A geoheritage treasure – a case study of the Hornsby Diatreme

Semeniuk T A1,2

1Western Sydney University, The College, Quakers Hill, Australia.  email: t.semeniuk@westernsydney.edu.au

The quarrying for bluestone at the Hornsby Quarry Site exposed a > 40 m-deep cross-section of a volcanic diatreme, showing a volcanic neck extruding through Sydney Basin sediments, complete with volcanic features visible at many scales, with post-volcanic features related to magma and gas extrusion at various depths, and with different host rocks. Globally, there are very few instances of such excellent exposure, revealing a full range of macroscale to microscale features in three dimensions. It is arguable that this exposure alone, makes it a site of international geoheritage significance.  Using the Geoheritage Tool-kit, applied at various scales shows that this diatreme is internationally to nationally significant.  For example, at the macroscale, there dish beds in all orientations of the quarry walls are visible, giving a three-dimensional picture of its structure, reflecting its volcanic accretion and later caldera collapse.  At the mesoscale, where breccia beds, bombs, and surge layers are visible, these show how the magma interacted with various host rocks and other post-volcanic processes that occurred prior to solidification.  Finally, at the microscale, lapilli (including accretionary lapilli), chilled margins and carbon-rich xenoliths are evident in hand specimen.  As such, this Quarry is a unique site worldwide. In fact, the Quarry, exposing the volcanic pipe of the Hornsby Diatreme, offers a snapshot in time of the Sydney Basin, preserving its volcanic and post-volcanic history on the quarry walls.


Trudi Semeniuk is a multidisciplinary scientist in the fields of general geology, metamorphic geology, geoheritage, wetland-, mangrove-, foraminiferal-, and tidal-flat sedimentology and ecology.  Her work experience is manifold ranging from fieldwork for VCSRG (a Research & Development Corporation), and a Research Officer for ANU, CSIRO, and Kings Park Botanical Gardens. More recently Trudi has focused on reviewing sites of geoheritage significance listed on, the now archived, Register of the National Estate (NSW) and work as a scientific editor.  Trudi was awarded a PhD from the Institute of Mineralogy and Petrography, ETH Zurich in 2003 in a study of Alpine mylonites, and completed a Postdoctoral Fellowship in 2004-2006 in aerosol chemistry at Arizona State University.  Trudi is active in Geoheritage, and is the co-convenor for the NSW Division for Geoheritage in the Geological Society of Australia.

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