Origin of color variations of thin, nano-sized layers of volcanic cinder from the Sierra Negra Volcano of the Galapagos Islands

Volcanic cinder, also known as scoria, is an extrusive igneous rock that forms when gas-rich magmas of basaltic or andesitic composition cool quickly. It is typically dark in color, ranging from black to red depending on its chemical composition. Sometimes fresh cinder samples show a variety of shiny metallic colors on its surface ranging from blue to gold to silver. The origin of these colors has remained unknown up to now. Cinder samples from an eruptive event occurred in October 2005 have been collected in the surroundings of the Sierra Negra volcano in the Galápagos Islands. The samples’ crystallographic structure, chemical composition, and surface morphology have been analyzed using X-Ray diffractometry (XRD), energy dispersive X-Ray spectroscopy (EDS) and a field gun emission scanning electron microscopy (SEM), respectively. Based on an extensive physical and chemical analysis, we were able to demonstrate that these colors are due to a light interference phenomenon. These results have a great potential to be used for a wide variety of purposes such as determining the temperature and composition of magma and evaluating volcanic samples for planetary studies.

Circulación y uso de obsidianas en la costa norte del golfo San Jorge (provincia de Chubut, Argentina) durante el Holoceno tardío: primeros resultados

En este trabajo se presentan los resultados del análisis físico-químico (ED-XRF) y tecnológico de 21 artefactos de obsidiana procedentes de la costa norte del golfo San Jorge. Se identificaron cinco grupos químicos correspondientes a cuatro fuentes secundarias distantes entre 435 y 280 km del área. Tres fuentes son de localización conocida: Pampa del Asador (PDA1) al Sur, y Sacanana (S1) y Telsen/Sierra Negra (grupos T/SN1 y T/SN2), al Norte. La cuarta, Meseta de Somuncurá (MS), aún desconocida, estaría también ubicada al Norte. Dos dataciones radiocarbónicas —1950 14C AP y 1640 14C AP— señalan el uso de esta roca desde por lo menos el Holoceno tardío. En los muestreos líticos las obsidianas muestran baja frecuencia, en contraste con las abundantes materias primas locales (sílices, xilópalos y calcedonias), de excelente calidad para la talla. Predominan los artefactos pequeños, la mitad presenta reserva de corteza y las lascas superan levemente a las láminas. Los grupos tipológicos están representados por desechos, instrumentos y un núcleo. Se interpreta la explotación de rodados chicos y la ejecución in situ de todas las etapas de la reducción lítica. Se comparan y discuten estos resultados con los de otros muestreos de obsidianas de Patagonia central argentina.

Observing a Gal�pagos Volcano from Buildup to Eruption

Insights from a 13-year monitoring program of Sierra Negra—one of the many volcanoes that dot the Gal�pagos Islands—shed light on the volcanic evolution of basaltic eruption.

Caldera resurgence during the 2018 eruption of Sierra Negra volcano, Galápagos Islands

Recent large basaltic eruptions began after only minor surface uplift and seismicity, and resulted in caldera subsidence. In contrast, some eruptions at Galápagos Island volcanoes are preceded by prolonged, large amplitude uplift and elevated seismicity. These systems also display long-term intra-caldera uplift, or resurgence. However, a scarcity of observations has obscured the mechanisms underpinning such behaviour. Here we combine a unique multiparametric dataset to show how the 2018 eruption of Sierra Negra contributed to caldera resurgence. Magma supply to a shallow reservoir drove 6.5 m of pre-eruptive uplift and seismicity over thirteen years, including an Mw5.4 earthquake that triggered the eruption. Although co-eruptive magma withdrawal resulted in 8.5 m of subsidence, net uplift of the inner-caldera on a trapdoor fault resulted in 1.5 m of permanent resurgence. These observations reveal the importance of intra-caldera faulting in affecting resurgence, and the mechanisms of eruption in the absence of well-developed rift systems.