Fierce winds, flooding downpours and freezing snowstorms blast away at the earth's surface over time, weathering away the former stability of the land, and setting the stage for collapse.
Weather, geology, gravity and time all acting together can prove catastrophic when the earth finally gives way, unleashing a sudden, fatal sweep, whether by slide or descent.
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Landslides and sinkholes can suck down structures, vehicles and thousands of people instantly in a cataclysmic dive. Hundreds of thousands of deaths and billions of dollars in damages caused by these events each year. Adding to the danger, it is difficult to predict where or when these events will occur.
A landslide is when a slope fails, pulling the land, rock and mud with it downhill. According to the U.S. Geological Survey, a landslide is caused by gravity acting on an over-steepened slope.
A variety of contributing factors are often responsible for their occurrence, but precipitation often plays a major role in failure.
"Excess weight from accumulation of rain or snow... may stress weak slopes to failure," USGS reported. "Almost every landslide has multiple causes."
In addition to erosion from waterways, glaciers and waves, earthquakes and volcanic eruptions can cause landslides. Human activities can also contribute to landslides.
The Appalachians, areas in the Rockies and mountain ranges near the Pacific Coast are more prone to landslide than other parts of the county.
Sinkholes differ from landslides, but are also a direct result of precipitation and gravity acting upon the earth. Unlike landslides, sinkholes are even harder to predict since erosion beneath the surface is a direct cause of failure.
"Geologically, a sinkhole is a depression in the ground that has no natural external surface drainage," the USGS reports. "Basically this means that when it rains, all of the water stays inside the sinkhole and typically drains into the subsurface."
Areas with rocks that are soluble to groundwater are at the highest risk for sinkholes.
"Soluble rocks include salt beds and domes, gypsum, and limestone and other carbonate rock," according to the USGS report, adding Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania are the most susceptible states.
Water sinks down into the soil and begins to slowly dissolve the rock away, creating spaces and caverns underground. As this process occurs, support for the land above eventually gives way to gravity and collapses.
According to USGS, there are two ways to classify a sinkhole. Within just hours, cover-collapse sinkholes can occur suddenly and cause immense damage. Other sinkholes, named cover-subsidence sinkholes, tend to form slower and are a result of a gradual sinking, however they are less noticeable and can go undetected.
The severity and depth of the collapse can also vary greatly. Some sinkholes may only be a few feet in depth and width, but others can span hundreds of acres and fall to more than 100 feet in depth.
"(Sinkholes) can contaminate water resources and have been seen to swallow up swimming pools, parts of roadways, and even buildings," the USGS reports.
Similar to landslides, humans can also cause sinkholes.
"Many sinkholes form from human activity," the USGS reports. "Collapses can occur above old mines, from leaky faucets when sewers give way or due to groundwater pumping and construction."
While it is very hard to know if a sinkhole is looming under a structure, or property, the USGS recommends individuals assess their land as best they can.
"It is recommended that people constantly observe their property for things such as small holes in the ground or cracks formed in a structure's foundation" according to the report. "People can also check to see if they live in areas underlain by soluble rock, and they can do so by checking with county offices, local or state geological surveys, or the USGS."
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