Natural Domes

A dome is a curved formation or structure. It is shaped like half of a sphere. Imagine cutting an orange in half, and placing it cut-side-down on a table. This is the shape of a dome, although most domes in nature are not     1     rounded.

Some natural domes develop when magma(岩浆)from deep within the Earth pushes up    2     rock layers. This type of geologic dome can     3     as magma intrudes between two layers of sedimentary rock. The magma creates a dome or triangle shape as it pushes the other layers apart. The hardened magma that forms this type of dome is called “laccolith”.

Lava domes form as magma     4     atop volcanic vents(火山口). In the Chaitn Volcano in Chile, an ongoing eruption that began in 2008 is forming a lava dome next to another one that filled the crater when the volcano erupted 9, 400 years ago.

Another kind of natural dome is shaped    5     by weathering and erosion, which cause curved sheets of rock to separate from a large rock mass. The Cima Dome in the MojaveDesert, in the U. S. state of California, was once a mountain. Over time, weathering and erosion     6     away the mountain and smoothed it into a rounded dome.

Salt domes result when rock salt rises through overlying sedimentary rock. Salt    7     up as ancient seas dry up over time. Eventually, sediments form over this layer of salt. Salt is less dense than most other rocks, and it slowly moves upward toward the surface, forming a dome-shaped hill. One of the most famous salt domes is Avery Island, in the U. S. state of Louisiana. Avery Island is a salt dome     8     by low-lying swamps(沼泽地)of the Mississippi River delta. Eventually, these salt domes can break through the surface rock layers. Actually, salt domes are important sites for industry. Salt is a valuable     9     used in the production of plastics, pesticides, preservatives, and fire extinguishing equipment. Moreover, Salt domes     10     oil between layers of rock so that oil wells can drill into it and extract oil.