HOME

TheInfoList



OR:

Dropstones are isolated fragments of rock found within finer-grained water-deposited
sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...
s or pyroclastic beds. They range in size from small pebbles to boulders. The critical distinguishing feature is that there is evidence that they were not transported by normal water currents, but rather dropped in vertically through the air or water column. Such deposition can occur i.e. during a volcanic eruption.


Background

When deposited into fine layered mud, such evidence includes an impact depression beneath the dropstone, and indication that the mud has been squeezed up around the edges of the falling rock. Subsequent deposits of mud drape over the dropstone and its crater.
Glacial A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
dropstones, involving rocks falling out of
iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...
s, are one of the most common types of dropstone preserved in the geological record, particularly when deposited in low-energy deep sea or lake environments. Dropstones differ from erratics found in glacial
till image:Geschiebemergel.JPG, Closeup of glacial till. Note that the larger grains (pebbles and gravel) in the till are completely surrounded by the matrix of finer material (silt and sand), and this characteristic, known as ''matrix support'', is d ...
in that they are deposited in a
lake A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much large ...
or marine environments. Dropstones may also be deposited by a variety of non-glacial means.


Origin of dropstones

There are five natural mechanisms that produce dropstones:


Glaciers

As glaciers move across a surface, they pluck rocks from it, and incorporate them into their mass. At the coast, fragments of glacier detach and float away as icebergs, which are often transported (
ice rafted Ice rafting is the transport of various materials by ice. Various objects deposited on ice may eventually become embedded in the ice. When the ice melts after a certain amount of drifting, these objects are deposited onto the bottom of the wate ...
) many miles into the ocean, where they melt and deposit their load. When entrained rocks sink to the ocean floor, they can be incorporated into the oceanic sediments, which are typically fine grained. Glacially deposited rock differing from the size and type of rock native to the area in which it rests is called a
glacial erratic A glacial erratic is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by glacial ice, often over distances of hundred ...
.


Volcanoes

Whilst dropstones were once thought to be diagnostic of glaciers, it has since been realised that they can also be formed via volcanic eruptions.
Volcanic bombs A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they l ...
are large fragments of rock, projected many miles by the force of an eruption. If these land in fine sediments or pumice-forming ash, they can form dropstones. Dropstones originating in this fashion are relatively rare in the geological record as most will land on high ground, which has a poor preservation potential as it is in an erosive environment. However, a large blast may spread bombs far enough for them to end up in a marine setting of fine enough sediment for them to be recognized, or may land in or be buried by
pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...
s and surges.


Turbidity currents

Dropstones can also be deposited through the action of strong ocean-floor
turbidity current A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. T ...
s. Boulders the size of a human have been found in relatively recent finely
laminated Lamination is the technique/process of manufacturing a material in multiple layers, so that the composite material achieves improved strength, stability, sound insulation, appearance, or other properties from the use of the differing materia ...
sediments near
Jamaica Jamaica (; ) is an island country situated in the Caribbean Sea. Spanning in area, it is the third-largest island of the Greater Antilles and the Caribbean (after Cuba and Hispaniola). Jamaica lies about south of Cuba, and west of His ...
, which has been a warm
tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
island entirely devoid of glaciers since it came into existence. Whilst turbidity currents are cited as the origin of the boulders, they are not found in association with deposits formed by them.


Biological rafts

Stones can also be transported large distances by becoming bound in a raft of floating plant material or in the roots of floating trees. When such a raft disintegrates due to waterlogging and sinking of its constituents, the transported rocks would also sink. Dropstones formed in this manner are typically associated with organic matter, especially logs – the fossilised remains of the raft that caused its transport. Vertebrates, including ancient dinosaurs, may also act as dropstone agents by ingesting
gastrolith A gastrolith, also called a stomach stone or gizzard stone, is a rock held inside a gastrointestinal tract. Gastroliths in some species are retained in the muscular gizzard and used to grind food in animals lacking suitable grinding teeth. In othe ...
s and depositing them on land or within standing bodies of water by regurgitation or when the organism dies. These rock clasts, usually siliceous, are anomalous compared to the eventual surrounding rocks, and are much more easily preserved than the bones and other organic material of the biological organism that deposited them. There are over 10 known examples of dinosaur remains actually with associated "dropstones", sometimes with as many as 200 clasts. And many other rounded clasts in some of the dinosaur era sediments are debated among scholars as to their origin - gastroliths(i.e.,biological dropstones) or ancient, strange river sediments.


Meteorites

Meteorites landing in marine depositional environments are a fifth category of dropstone. A number of meteorites have been found in Sweden's Thorsberg quarry, where they sank to the bottom of a shallow sea and were entombed in limestone 470 million years ago.


See also

*
Glacial erratic A glacial erratic is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by glacial ice, often over distances of hundred ...
*
Ice rafting Ice rafting is the transport of various materials by ice. Various objects deposited on ice may eventually become embedded in the ice. When the ice melts after a certain amount of drifting, these objects are deposited onto the bottom of the wate ...


References


External links

{{Authority control Glaciology Glacial deposits Sediments