Burning Bush. Stephen J. Pyne

      Dust to Dust

      … Flood, fire, and cyclone in successive motionComplete the work the pioneers beganOf shifting all the soil into the ocean.

      —JAMES MCAULEY, “The True Discovery of Australia”

       GONDWANA GENESIS

      In the beginning—even 250 million years ago—all lands were one. Then they began to break apart. Great rifts appeared that opened and closed uncertainly over a period of tens of millions of years, and when the final tear ceased, Pangaea had become two continents. One, Laurasia, drifted north. The other, now known as Gondwana, moved south.

      Over the coming eons the continental masses migrated, fractured, grew, and reoriented themselves. They massed mountains to their flanks, fresh lands swelled out of submerged basins, volcanic eruptions piled new rock onto old crust. Laurasia broke cautiously into Old World and New, Eurasia and North America. Gondwana was more prolific. As it fissioned, it spawned continents, subcontinents, and microcontinents. New island arcs broke through Gondwana’s peripheral oceans as colossal plates of crust grated, subducted, melted, and bubbled upward into chains of fiery volcanoes. Greater Australia, which included Papua New Guinea and Tasmania, moved northeast into an empty Pacific. By 80–60 million years ago, with the opening of the Tasman Sea, Australia segregated from New Zealand, the last of its Gondwana affiliates. In its travels it had rotated nearly 90 degrees, and it had rafted northward at roughly six or seven centimeters per year until it rammed into the submerged Pacific margins of Asia and helped raise the Sunda arc, punctuated by the towering mountains of New Guinea.

      Australia’s geology preserved a Gondwana core, a continental craton, in the enormous plateau that sprawled over the western and central thirds of the continent. Its odyssey, however, had raised mountains along its eastern flanks—the Flinders Range, the Tasmanian Alps, and the Great Dividing Range, a plateau eroded into dramatic escarpments. Continental warping had raised basins and bulged crust into domed, long-wasted mountains in the center. Associated with the eastern uplift, too, were local outpourings of volcanics, largely Tertiary in age, occasionally sputtering into the Holocene.

      But overall the post-Gondwana history of Australia was one of geologic quiescence. Australian tectonics were muted; mountains were relatively low in elevation; the principal periods of vigorous activity were old and circumscribed. Australia became the most level of continents. Small rises in sea level resulted in massive incursions of ocean, while small recessions exposed vast regions of land with catastrophic suddenness. The geologic story of Australia required geologic time to record—the minuscule migration of the craton, the relentless leaching and the implacable erosion of its surface. The rejuvenation proposed by the first failed to match the degradation of the second.¹

      Where rainfall remained plentiful, deep weathering became the norm. Soils edged into acidity and deteriorated in those physical properties vital to groundwater. Laterization bound phosphorus to iron and aluminum complexes, chemically inaccessible to most organisms. Minerals once distributed more or less uniformly were reworked, concentrated, or removed from the scene. In some instances this led to localized lodes of gold, bauxite, and uranium, but overall the processes only encouraged the generic pauperization of soils, the loss of phosphorus and such critical trace elements as molybdenum, copper, cobalt, and boron. Without renewal by major tectonic forces, without intervention by some process like glaciation that could overturn or scrape away the surface debris or recharge lowlands with new minerals derived from mountain catchments, there was little chance to reverse the inexorable tread toward an entropy of emptiness. The process might be slowed, even locally defied, but it could not be resisted. In Western Australia, at Mount Narryer, weathering exposed zircon some 4.1–4.2 billion years old, incorporated into rocks formed 3.6 billion years ago. New landscapes emerged from the successive exfoliation, by erosion, of old landscapes. Australia’s biologic renaissance had to rise out of a geologic decadence.²

      THE BREAKUP OF GONDWANA freed Greater Australia to pursue a separate destiny. That burden fell to its biota: its curious flora and fauna would proclaim the unique character of the island continent. Old Australia’s geology and its biology coexisted in weird counterpoint. Where one degraded and removed, the other exploded into a biotic efflorescence, a proliferation of species unlike those found anywhere else on Earth. Instead of much devolving into less, a relatively small Gondwanic inheritance evolved into more. Some components of green Gondwana not only accepted the geologic legacy—soil depletion and geographic isolation—but turned them to advantage.³

      Its originating biota was one Old Australia shared with most of Gondwana. The ancestral forest was dominated by the gymnosperms—the southern conifers, the araucarias and podocarps—but just as Gondwana began to break up, and perhaps in partial response to the stimulus of that profound dislocation, the angiosperms—the flowering plants—proliferated. From a projected point of origin where Africa joined South America, the angiosperms spread throughout Gondwana. Their migration was selective, and the resulting geographies of the conifers and the flowering plants varied. Gondwana was too large, too unbroken a landmass for a single climate to characterize it everywhere, and as the giant continent fragmented, separate lines of biotic advance and exchange danced in slow choreography with geologic cratons. This meant that a single pan-Gondwanic biota did not exist everywhere in equal composition. What part of the ancestral forest the different continental fragments took with them depended on their relationship to Greater Gondwana and on the sequence of their fissioning, all played against the larger drama in which the angiosperms invaded and claimed greater proportions of the supercontinent.

      It appears that Old Australia embarked with a solid complement of the ancestral rainforest, a Gondwanic ark. Among the dominant conifers were the araucarias and the podocarps; among the angiosperms, the dominant genus was