Subsection 1.1: The Role of Subduction in Geological History

Geologists have become skilled in piecing together clues, often using analogies or examining large-scale processes under a microscope. In Papua New Guinea, the narrative of plate tectonics is encapsulated in grains of sand. Professor Suzanne Baldwin and her team from Syracuse University recently shared their findings on fluid and mineral inclusions within garnet sands. These garnet beaches, while rare, offer a striking hue to the coastline, while the azure waters of the Pacific lap at its shores.

The Australian and Pacific plates, along with several smaller plates, are engaged in a collision beneath the ocean. This creates a subduction zone where denser oceanic rock is forced beneath lighter continental rock. The dynamics here are intricate, as both major and minor faults work to release the energy produced by these colossal colliding forces. As one researcher aptly noted, “Papua New Guinea hosts every conceivable type of plate boundary,” making it a fascinating yet challenging site for geological studies.

As the subducting plate is driven downward, the rocks undergo transformation due to extreme temperature and pressure. These changes leave behind evidence in the form of new mineral combinations and in the structure of the rocks themselves. This metamorphic process occurs globally wherever oceanic crust collides with continental land, whether along South America's coast or at New Zealand's Macquarie triple junction. What distinguishes Papua New Guinea is the subsequent process.

Subsection 1.2: Metamorphosis and Its Geological Impact

Typically, metamorphosis occurs deep within the Earth, beyond the reach of geologists. The transformation of rocks is a slow process, spanning tens of millions of years, which results in a disconnect between the changes at depth and their surface manifestations. Meanwhile, continents may drift significantly. In Papua New Guinea, however, this transformation occurs relatively swiftly in geological terms—within less than ten million years—allowing geologists to compare metamorphic rocks with their sedimentary counterparts within the same rock cycle and geographical setting. The changes did not vanish; they remained present.

As metamorphic rocks ascend to the surface, they face erosion from wind, rain, and ocean waves. Boulders break down into pebbles, eventually yielding sand. Minerals that were once fused by chemical interactions and shared histories are now separated by mechanical forces. Some minerals are too fragile to withstand this erosion, while others, like garnet, prove resilient, allowing them to be sorted by the ocean's currents. Minerals of similar densities are concentrated into placer deposits, turning what might have been a minor component in the original rock into abundant beach sand.

Section 1.3: Erosion and the Revelation of Earth's History

Despite the extensive weathering and transportation faced by these garnets, the grains remain largely intact. Encased within are tiny crystals of rare minerals along with fluid and gas inclusions—these are the telltale signs that Professor Baldwin and her team are investigating. One such rare mineral is coesite, a form of silicon dioxide akin to the quartz found in gardens. Coesite's rarity is attributed to its atomic structure, which can only form under extreme temperatures and pressures. This mineral serves as an indicator, revealing that the garnet and its source rock reached depths of approximately 75 miles before resurfacing.

Equally telling is what was absent: diamonds. The depth of 75 miles is nearly sufficient for diamond formation, yet the researchers discovered graphite, which can transform into diamonds under high temperatures and pressures. These rocks could not have descended much deeper without surpassing that threshold.

The geological scene unfolded as follows: after reaching their maximum depth, the rocks returned to the surface via lithospheric deformation. Erosive forces then broke down the rock, forming the garnet sands that Baldwin and her team studied. The cycle of rock formation and erosion was completed.