那位能帖一下钟乳石(Stalactite)形成的背景资料

25号听力考这个了

关于钟乳石的形成
石灰岩洞中的钟乳石、石笋是怎样形成的

　　桂林是世界闻名的风景区，山奇水秀，风景美丽，因此自古就有“桂林山水甲天下”之称。桂林除山水美之外，还有更具特色的石灰岩洞，七星岩是比较有代表性的溶洞。洞内到处悬垂着美丽的钟乳石，有的像累累的果实，有的像盛开的花朵。与之相对应的石笋拔地而起，一个个像春天从地面下“冒”出来的竹笋。
　　那么这些奇丽的钟乳石和石笋是怎样形成的呢？
　　原来“盛产”钟乳石和石笋的溶洞都是石灰岩构成的。洞顶有很多的裂隙，每一处裂隙里都有水滴渗透出来。每当水分蒸发掉后，那里就会留下一些石灰质的沉淀，日积月累，天长日久洞顶上的石灰质愈积愈多，终于形成了乳头。以后，乳头外面又包起一层石灰质，以至越垂越长，就形成了姿态万千的钟乳石。
　　石笋其实就是钟乳石的孪生兄弟。当洞顶上的水滴落下来时，石灰质也在地面上沉积起来，就这样石笋对着钟乳石向上长起来，若是说钟乳石是“兄长”，那石笋就是“孪生弟弟”了。
　　而石笋底盘大，本身比较稳定不易折断，所以它比钟乳石的生长速度还要快，还要粗壮呢。

Stalactite, pronounced stuh LAK tyt or STAL uhk tyt, is a beautiful stone formation found in some limestone caves. Stalactites hang from the walls or roofs of the caves. Most look like large icicles, but some resemble draperies or straws with a hole through their center.

Most stalactites form when ground water rich in carbon dioxide dissolves the mineral calcite (calcium carbonate) from limestone directly above the cave. As the water drips into the cave, it loses carbon dioxide to the cave atmosphere and leaves behind minute quantities of calcite. The calcite accumulates very slowly, forming stalactites. In many cases, this process occurs over thousands of years.

Formations that build up from the floor of a cave are called stalagmites (see STALAGMITE). In the United States, excellent examples of stalactites and stalagmites exist in Carlsbad Caverns in New Mexico, Luray Caverns in Virginia, Mammoth Cave in Kentucky, Cumberland Caverns in Tennessee, and Blanchard Springs Caverns in Arkansas.

Stalactite and Stalagmite, mineral formation frequently found in caves. A stalactite is an icicle-shaped mass of accumulated calcium carbonate hanging from the roof or sides of a limestone cavern; it is formed by mineral precipitation from groundwater that has seeped, very slowly, through the roof of the cavern. Water circulating through the ground above a cavern picks up calcium bicarbonate as it percolates through the limestone. When the water trickles through the ground to the roof of a cavern, it tends to cling to the roof in droplets; as the droplets lose some of their water and carbon dioxide, calcium carbonate is precipitated and forms a deposit. As more water seeps through the roof, the precipitation of calcium carbonate continues and the deposits grow in length and width, forming stalactites; these are often of enormous size and beautiful shape.

Pure calcium carbonate is white, but stalactites are often variously colored by impurities in the mineral. Some of the water seepage also falls to the floor and there accumulates into calcium carbonate masses resembling inverted stalactites. These deposits, growing upward from the floor, are called stalagmites. Stalactites and stalagmites often occur in pairs and meet, forming columns that may grow as large as 1.5 m (5 ft) in diameter.