Four billion years ago, parts of the Earth’s basaltic crustal lid melted to form areas of continental granitic crust; Alan Hastie and his team will investigate the mechanisms behind this process through the study of analogous granites in Iceland
Our Earth is unique among known planets in having liquid-water oceans, and continents made of granitic crust that cover about forty percent of its surface. The remaining sixty percent of the planet’s surface is covered by oceanic crust, which is thinner (7 km), composed of basaltic rock and is continually renewed. Plate-tectonic processes constantly re-shape the Earth’s surface today by forming new oceanic crust at mid-ocean ridges and destroying it by subduction (where one tectonic plate slides beneath another). Oceanic crust is hydrated by interaction with sea water and this water is then released as the crust is dragged down into the Earth’s interior, to trigger subduction-related magmatism as, for example, along the Pacific Ring of Fire. The resulting magmas solidify as new continental crust.
The oldest continental crust is four billion years old and, although plate tectonics explains crust formation on the present-day Earth, the tectonic processes operating on the early Earth are very poorly understood. Unlike the modern planet, the surface of the early Earth was covered with a world-wide 30 km thick basaltic crustal lid. Parts of this lid subsequently melted and formed the oldest continental landmasses. However, the mechanism(s) for how the crustal lid melted to form continental crust four billion years ago is unknown. The lack of knowledge about the early crust-forming processes means that we do not know how the oldest continents began to form, how the Earth’s surface differentiated and why the organisms that colonised the early land surface were able to evolve. Furthermore, although other rocky planets in our solar system lack life-supporting oceans, landmasses and plate tectonics, some of them have both basaltic and granitic crust. Nevertheless, scientists also do not understand how granite-like rocks on other planetary surfaces formed.
An analogue of the early Earth and other rocky planets (Venus and Mars) is found in Iceland where unstudied granites are encased in thick basaltic crust. We will investigate these Icelandic granites to understand the mechanism that generated the Earth’s oldest continents and similar granitic rocks on Mars and Venus. Iceland is our chosen area because it is uniquely composed of approximately 30 km thick basaltic crust (like the early basaltic crustal lid) that contains relatively unstudied granitic rocks away from any subduction zones. Investigation of the mechanism(s) that generated these Icelandic granites will allow us to better understand how non-subduction processes could have formed the Earth’s oldest continents and granitic crust on other rock planets.