Saturday, 9 November 2019

Beach in Finland covered by 'ice eggs'.

Thousands of ice eggs have been observed on a beach in Finland this week. The 'eggs', roughly spherical balls of ice, were found on Hailuoto Island in the Gulf of Bothnia, which separates Finland and Sweden, on Sunday 3 November 2019, covering an area of beach about 30 m in extent.

Thousands of ice eggs on a beach on Hailuoto Island. Risto Mattila/Instagram.

Ice eggs form under slightly specific conditions, when the temperature is about -1°C, to cold for ice to melt and to warm freeze seawater (which, because of its salt content, does not freeze till the temperature drops to about -4°C), and the wind is high enough to make the water choppy. Under these circumstances any ice fragments in the water will constantly know against one-another, slowly becoming rounded and small chips are knocked away.

Thousands of ice eggs on a beach on Hailuoto Island. Risto Mattila/Instagram.

This rounding by friction is a common phenomenon in geology and physical geography, and is responsible for the shapes of a variety of structures. The most obvious of these are the rounded pebbles also seen of beaches. Typically made of chert or similar hard rock, these pebbles are rounded by the same process as the ice eggs, and while it takes a lot longer to form rock into such shapes, the time is available because the rock does not melt seasonally.

Rounded pebbles on a beach in Kent, southern England. Joe Bauwens.

However, this rounding is not restricted to large pieces of rock such as pebbles, but also effects smaller grains of sand. Sand is essentially derived from larger chunks of rock by erosion or fracturing, and tend to be angular in shape when formed. However, over time grains become rounded as they are knocked together by the action of wind or water. Geologists call this process 'maturing' with grains of sand being more 'mature' if they are more rounded, and sandstone being more 'mature' if it contains more rounded sand grains.

Mature sand grains. Brookfield (2011).

With calcareous sands in warm seas this process can take on an extra level of complexity, as under such circumstances, in enclosed waters such as lagoons, the water can become concentrated in calcium carbonate which then precipitates out onto available surfaces, with other pieces of limestone being ideal. If this happens to be grains of sand being rolled in shallow waters, then more limestone will accumulate on the outside, causing them to grow, forming structures called 'oolites'. Such oolites can form sediments on tropical beaches, and when eventually berried, fuse together to form a type of rock called oolitic limestone.

Oolites on a beach on Joulter's Cay, The Bahamas. Mark Wilson/Department of Earth Sciences/College of Wooster/Wikimedia Commons.

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