Cooling rate
A cooling rate is how quickly a rock or metal lost heat, often measured in degrees per million years for meteorites.
A cooling rate tells us how fast a material cooled through a range of temperatures. Because crystal sizes, mineral zoning and exsolution all depend on cooling speed, scientists can work backwards from these features to estimate the rate.
The range in meteorites is enormous. Chondrules cooled at perhaps tens to thousands of degrees per hour, recorded in the shapes of their crystals. Glassy impact melts cooled in seconds. Iron meteorites cooled extremely slowly, at roughly 1 to several thousand degrees per million years, as asteroid cores lost heat through their rocky mantles.
Cooling rates also indicate size and depth. A slowly cooled iron points to a core buried under a thick insulating mantle, while a faster rate implies a smaller body or one whose mantle was stripped away. The wide spread of cooling rates in IVA irons led to the idea that their parent body was broken up and reassembled while still hot.
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