- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of Geology Exclusive of North America, Geological Society of America, Boulder, CO, United States
Cooling cracks form at high temperatures when a basic magma is either viscous or crystalline and hot. It is postulated that fine-grained margins form along the fractures by release of gas through the fracture network. This takes place (a) by raising the liquidus-solidus boundary due to release of volatiles, (b) by expansion of the released gas with consequent cooling. Granitic material introduced along the fractures post-dates and may recrystallize the 'chilled' margins.