The majority of Earth’s volcanism is concentrated at tectonic plate boundaries, where plates move away from one another to create mid-ocean ridges, or where one plate slides beneath another to form a subduction zone. However, an important and widespread class of volcanism occurs within plates, or across plate boundaries. These so-called intra-plate volcanic provinces, which include the most rapid and voluminous volcanic episodes recorded in Earth’s history, are often associated with mantle plumes, hot buoyant columns that rise from Earth’s core-mantle-boundary to its surface.Classic examples include the Hawaiian–Emperor chain in the Pacific, the Yellowstone–Snake river plain province on the North American continent, and the volcanic system centered upon Iceland. It is becoming increasingly evident, however, that several of Earth’s intra-plate volcanic provinces cannot be explained by the mantle plume hypothesis.

The Australian continent, which hosts one of the world’s largest Cenozoic intra-plate volcanic provinces, includes many examples that are better explained by alternative mechanisms, involving the interplay between mantle flow and the base of Earth’s heterogeneous lithosphere – its rigid outermost shell.

In this talk, I will provide an overview of the geodynamical mechanisms underpinning intra-plate volcanism on Earth, with a focus on Cenozoic volcanism in Eastern Australia. I will subsequently present results from a series of recent studies from the ANU where we integrate observational constraints from surface geology, geochronology, plate-motion reconstructions, geochemistry and seismology to ascertain mantle plume melting depths beneath the Cosgrove track in eastern Australia, Earth’s longest continental hotspot track.

Our analyses highlight a strong correlation between lithospheric thickness and magma composition along this track, placing the first observational constraints on the sub-continental melting depth of mantle plumes and providing direct evidence that lithospheric thickness has a dominant influence on the volume and chemical composition of plume-derived magmas. Our results also hint at complex interactions between mantle plumes and sub-lithospheric mantle flow, exemplified by the Newer Volcanics Province of Victoria and South Australia.

A/Prof Rhodri Davies