. Estimate the height and width of Tianmen cave？
2. According to the joint analysis of the rock on both sides and top of Tianmen cave, how long do you think the Tianmen cave will collapse?
3. Why do you think there are two colors in this rock?