FAYETTEVILLE, Ark. – A certain type of rock that records the changing distance between the Earth and the Moon will be the topic of a talk at 3:30 p.m. Monday, Feb. 23, in the Space Center Theater in the Old Museum Building on the University of Arkansas campus.

Professor Allen Archer of Kansas State University will speak about a type of rock that can show the distance of the Earth to the Moon over the course of history. This rock, known as a rhythmite, has a common type called varve, a pair of thin sedimentary layers formed annually by seasonal climatic changes. Usually found in glacial lake deposits, varves consist of a coarse-grained, light-colored summer deposit and a finer-grained, dark-colored winter deposit formed when fine sediment settles out from the water under the ice cover. Varves, and the pollen they contain, are useful for interpreting recent climatic history.

Archer has documented tidal rhythmites in the modern and Paleozoic period. Earth’s orbital dynamics, including its elliptical shape, axial tilt, and the movement of the rotational axis, may be revealed in rhythmites through the ages, but it is apparent that daily, 12-hour and fortnightly sedimentary successions are also preserved. Recognizing these rhythms in the rocks may lead to better recognition of early Earth-Moon distances and past climate changes.

Tidal effects between the Earth and Moon result in the Moon moving away from us at a constant rate. During the Apollo missions, astronauts placed mirrors on the moon, allowing us to use lasers to determine the exact distance between the Earth and Moon. This method has shown us that the Moon is receding at a rate of 3.8 centimeters per year.