A pressure ridge is the term for a protruding formation made of either ice or lava. When such a ridge forms in ice, it is usually the result of fluctuating temperatures on the surface of a lake or, in the case of an ocean, sea ice. Pressure ridges composed of lava generally form after a top layer of hardened lava is pushed up by molten lava below. These occurrences may be of interest to glaciologists, who study the natural processes of ice, or volcanologists, who study volcanoes and lava as well as the resulting formations.
When the surface of a body of water freezes in cold temperatures, whether on lake or the surface of the ocean, a pressure ridge can form. If the surface of the frozen water repeatedly melts and cools, this will cause contraction and expansion, and a crack can occur as a result. In extremely cold temperatures, it is possible that the ice will contract further, causing gaps between these cracks. These cracks will sometimes fill with water, which also freezes. If the temperature rises at a later time, this ice will expand and force itself out of the crack, causing a protruding ridge which can extend upwards for 10 feet (about 3 meters) or more.
Pressure ridges made of ice should not be confused with the occurrence of rubble ice. Rubble ice is a grouping of fragmented ice that covers a much larger expanse of a frozen surface. It is also much lower in height than a pressure ridge.
When lava is flowing, it is possible for it to form a pressure ridge, which may also be referred to as a tumulus. This can occur when the outermost surface of the lava begins to become hard and the still molten lava below begins to push against it from beneath. As a result, elevated mounds of hardened lava can appear, varying in height and length.
Small enclosures can occur as a result of a lava pressure ridge, and are usually identified as inflationary caves. This is not common but can occur when the lava hardens with a hollow inner section. The inside of these small caves usually become hollow when the outer surface hardens and the molten lava that caused the pressure ridge to occur finds a way to escape.