**5. Geoheritage significance of the eastern Australian Cosgrove volcanic chain**

The Cosgrove Volcano Chain provides a globally-unique system to explore an intra-plate migratory volcanic system and, additionally, in this context, with its globally-distinct suite of sedimentary basins that it has over time intruded through, it has global geoheritage significance. From north to south, in its younging direction, the Cosgrove Volcano Chain has a rich history and expression of volcanic activity with a range of lava types and eruptive volcanic rocks (dominantly basaltic lava [alkali olivine basalts, hawaiites, mugearites], but with occurrences of leucitite, trachyte, rhyolite, andesite, andesitic basalt, trachyandesite, and, where explosive and where expressed pyroclastically, tuff, breccia and agglomerate). Geomorphically, these volcanic eruptions are expressed as a range of primary volcanic landforms as well as eroded-residual geometry types (shield volcanoes, stratovolcanoes, domes, plugs, spires, *etc*.), and stratiform ash deposits, dykes, plugs, and sills. There are also valley fills, conic accumulations of tephra (*e.g.*, accretionary pyroclastic cones), ash sheets, and lava tubes.

In a sub-global context, the Cosgrove Volcano Chain presents a range of magma types, volcanic expressions, a history of interactions with the variable but regionally diagnostic lithosphere (*i.e.*, regionally-specific and lithologically-diagnostic sedimentary basins). It also presents a landscape-*cum*-climate response as the various volcanic and eruptive centres passed progressively through a climate gradient as Australia migrated from artic/boreal climates through to subtropical/tropical

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

**354**

**Figure 4.**

*are listed in Table 1.*

*Map of eastern Australia showing occurrence of the sedimentary basins and fold belts (modified from geoscience Australia https://www.ga.gov.au/\_\_data/assets/image/0020/13943/GA14654.gif) that the trackway of the Cosgrove volcanic chain will have intersected. Dominant lithologies that will yield xenoliths/xenocrysts*  climates with its attendant influence on erosion and weathering styles and transformation of volcanic rock and tephra to climate-specific landforms and climate specific soils. This variability in volcanic expression, in a north-to-south gradient influences content of xenoliths and xenocrysts and magma composition, as well as volcanic landscape types, and in climate-influenced weathering styles changing progressively over 33 million years – it thus provides a globally-unique natural laboratory as a window into Earth processes, and a teaching and research resource in perpetuity.

The key elements of the Cosgrove Volcano Chain of geoheritage significance are as follows:


To further the disciplines of geoheritage and geoconservation, Brocx & Semeniuk provided a Geoheritage Tool-kit to systematically compile an inventory of the full diversity at various scales of geological and geomorphological features in a given area, assess their levels of significance, and address whether geoheritage features are treated in isolation or as inter-related suites that should be conserved as an ensemble. Utilising the Geoheritage Tool-kit, many of the features listed 1–10 above individually would be evaluated as internationally to nationally significant, but specifically (1), (2), (3), and (8) are globally significant features of this volcanic chain. However, the cross-continent assemblage of volcanic features of the Cosgrove Volcano Chain should not be viewed as isolated separate geological phenomena spread across the north-to-south length of Australia but, from a geoheritage perspective, should be viewed as a single, integrated ensemble recording 1. a gradient of magma types, 2. the effect of a variable thickness of lithosphere, 3. the variety of volcanic landform expression, 4. the rock types intersected by the ascending plume, and 5. the changes in styles of erosion and weathering as the volcanoes passed through the various climate zones. However, while the Cosgrove Volcanic Chain is suggested as a global-significant mega-park, there is much planning, valorisation, community involvement, and government support required to achieve this (matters that are beyond the scope of this Chapter).

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*A Globally Significant Potential Megascale Geopark: The Eastern Australian Mantle Hotspot…*

**6. Discussion and conclusions: a globally significant mega-geopark** 

As reiterated above, Australia, having finally split from Antarctica some 33 million years ago, has been migrating northwards towards the Eurasian plate progressively passing on its eastern side over a mantle hotspot (magma plume) which intersected various, variable lithologic packages such as Phanerozoic sedimentary basins, fold belts and metamorphic terranes, and Precambrian rocks. With xenolith plucking, enclave plucking, and xenocryst incorporation, there had been compositional evolution of the magma plume. The volcanic chain, the individual volcanoes, and the lava fields that are spread latitudinally along 2000 km of eastern Australia present a globally-significant volcanic system that provides insights into Earth magma and crust interactions, into the variability of xenoliths and xenocrysts, into magma evolution dependent on setting, and into the story of the Earth. The Cosgrove Volcano Chain stands as an example of a globally-unique example of these

In summary, this Chapter has described the significance of a magma plume pulsing though a heterogeneous continental crust, with lithospheric heterogeneity giving rise to a mix in magma composition and a mix of xenoliths/xenocrysts. The movement of the Australian plate to northward gives clues (as a window) to subcontinental lithology via the enclosed xenoliths and xenocrysts and the influence of the thickness of crust as to whether there is volcanic expression of the magma plume at the Earth's surface [13]. And while they have been noted in previous studies and their occurrence is important, xenoliths and xenocrysts have not been a detailed focus of many studies, yet they are important as they provide clues to the sub-crustal influences and processes at the base of a magma plume. The phenomena of magma mixing, nature of xenoliths and xenocrysts, the influence of lithospheric lithologies in generating xenolith/xenocryst types and on magma contamination, and the relationship of volcanic expression to crust thickness are applicable to other

volcanic trackways here in eastern Australia and elsewhere globally.

Also, it is worth pointing out that a similarly large volcanic field has been recognised for its geoheritage values in Harrat Khaybar, Kingdom of Saudi Arabia [60, 61] which has local and global implications for geo-education, research, and

On a final note, we emphasise that we consider the Cosgrove Volcano Chain to be a megascale geological feature of global geoheritage significance and should be considered as a potential megascale geopark. In effect, it would be the volcanic equivalent of the latitudinally extensive Great Barrier Reef which is accepted as a

*DOI: http://dx.doi.org/10.5772/intechopen.97839*

**centred on the Cosgrove volcano chain**

processes and a potential megascale geopark.

reference site for global volcanism.

World Heritage Site.

*A Globally Significant Potential Megascale Geopark: The Eastern Australian Mantle Hotspot… DOI: http://dx.doi.org/10.5772/intechopen.97839*
