The hypothesis stating that mountain building is caused by the contraction of the Earth. Conventionally, the wrinkles that form on a drying apple are used as an analogy for the formation of mountains on Earth. The hypothesis was prevalent in the 19th and early 20th centuries, prior to the discovery of radiogenic heating within the Earth. The Earth has probably reduced by some 5% in volume since its original formation, but mountains mainly result from plate tectonic motions.