The African continent is geologically significant, divided into tectonic plates at the heart of Ethiopia. Recent advancements in geophysics have shed light on the mechanisms of tectonic plate separation. Research has revealed that the continents started to fragment due to cracks and misalignments in the crust and upper mantle, known as the lithosphere. As magma ascends through these fissures, it reaches the Earth’s surface, leading to volcanic formations. While scientists understand the association between volcanoes and continental rifts, the rate of their formation remains unclear, complicating volcanic hazard assessments in rift zones.
A research team, led by Kevin Wong, aimed to resolve this question by analyzing the minerals formed during magma cooling, specifically olivine. They examined 72 olivine crystals, each measuring between 1 and 4 millimeters (0.04 to 0.16 inches), sourced from the Bok and Jiwei volcanoes located within Africa’s Main Ethiopian Rift (MER). Their findings indicate that the lithosphere in this area maintains a thickness of approximately 35-40 kilometers (21-25 miles). This substantial lithosphere hints at the MER’s position as an intermediate stage in continental separation, offering a unique perspective on the transition from tectonic deformation to magmatic fractures.
Wong and his team chose to analyze olivine due to its role as one of the earliest minerals to crystallize from magma, continuing to grow as the magma cools and rises. As the magma ascends, its composition alters, creating distinct chemical “zones” within the growing crystals, akin to the rings of a tree. Fluctuations in temperature and magma composition cause various elements, like magnesium and iron, to diffuse at differing rates, allowing scientists to model these chemical zones and their boundaries to determine the speed of magma ascent from the upper mantle to the surface.
The researchers utilized high-magnification imaging and chemical analysis through an electronic microprobe to study olivine crystals from the MER volcanic field. They meticulously mapped 10 to 15 points within each crystal, spaced approximately 5 to 15 microns (about 10% the thickness of a human hair) across a cross-section that spanned the growth zone from the inner core to the outer edge.
Their analysis identified two distinct categories of olivine crystals. The first displayed a normal zone crystal characterized by a magnesium-rich inner core, while the second was identified as a reverse zone crystal with a magnesium-poor core. The research indicated that freshly formed magma deep within the Earth is richer in magnesium than iron. The boundary between the magnesium-rich and magnesium-poor zones can become indistinct due to diffusion. This gradual smoothing of crystal boundaries over time operates at a known rate, allowing researchers to extract valuable information regarding the rate of magma ascent and its interaction with adjacent rock.
Employing a numerical model, the team estimated the diffusion rates of magnesium and iron across these chemical boundaries, factoring in varying temperatures and magma compositions. By comparing thousands of simulated diffusion profiles with actual olivine diffusion profiles, the researchers estimated that the crystals ascended from deep within the Earth and mixed with the surrounding magma over an average of 40 days during the Bok eruption and 17 days during the Jiwei eruption. They further cross-validated these estimates using a growth-diffusion model, which better mirrors the natural behavior of crystals, yielding an approximate rise time of 27 days while accurately replicating the observed crystal band pattern.
Based on their findings, the researchers concluded that intermediate-stage rifting events occur at surprisingly short time scales. On average, magma can ascend up to 40 kilometers (25 miles) from deep within the Earth to the surface in about one month. This timeline aligns more closely with human time frames than geological ones. They suggested that such rapid ascent is likely due to a sophisticated magmatic plumbing system embedded within the lithosphere, which develops before substantial thinning occurs. However, the researchers cautioned that these findings imply that the ascension timescale could vary significantly, impacting disaster mitigation and prediction efforts.
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Source: sciworthy.com
