The existence of an ~26–36 m.y. rhythm in interrelated global tectonism, sea-level oscillations, climate, and resulting sedimentation patterns during Phanerozoic time (the last 541 m.y.) has long been suspected. A similar underlying ~26.4–27.5 m.y. cycle was reported independently in episodes of extinctions of marine and non-marine species. Subsequent spectral analyses of individual geologic events of the last 260 m.y., including changes in seafloor spreading and subduction, times of hotspot initiation and intraplate volcanism, eruptions of Large Igneous Provinces (LIPs), tectonic events, sea-level fluctuations, oceanic anoxia, atmospheric carbon dioxide levels, and global climate have revealed evidence for the 26–36 m.y. cycle and the temporal association of events with an apparent overall periodicity of ~27.5 m.y. modulated by an ~8–9 m.y. cycle.

The proposed episodes of geologic activity and environmental and biotic change may result from cyclical internal Earth processes that affect changes in mantle convection, plate motions, intraplate stresses, and/or periodic pulses of mantle-plume activity. Recently, the ~30 m.y. cycle has been linked to Earth’s long-term orbital changes within the Solar System, and it may also affect tectonism and climate. I also note considerable evidence for a similar ~30 m.y. cycle in the ages of terrestrial impact craters, which suggests possible astronomical connections. The shared geologic cycle time, formally ranging from ~26 to 36 m.y. (depending partly on varying data sets, geologic timescales, and statistical techniques utilized) is close to the estimated interval (~32 ± 3 m.y.) between our cyclical crossings of the crowded mid-plane region of the Milky Way Galaxy. Here I outline a proposed astrophysical pacing for the apparent pulses of both impact cratering and rhythmic geological episodes.

Continental drift, rifting, and compression, earthquakes, volcanism, transgression and regression, and polar wander have undoubtedly a grandiose causal interconnection… Which, however, is cause and which is effect, only the future will reveal.

—Alfred Wegener, The Origin of Continents and Oceans, 1929

An outstanding consequence of the Hypothesis (Continental Drift) is the orderly and interrelated nature of all associated phenomena.

—Alex. Du Toit, Our Wandering Continents, 1947

While historical geologists since Lyell have given lip service to the principle of uniformitarianism, in which the Earth is viewed as having developed in a gradual and steady manner, a majority of stratigraphers and tectonicists, going back to Cuvier and d’Orbigny, including Chamberlin, Grabau, and Umbgrove, have been impressed with the segmentation of geologic history into episodes. Some of these changes appear to be rhythmic, and one of the rhythms represented lies at the 30–36-m.y. level.

—Alfred G. Fischer, Climate in Earth History: Studies in Geophysics, 1982