South Island, or Hohnel Island (named after

Ludwig von Höhnel Ludwig Ritter von Höhnel (6 August 1857, Preßburg – 23 March 1942, Vienna) was an Austrian naval officer and explorer. He was trained at the naval academy in Fiume, then part of the Austrian empire. His brother was the naturalist Franz Xaver ...

who helped with Teleki's expedition), is a fissure-formed volcanic island and the largest one out of 3 other major islands in Lake Turkana, Kenya. This volcano was discovered in 1888 while it was erupting by Count Teleki, a Hungarian explorer who led the first expedition into the region. It is a part of the Lake Turkana National Parks, which is a UNESCO World Heritage Site.

Geography

The island is a 12 km by 5 km island, covered almost completely by recent

lava flows Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

erupted by the volcano. Around the island can be found some smaller islets.

Climate

The island generally has a hot and arid climate which lasts from December to March. The coolest months on the island are June and July. Strong winds are a common occurrence from May to September. The annual rainfall at the island averages at 250mm.

Discovery

In 1888, Samuel Teleki, a Hungarian explorer, set off to a trip in Eastern Africa specifically in northern Kenya and southern Ethiopia with Ludwig von Höhnel, an Austrian naval officer and explorer. They were the first Europeans to see many places including Lake Turkana which was named Lake Rudolf (named after the

Crown Prince Rudolf of Austria en, Rudolph Francis Charles Joseph , caption = Rudolf in 1887 , spouse = , issue = Elisabeth Marie, Princess Otto of Windisch-Graetz , house = Habsburg-Lorraine , father = Franz Joseph I of Austria , mother ...

) when it was discovered,

Lake Chew Bahir Lake Chew Bahir (Amharic: ጨው ባሕር ''č̣ew bāhir'', "salty lake") or Lake Istifanos, also called Stefanie, Basso Naebor and Chuwaha, is a lake in southern Ethiopia, located on the southwestern end of the Southern Nations, Nationalities ...

which was named Lake Stefanie (named after

Princess Stephanie of Belgium Princess is a regal rank and the feminine equivalent of prince (from Latin ''princeps'', meaning principal citizen). Most often, the term has been used for the consort of a prince, or for the daughter of a king or prince. Princess as a subst ...

) back then, the Teleki's Volcano (named after Teleki) and more including South Island. Teleki and Höhnel made many observations and discoveries about the local geography, the tribe community and others. All of their work was published in a book called ''The Discovery of Lakes Rudolf and Stefanie'' in 1892 by Höhnel himself.Balàzs Borsos, "Whose merit is it anyway? The Evaluation of Count Teleki and Ritter von Höhnel's roles in the Teleki expedition to East Africa in 1887–1888." In ''Archiv für Völkerkunde.'' 54, 2004, ISSN 0066-6513 S. 27–47.

Geological setting

The island sits in the southern portion of Lake Turkana which was formed by the

East African Rift System East or Orient is one of the four cardinal directions or points of the compass. It is the opposite direction from west and is the direction from which the Sun rises on the Earth. Etymology As in other languages, the word is formed from the f ...

(EARS). Along the EARS sits many volcanoes, the closest ones to South Island including Central Island, North Island,

The Barrier The Barrier is a lava dam retaining the Garibaldi Lake system in southwestern British Columbia, Canada. It is over thick and about long where it impounds the lake. The area below and adjacent to The Barrier is considered hazardous due to the u ...

and more. The EARS, a rift zone which extends all the way from

Mozambique Mozambique (), officially the Republic of Mozambique ( pt, Moçambique or , ; ny, Mozambiki; sw, Msumbiji; ts, Muzambhiki), is a country located in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi ...

to Eritrea, is a divergent boundary that is separating the

African Plate The African Plate is a major tectonic plate that includes much of the continent of Africa (except for its easternmost part) and the adjacent oceanic crust to the west and south. It is bounded by the North American Plate and South American Plate ...

and the

Somali Plate The Somali Plate is a minor tectonic plate which straddles the Equator in the Eastern Hemisphere. It is currently in the process of separating from the African Plate along the East African Rift Valley. It is approximately centered on the isl ...

apart at a rate of a few millimeters per year. As a result, seismic activity is caused which can sometimes have larger earthquakes as well as a belt of volcanoes being fed by the rift since in this case the tectonics and the volcanism are closely related.

Magma processes

The volcano sits on a rift zone, which means that tectonic movement is one of the main factors which generates magma. In this case, tectonic movement is a factor as volcanic edifice or materials underground loads into the crust, which modifies the

differential stress Differential stress is the difference between the greatest and the least compressive stress experienced by an object. For both the geological and civil engineering convention \sigma_1 is the greatest compressive stress and \sigma_3 is the weakest, ...

and affects the upper crust relative to the volcanic materials' radius. Meanwhile, the

principal stress In continuum mechanics, the Cauchy stress tensor \boldsymbol\sigma, true stress tensor, or simply called the stress tensor is a second order tensor named after Augustin-Louis Cauchy. The tensor consists of nine components \sigma_ that completely ...

' trajectory focuses to the greatest load within the materials. This focusing action (mostly with an asymmetrical stress field) leads to the creation of a

magma chamber A magma chamber is a large pool of liquid rock beneath the surface of the Earth. The molten rock, or magma, in such a chamber is less dense than the surrounding country rock, which produces buoyant forces on the magma that tend to drive it up ...

Structure

On South Island can be found a N-S (north to south) trending volcanic ridge which rises to about a height of 300 m. Along this ridge can be found several

volcanic cones Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and ...

, which some rise to about a height of 800 m ( above sea level, 320 m above the lake level). Several N-S trending faults can also be found on the east and west side of the main ridge, which proves that the island sits on a horst. Morphologically, the east and western side of the island differs from each other. To the east,

basaltic Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90% of a ...

lava flows originating near the ridge spill all the way down to the shore which completely covers the eastern portion. In comparison, the western portion is mostly covered in unconsolidated ash, likely from southwesterly winds. The lava flows and

pyroclastic rocks Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroc ...

are built upon older tuffs and smaller

pillow lavas Pillow lavas are lavas that contain characteristic pillow-shaped structures that are attributed to the extrusion of the lava underwater, or ''subaqueous extrusion''. Pillow lavas in volcanic rock are characterized by thick sequences of disconti ...

. Sandy beaches can be seen along the west side of the island. Elsewhere, small, isolated

black sand Black sand is sand that is black in color. One type of black sand is a heavy, glossy, partly magnetic mixture of usually fine sands containing minerals such as magnetite, found as part of a placer deposit. Another type of black sand, found on ...

beaches and rocky areas are located at small coves along the western and northern shores. The oldest rocks and minerals on the island are sequences of basaltic lavas found on the terrace of the northeastern shore and the Enwoiti Islet (located 100m east of South Island). The sequence of rocks, from the base to top, includes hyaloclastites,

, and massive lava flows. This sequence is estimated to be 30-40 meters thick. These units of older rocks are covered by an

alluvium Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. ...

terrace which goes all the way to the western flank of the ridge.

Geologic history

Much isn't known about the geological and eruptive history of South Island, however 1 eruption was witnessed in the 19th century.

1888 eruption

When Count Teleki was in an expedition in Kenya, an explosive eruption of the Teleki's Volcano (

) was observed, which reportedly triggered lava flows and pyroclastic flows. Reportedly, meanwhile this eruption was ongoing, Count Teleki witnessed another but smaller eruption from a scoria cone in South Island from a volcano which prompted a lava flow, perhaps in relation to the eruption at The Barrier volcano.

References

{{Turkana Basin Volcanoes of Kenya National parks of Kenya World Heritage Sites in Kenya Lake islands of Kenya Lake Turkana