Keurusselkä

Keurusselkä covers an ancient impact crater remnant,^[6] which was discovered in 2003 by amateur geologists.^[4] Shatter cones, horsetail-shaped formations in rocks specifically formed in meteor impacts, have been found in an 11.5 km (7.1 mi) wide area, but it is possible that the area containing shatter cones may be only the central uplift of the crater. Weak traces based on digital elevation data suggest possible ring structures from 10 km (6.2 mi) to as wide as about 30 km (19 mi) in diameter. This would make Keurusselkä the largest impact structure in Finland surpassing the Lappajärvi crater.^[3] In addition to the shatter cones, microscopic studies of samples from a breccia boulder have revealed shock metamorphic features, (planar fractures and planar deformation features) in quartz grains, which formed when the rock underwent extreme shock pressures of between 7–35 GPa (1,000,000–5,100,000 psi).^[4]

• http://en.wikipedia.org/wiki/Keurusselkä