Emi Koussi

Emi Koussi is a massive high pyroclastic shield volcano.

It is located at the southeast end of the Tibesti Mountains. This mountain range sits in the central Sahara within northern Chad. Emi Koussi is the highest mountain in the entire Sahara Desert. It reaches a peak elevation of 3,447 metres.

The volcano is very wide, measuring between 60 and 70 kilometres across. Its total volume is estimated at 2,500 cubic kilometres. Because of its size and shape, some astronauts consider it a very recognizable landmark from orbit.

The structure of the volcano is defined by its unique shape and nested pits. It is a shield volcano, which means it has broad, sloping sides. The summit features two overlapping calderas, which are large pits formed by volcanic activity.

The outer caldera is an elliptical shape about 15 by 11 kilometres wide. This outer ring was formed in stages, with a northern caldera forming before a southern one. The southern caldera is about 50 metres deeper than the northern one. Nested inside this large area is a smaller caldera called Era Kohor.

Era Kohor is 2 kilometres wide and 350 metres deep. It looks like a giant hole and is also known as the Natron Hole. Its floor sits at 2,670 metres and is covered in white sodium carbonate deposits and a salt lake.

Geologists believe the volcano was built through three distinct eruptive stages.

The first stage occurred between 2.42 and 2.33 million years ago. During this time, the volcano erupted trachytes and trachyandesites as ignimbrites and basaltic lapilli. The second stage involved trachytic and phonolitic lava domes along with basaltic lava flows. The third stage was dominated by phonolitic eruptions, including the Kohor ignimbrite. Each of these stages was accompanied by the formation of a caldera. Recent volcanic activity includes the creation of maars, which are low-relief volcanic craters. Long lava flows and several scoria cones also sit on the slopes.

The history of the Tibesti Mountains involves complex tectonic movements. Scientists suggest that a mantle plume, which is a rising column of hot rock, helped form these volcanoes. Another theory suggests that the collision between Africa and Europe caused tectonic effects that created the range. The oldest rocks beneath the mountains are Precambrian granites and schists. The volcanic rocks themselves rest on an uplifted basement of sandstone. Emi Koussi has erupted many specific types of rock, such as phonolite and basalt. These rocks contain various minerals, including olivine, magnetite, and alkali feldspar. Even ancient Neolithic societies used these volcanic rocks as raw materials.

Climate and biology on Emi Koussi are shaped by its high elevation. The mountain has a desert mountain climate with very little rain. Annual precipitation is estimated to be only between 80 and 120 millimetres. However, the environment was different during the Holocene period. About 5,400 years ago, the Era Kohor caldera held a large lake. This lake was 130 metres deep and covered 4.1 square kilometres.

Today, the area is much drier, but the lake left behind diatom beds in the sediment. These are remains of tiny, single-celled algae that lived in the water.

Despite the harsh desert conditions, different types of life exist on the mountain. Vegetation is organized into five different belts based on how high they grow. At lower elevations, you can find the plant Erica arborea. The summit region contains Artemisia-Ephedra steppes. On the high lava flows, you can find ferns, mosses, and liverworts. Some plants are very rare and are only found here. For example, the wood rush Luzula tibestica is endemic, meaning it grows only on Emi Koussi. You can also find active fumaroles, which are openings that release volcanic gases. Near these vents, hot springs produce water that stays at 37 degrees Celsius.

Emi Koussi is highly significant for modern space science. Scientists use the volcano as an analogue to study other planets. An analogue is a place on Earth that looks and acts like a place in space. Specifically, Emi Koussi is compared to Elysium Mons, a volcano on Mars. By studying how eruptions and minerals work here, researchers can better understand Martian geology. This connection makes the remote mountains of Chad a very important site for understanding the solar system.