Isua, in southwestern Greenland, contains some of Earth’s most ancient rock formations. Studying these rocks provides insights into the planet’s earliest history.
The Ancient Isua Greenstone Belt
The Isua Greenstone Belt forms part of the Itsaq Gneiss Complex. This complex is composed of felsic orthogneisses, aged between 3.85 and 3.6 billion years. The greenstone belt itself dates back approximately 3.7 to 3.8 billion years, making it one of Earth’s oldest and largest exposures of supracrustal rocks.
The belt stretches about 35 kilometers (22 miles) in length and consists of metamorphosed mafic volcanic and sedimentary rocks. These include basaltic pillow lavas, volcanic breccias, chert, and banded iron formations. The presence of pillow lavas indicates that these volcanic eruptions occurred underwater.
The rocks within the Isua Greenstone Belt have undergone at least two major metamorphic episodes. The first phase, between 3.7 and 3.6 billion years ago, involved amphibolite-facies conditions, transforming the original rock compositions. A later metamorphic event between 2.9 and 2.6 billion years ago further altered these ancient rocks.
Evidence of Early Life
The Isua Greenstone Belt is a focal point for researchers seeking evidence of early life. One significant finding involves potential fossilized microbial structures, specifically stromatolites, discovered in dolomites. These structures exhibit wavy and dome-shaped forms, suggesting their formation by ancient microbial colonies approximately 3.7 billion years ago.
If these structures are indeed stromatolites, they would predate the previously oldest known stromatolites from the Dresser Formation in Western Australia by about 220 million years. The complexity of these Isua structures implies that life was already relatively developed by this early point in Earth’s history, suggesting that life may have originated over 4 billion years ago. However, some geologists propose that these structures might be the result of geological deformation and alteration of the original rock rather than biological activity, a subject of ongoing scientific discussion.
Further evidence comes from geochemical analysis, particularly the isotopic signatures of carbon found in the belt’s carbon-rich layers. Scientists have hypothesized that isotopically light carbon, with a specific δ13C signature, in graphite inclusions within these metasedimentary rocks is indicative of biological activity. Microorganisms tend to preferentially utilize lighter isotopes of elements for their metabolic processes, leading to distinct isotopic fractionations in their remains. While these findings are compelling, the interpretation of some carbon isotope data has been reassessed, with some graphite now understood to have formed through non-biological, metasomatic reactions from hot fluids interacting with crustal rocks.
Insights into Early Earth
The Isua Greenstone Belt offers insights into the conditions and geological processes of the early Earth. The rock formations provide clues about the planet’s initial crust, which was likely predominantly mafic, with little felsic crust present during the belt’s formation. The presence of pillow lavas confirms that surface water bodies, such as oceans, existed during the Eoarchean, with temperatures likely less than 100°C.
The Isua rocks also contribute to the understanding of early plate tectonics. Some geological features, such as duplex structures and “oceanic plate stratigraphy” observed in the belt, suggest it may represent the oldest known accretionary complex, formed in an intraoceanic environment similar to parts of the modern Pacific Ocean. This implies that modern-style plate tectonics, involving the subduction of oceanic crust, may have been operating as early as 3.8 to 3.7 billion years ago. However, other theories propose that the belt’s formation could be explained by different, non-plate tectonic models, such as vertical plate tectonics, adding to the ongoing scientific debate about the specific style of tectonics on the early Earth.