Precarious boulder

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Precarious boulder refers to a large rock or boulder resting either on the ground or on another rock in an apparently unstable position. This can arise by a few mechanisms:

• Erosion may have removed the basal rock or sediment, leaving the boulder exposed
• The rock may have been transported from an ice sheet and was left exposed when the ice retreated—this is more accurately called a glacial erratic
• The rock may have been transported and placed at the location from a catastrophic geologic process, such as:
  • Through a rockfall
  • From a volcanic eruption, which results in lava bombs
  • From a cataclysmic flood, such as a glacial lake outburst flood

Yeager Rock, a 400 ton erratic on the Waterville Plateau, Washington, USA.

A precarious boulder, which was first transported from a rockfall, then exposed on a column of sediment from erosion.

A lava bomb from the Capelinhos Volcano, Faial Island, Azores.

See also

• Glacial erratic
• Hoodoo (geology)

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