The geological history of Fiji is complex owing to its proximity to the Australian–Pacific plate boundary.
The oldest rocks found in western Viti Levu are island-arc volcanics 56-34 million years old (Late Eocene). Subsequent uplift caused deposition of shallow-water limestones around these island arcs. About 28 million years ago tectonic movements shifted causing more volcanic islands and sedimentary basins to form (now exposed in southern Viti Levu, the Yasawa, and Mamanuca Islands). The first significant land mass (Viti Levu) developed 16 to 5 million years ago (Middle to Late Miocene) from plutonic lava intrusions and uplift. Vanua Levu was formed about 7 million years ago while Taveuni formed less than 2.5 m million years ago as a basaltic volcanic island with over 150 vents along its 40 km length. At least 2.7 km2 of magma has been erupted from over 100 events on Taveuni during the Holocene (since 11,000 years ago).
Given this complex history, it is easiest to view the connection between soil formation and parent material in terms of mineral composition:
- Acidic parent materials with high silica low ferromagnesium mineralogy are resistant and weather to soils with low clay content and low fertility.
- Basic parent materials are lower in silica but relatively higher in ferromagnesium minerals and weather more easily resulting in soils that tend to be clayey and fertile.
Where parent material has been “reworked” the composition of the resulting sedimentary parent material and the soils that form on them depend on the original rock from which they formed and the amount of weathering they have undergone. Sandstones tend to be silicious (acid) while limestones and calcareous tuffs and marls are more intermediate or basic.
The range of soil forming parent materials in Fiji includes:
- Siliceous – Acid volcanic rocks (rhyolite, granite and quartz-rich tuffs); siltstones, sandstones and silicified sedimentary rocks.
- Intermediate – older chrystalline rocks; younger coral limestones; marls and calcareous mudstones; andesites.
- Basic – basalts, gabbros and other basic volcanic rocks; younger andesitic and basaltic volcanic ash.
- Colluvial (hillslope/gravity transport), alluvial (water transport), aolian (wind transport) and littoral (coastal) materials may be derived from any of the above.
The two major groups of parent materials are tuffaceous sedimentary rocks that occupy about 36% of the landscape and basic volcanic rocks that occupy 48% of the landscape.
