Straws sometimes reach lengths of over 6 metres. Stalactites grow from water dripping off a cave roof. A long, thin hollow tube, a straw, forms first, but its internal channel becomes blocked, forcing water to flow over its outer surface, depositing successive layers of calcite, resulting in a stalactite with typical pointed carrot shape.
Stalagmites grow from splashing drips beneath the stalactites. Stalagmite shapes tend to be 'stumpy' or rounded, compared to the more pointed shape of stalactites. Shawls grow from water trickling down sloping surfaces.
Layers of calcite build up into fluted curtains. Helictites seem to defy gravity. They grow when water slowly seeps through porous roof or wall surfaces, producing tiny droplets that precipitate calcite in the form of fine tubes with minute internal channels. The water flows through the tubes by capillary action, and changes in the direction of growth may be due to blockages in the internal channels.
As the calcite grows from the tip at various angles, interesting twisted shapes result. Flowstone forms from layers of calcite deposited when thin films of water flow down walls and across cave floors. Some resemble 'frozen waterfalls'. But most caves form in karst, a type of landscape made of limestone, dolomite, and gypsum rocks that slowly dissolve in the presence of water with a slightly acidic tinge. Rain mixes with carbon dioxide in the atmosphere as it falls to the ground and then picks up more of the gas as it seeps into the soil.
The combination is a weak acidic solution that dissolves calcite, the main mineral of karst rocks. Continue reading from National Geographic. Caves are formed by the dissolution of limestone. Rainwater picks up carbon dioxide from the air and as it percolates through the soil, which turns into a weak acid.
This slowly dissolves out the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves. The largest caves form where water flows onto the limestone from the adjacent impermeable Portishead Formation Old Red Sandstone , and Avon Group mudstones. The water sinks underground into holes known locally as 'swallets' or 'slockers'. The streams reappear at the base of the limestone outcrop at large springs, for example at Cheddar and Wookey Hole.
Over time, the water finds new lower routes leaving some caves high and dry. This type of sedimentary rock forms large, solid, rectangular blocks. Rainwater and snowmelt seeping into the ground absorb large amounts of carbon dioxide, which is a product of decaying organic matter. This carbonic acid continues to seep into the soil and through the limestone until it reaches the water table, which is the upper limit at which ground is saturated with water.
The acid moves fast enough through the limestone that it does not create channels as it makes its way to the water table. The carbonic acid eats away at the limestone at a level directly below the water table, eventually forming channels.
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