Erta Ale is classified as a shield volcano, a type of volcanic landform defined by its broad, low-profile structure. Located in the Afar Region of northeastern Ethiopia, it is one of the most active volcanoes in the country. It has gained international recognition for hosting one of the world’s longest-existing, persistent lava lakes in its summit caldera.
Defining the Shield Volcano
A shield volcano earns its name from its physical resemblance to a warrior’s shield lying on the ground, characterized by its wide, gently sloping flanks. This distinctive shape is a direct result of the type of magma it erupts, which is almost exclusively basaltic. Basaltic lava possesses a low viscosity, meaning it is highly fluid and flows easily over long distances before cooling and solidifying.
The continuous outpouring of this runny lava from a central vent or fissures builds up successive layers, creating a massive, broad cone with slopes that typically do not exceed 10 degrees. This contrasts sharply with the steep-sided, conical structure of stratovolcanoes, which are built from thicker, more viscous lava and fragmented rock. The eruption style of a shield volcano is primarily effusive, meaning the lava flows out relatively gently rather than exploding violently.
The low-viscosity, basaltic composition ensures that gases within the magma can escape relatively easily, preventing the pressure buildup that leads to explosive eruptions. Over time, these effusive eruptions can create truly immense volcanic edifices, making shield volcanoes some of the largest on Earth in terms of volume. Erta Ale’s base, for example, spans approximately 50 kilometers in diameter.
The low-silica content of the basaltic magma, combined with its high temperature, maintains its fluidity and allows for the extensive lava fields that surround the volcano’s core. This consistent, non-explosive activity is the defining characteristic that categorizes Erta Ale within the shield volcano family.
The Characteristics of Erta Ale’s Eruptive Activity
The eruptive character of Erta Ale is a specific manifestation of its shield volcano classification, centered around its long-lived lava lake. The volcano’s summit is truncated by an elongated, elliptical caldera, a collapse feature measuring approximately 1.6 kilometers by 0.7 kilometers. Within this caldera are two main pit craters, known as the North and South pits, which often contain the active lava.
Erta Ale is most famous for being one of the only volcanoes on Earth to host an almost persistent lava lake, a feature that has been active since at least 1906. This enduring presence is possible due to a continuous and stable supply of high-temperature, low-viscosity basaltic magma from below. The lava lake constantly circulates, with cooler crustal slabs sinking and hotter magma rising in a process of convection.
The activity within the pit craters is typically a low-level, effusive style, ranging from slow circulation to minor fountaining, often described as Hawaiian or Strombolian activity. This constant heat release, with relatively small volumes of lava being erupted over long time frames, prevents the magma conduit from solidifying. On occasion, the lake level may rise and overflow the rim of the pit crater, sending pāhoehoe lava flows across the broader caldera floor.
The draining of the lava lake, such as the major event that occurred in 2017, is usually associated with the opening of new flank fissures that divert the magma supply. However, the lake often reforms, demonstrating the persistent connection to a deep-seated magma reservoir within the rift zone.
Geological Context: The Afar Rift Zone
Erta Ale’s existence and eruptive nature are inextricably linked to its location within the Afar Rift Zone, a tectonically dynamic area in East Africa. The volcano sits at the northern end of the East African Rift Valley, specifically at the Afar Triple Junction. This junction marks the meeting point of three divergent plate boundaries: the Arabian, Nubian (African), and Somalian plates.
The geological environment here is one of continental rifting, where the Earth’s crust is being slowly pulled apart at an average rate of less than one centimeter per year. This extension is generating a new plate boundary, effectively transitioning from a continental rift to what will eventually become an oceanic spreading ridge. The stretching and thinning of the lithosphere create pathways for magma to ascend to the surface.
The tectonic forces cause a reduction in pressure on the underlying mantle, a process known as decompression melting. This mechanism generates large volumes of basaltic magma, which is naturally low in silica and highly fluid. This low-viscosity magma is the exact type required to build a shield volcano, explaining why the Afar region is characterized by extensive basaltic volcanism and flood basalts.
Erta Ale is part of a volcanic ridge system within the rift zone that runs parallel to the Red Sea. The magma chamber feeding the volcano is continuously supplied by this rifting process, which accommodates the crustal extension through the injection of dikes and sills.