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47-508: Lake Naivasha is at the highest elevation of the Kenyan Rift valley at 1,884 metres (6,181 ft) in a complex geological combination of volcanic rocks and sedimentary deposits from a larger Pleistocene Era lake. Apart from transient streams, the lake is fed by the perennial Malewa and Gilgil rivers. There is no visible outlet, but since the lake water is relatively fresh it is assumed to have an underground outflow. The lake had

94-457: A vesicular texture caused by voids left by volatiles trapped in the molten lava . Pumice is a highly vesicular rock produced in explosive volcanic eruptions . Most modern petrologists classify igneous rocks, including volcanic rocks, by their chemistry when dealing with their origin. The fact that different mineralogies and textures may be developed from the same initial magmas has led petrologists to rely heavily on chemistry to look at

141-411: A volcano . Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks . For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic"

188-430: A black glassy substance called obsidian . When filled with bubbles of gas, the same lava may form the spongy appearing pumice . Allowed to cool slowly, it forms a light-colored, uniformly solid rock called rhyolite. The lavas, having cooled rapidly in contact with the air or water, are mostly finely crystalline or have at least fine-grained ground-mass representing that part of the viscous semi-crystalline lava flow that

235-410: A close analogy to hot solutions of salts in water, which, when they approach the saturation temperature, first deposit a crop of large, well-formed crystals (labile stage) and subsequently precipitate clouds of smaller less perfect crystalline particles (metastable stage). In igneous rocks the first generation of crystals generally forms before the lava has emerged to the surface, that is to say, during

282-459: A good supplier will know what sort of volcanic rock they are selling. The sub-family of rocks that form from volcanic lava are called igneous volcanic rocks (to differentiate them from igneous rocks that form from magma below the surface, called igneous plutonic rocks ). The lavas of different volcanoes, when cooled and hardened, differ much in their appearance and composition. If a rhyolite lava-stream cools quickly, it can quickly freeze into

329-415: A lakeside human population now approaching a million people. In 1981, the first geothermal plant for Lake Naivasha was commissioned and by 1985, a total of 45 MW of electricity was being generated in the area. About 10,000 years ago, Lake Naivasha, together with neighboring Lake Elementaita and Lake Nakuru (60 km further north), formed one single, deep freshwater lake that eventually dried up, leaving

376-404: A normal surface area of 139 square kilometres (54 sq mi) before 2010 and had increased to 198 square kilometres (76 sq mi) by 2020. It is surrounded by a swamp which covers an area of 64 square kilometres (25 sq mi), but this can vary largely depending on rainfall. Situated at an altitude of 1,884 metres (6,181 ft), the lake has an average depth of 6 metres (20 ft), with

423-462: A paramorph of augite and magnetite, which may partially or completely substitute for the original crystal but still retains its characteristic outlines. The mechanical behaviour of volcanic rocks is complicated by their complex microstructure. For example, attributes such as the partitioning of the void space (pores and microcracks), pore and crystal size and shape, and hydrothermal alteration can all vary widely in volcanic rocks and can all influence

470-635: A series of lakes in the East African Rift valley that runs through eastern Africa from Ethiopia in the north to Malawi in the south, and includes the African Great Lakes in the south. These include some of the world's oldest lakes, deepest lakes , largest lakes by area , and largest lakes by volume . Many are freshwater ecoregions of great biodiversity , while others are alkaline "soda lakes" supporting highly specialised organisms. The Rift Valley lakes are well known for

517-437: A sizable population of hippos . The fish community in the lake has been highly variable over time, influenced by changes in climate, fishing effort and the introduction of invasive species. The most recent shift in the fish population followed the accidental introduction of common carp in 2001. Nine years later, in 2010, common carp accounted for over 90% of the mass of fish caught in the lake. There are two smaller lakes in

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564-502: A volcanic rock's origin. The chemical classification of igneous rocks is based first on the total content of silicon and alkali metals ( sodium and potassium ) expressed as weight fraction of silica and alkali oxides ( K 2 O plus Na 2 O ). These place the rock in one of the fields of the TAS diagram . Ultramafic rock and carbonatites have their own specialized classification, but these rarely occur as volcanic rocks. Some fields of

611-401: Is a molten liquid and rock is solid). "Lava stone" may describe anything from a friable silicic pumice to solid mafic flow basalt, and is sometimes used to describe rocks that were never lava , but look as if they were (such as sedimentary limestone with dissolution pitting ). To convey anything about the physical or chemical properties of the rock, a more specific term should be used;

658-411: Is dependent on two things: the initial composition of the primary magma and the subsequent differentiation. Differentiation of most magmas tends to increase the silica ( SiO 2 ) content, mainly by crystal fractionation . The initial composition of most magmas is basaltic , albeit small differences in initial compositions may result in multiple differentiation series. The most common of these series are

705-453: Is often applied to what are strictly metavolcanic rocks . Volcanic rocks and sediment that form from magma erupted into the air are called "pyroclastics," and these are also technically sedimentary rocks. Volcanic rocks are among the most common rock types on Earth's surface, particularly in the oceans. On land, they are very common at plate boundaries and in flood basalt provinces . It has been estimated that volcanic rocks cover about 8% of

752-446: The African Great Lakes . The Western Rift Valley lakes are fresh water and home to an extraordinary number of species. Approximately 1,500 cichlid fish (Cichlidae) species live in the lakes. In addition to the cichlids, populations of Clariidae , Claroteidae , Mochokidae , Poeciliidae , Mastacembelidae , Centropomidae , Cyprinidae , Clupeidae and other fish families are found in these lakes. They are also important habitats for

799-408: The pyroxenes . Bowen's reaction series correctly predicts the order of formation of the most common minerals in volcanic rocks. Occasionally, a magma may pick up crystals that crystallized from another magma; these crystals are called xenocrysts . Diamonds found in kimberlites are rare but well-known xenocrysts; the kimberlites do not create the diamonds, but pick them up and transport them to

846-562: The tholeiitic , calc-alkaline , and alkaline . Most volcanic rocks share a number of common minerals . Differentiation of volcanic rocks tends to increase the silica (SiO 2 ) content mainly by fractional crystallization . Thus, more evolved volcanic rocks tend to be richer in minerals with a higher amount of silica such as phyllo and tectosilicates including the feldspars, quartz polymorphs and muscovite . While still dominated by silicates, more primitive volcanic rocks have mineral assemblages with less silica, such as olivine and

893-713: The Central African lakes. Lakes Albert, Victoria, and Edward are part of the Nile River basin. Lake Victoria (elevation 1,134 metres (3,720 ft)), with an area of 68,800 square kilometres (26,600 sq mi), is the largest lake in Africa. It is not in the Rift Valley, instead occupying a depression between the eastern and western rifts formed by the uplift of the rifts to either side. Lakes Victoria, Tanganyika, and Malawi are sometimes collectively known as

940-479: The Earth's current land surface. Volcanic rocks are usually fine-grained or aphanitic to glass in texture. They often contain clasts of other rocks and phenocrysts . Phenocrysts are crystals that are larger than the matrix and are identifiable with the unaided eye . Rhomb porphyry is an example with large rhomb shaped phenocrysts embedded in a very fine grained matrix. Volcanic rocks often have

987-608: The Ethiopian Rift Valley lakes are of great importance to Ethiopia's economy, as well as being essential to the survival of the local people, there were no intensive and extensive limnological studies undertaken of these lakes until recently. The major ones are Lake Tana , the source of the Blue Nile , lies in the Ethiopian highlands north of the Rift Valley; however, it is not a Rift Valley lake. South of

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1034-724: The Ethiopian highlands, the rift valley splits into two major troughs. The Eastern Rift is home to the Kenyan Rift Valley lakes, while most of the Central African Rift Valley lakes lie in the Western Rift. This area includes the Gregory Rift in Kenya and Tanzania. The Kenyan section of the Rift Valley is home to eight lakes, of which three are freshwater and the rest alkaline. Of the latter,

1081-624: The TAS diagram are further subdivided by the ratio of potassium oxide to sodium oxide. Additional classifications may be made on the basis of other components, such as aluminum or iron content. Volcanic rocks are also broadly divided into subalkaline, alkaline, and peralkaline volcanic rocks. Subalkaline rocks are defined as rocks in which SiO 2 < -3.3539 × 10 × A + 1.2030 × 10 × A - 1.5188 × 10 × A + 8.6096 × 10 × A - 2.1111 × A + 3.9492 × A + 39.0 where both silica and total alkali oxide content (A) are expressed as molar fraction . Because

1128-436: The TAS diagram uses weight fraction and the boundary between alkaline and subalkaline rock is defined in terms of molar fraction, the position of this curve on the TAS diagram is only approximate. Peralkaline volcanic rocks are defined as rocks having Na 2 O + K 2 O > Al 2 O 3 , so that some of the alkali oxides must be present as aegirine or sodic amphibole rather than feldspar . The chemistry of volcanic rocks

1175-428: The ascent from the subterranean depths to the crater of the volcano. It has frequently been verified by observation that freshly emitted lavas contain large crystals borne along in a molten, liquid mass. The large, well-formed, early crystals ( phenocrysts ) are said to be porphyritic ; the smaller crystals of the surrounding matrix or ground-mass belong to the post-effusion stage. More rarely lavas are completely fused at

1222-719: The author of Born Free , lived on the shores of the lake in the mid-1960s. On the shores of the lake is Oserian ("Djinn Palace"), which gained notoriety in the Happy Valley days between the two world wars . In 1999, the Lake Naivasha Riparian Association received the Ramsar Wetland Conservation Award for its conservation efforts regarding the Lake Naivasha Ramsar site. Floriculture forms

1269-564: The catchment area. Lake Naivasha , once described as the Jewel in the Crown of all the East African lakes, has in recent decades been subjected to a series of devastating human pressures, not least the establishment of a vast horticulture and agriculture industry along its shorelines, but also the ever-increasing inflows of nutrients from siltation, sewage and other effluents emanating from

1316-433: The crystal. It is clear that after the mineral had crystallized it was partly again dissolved or corroded at some period before the matrix solidified. Corroded phenocrysts of biotite and hornblende are very common in some lavas; they are surrounded by black rims of magnetite mixed with pale green augite. The hornblende or biotite substance has proved unstable at a certain stage of consolidation, and has been replaced by

1363-417: The deepest area being at Crescent Island, with a maximum depth of 30 metres (98 ft). Njorowa Gorge once formed the lake's outlet, but it is now high above the lake and forms the entrance to Hell's Gate National Park . The town of Naivasha (formerly East Nakuru) lies on the north-east edge of the lake. The lake is home to a variety of types of wildlife including over 400 different species of bird and

1410-527: The densely populated areas round the lake. Lake Naivasha still supports many local companies. Some of them like the renowned Watamu Boat Rides Naivasha (watamuboatridesnaivasha.com) make a living by taking tourists and locals around the lake, showing them the major hippo hubs as well showcasing the Crescent Island. Volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from

1457-467: The depth reached 2.25 m above the lake bottom. The decline of the lake water level in 1987 increased concern in the future of geothermal industry, and it was speculated that Lake Naivasha underground water might be feeding the geothermal reservoir at Olkaria . Hence, the decline in the lake water would affect the future of the geothermal industry. The surface area of the lake had increased by over 50% by 2020 causing many people to be displaced from

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1504-508: The evolution of at least 800 cichlid fish species that live in their waters. More species are expected to be discovered. The World Wide Fund for Nature has designated these lakes as one of its Global 200 priority ecoregions for conservation. Lake Malawi and Lake Tanganyika have formed in the various valleys of the East African Rift zone. Lake Kivu's "still waters ... hide another face: dissolved within are billions of cubic meters of flammable methane and more still of carbon dioxide,

1551-404: The ground-mass . Microscopic examination of the phenocrysts often reveals that they have had a complex history. Very frequently they show layers of different composition, indicated by variations in color or other optical properties; thus augite may be green in the center surrounded by various shades of brown; or they may be pale green centrally and darker green with strong pleochroism (aegirine) at

1598-400: The main industry around the lake. However, the largely unregulated use of lake water for irrigation is reducing the level of the lake and is the subject of concern in Kenya. Fishing in the lake is also another source of employment and income for the local population. The lake varies in level greatly and almost dried up entirely in the 1890s. Lake levels in general follow the rainfall pattern in

1645-458: The mechanical behaviour of volcanic rocks can help us better understand volcanic hazards, such as flank collapse. Volcanic rocks : Subvolcanic rocks : Plutonic rocks : Picrite basalt Peridotite Basalt Diabase (Dolerite) Gabbro Andesite Microdiorite Diorite Dacite Microgranodiorite Granodiorite Rhyolite Microgranite Granite Rift Valley lakes The Rift Valley lakes are

1692-714: The moment of ejection; they may then cool to form a non-porphyritic, finely crystalline rock, or if more rapidly chilled may in large part be non-crystalline or glassy (vitreous rocks such as obsidian, tachylyte , pitchstone ). A common feature of glassy rocks is the presence of rounded bodies ( spherulites ), consisting of fine divergent fibres radiating from a center; they consist of imperfect crystals of feldspar, mixed with quartz or tridymite ; similar bodies are often produced artificially in glasses that are allowed to cool slowly. Rarely these spherulites are hollow or consist of concentric shells with spaces between ( lithophysae ). Perlitic structure, also common in glasses, consists of

1739-409: The periphery. In the feldspars the center is usually richer in calcium than the surrounding layers, and successive zones may often be noted, each less calcic than those within it. Phenocrysts of quartz (and of other minerals), instead of sharp, perfect crystalline faces, may show rounded corroded surfaces, with the points blunted and irregular tongue-like projections of the matrix into the substance of

1786-454: The presence of concentric rounded cracks owing to contraction on cooling. The phenocrysts or porphyritic minerals are not only larger than those of the ground-mass; as the matrix was still liquid when they formed they were free to take perfect crystalline shapes, without interference by the pressure of adjacent crystals. They seem to have grown rapidly, as they are often filled with enclosures of glassy or finely crystalline material like that of

1833-561: The product of explosive volcanism. They are often felsic (high in silica). Pyroclastic rocks are often the result of volcanic debris, such as ash , bombs and tephra , and other volcanic ejecta . Examples of pyroclastic rocks are tuff and ignimbrite . Shallow intrusions , which possess structure similar to volcanic rather than plutonic rocks, are also considered to be volcanic, shading into subvolcanic . The terms lava stone and lava rock are more used by marketers than geologists, who would likely say "volcanic rock" (because lava

1880-749: The result of volcanic gases seeping in." The Ethiopian Rift Valley lakes are the northernmost of the African Rift Valley lakes. In central Ethiopia , the Main Ethiopian Rift , also known as the Great Rift Valley, splits the Ethiopian highlands into northern and southern halves, and the Ethiopian Rift Valley lakes occupy the floor of the rift valley between the two highlands. Most of the Ethiopian Rift Valley lakes do not have an outlet, and most are alkaline . Although

1927-478: The resultant mechanical behaviour (e.g., Young's modulus, compressive and tensile strength, and the pressure at which they transition from brittle to ductile behaviour ). As for other crustal rocks, volcanic rocks are brittle and ductile at low and high effective confining pressures, respectively. Brittle behaviour is manifest as faults and fractures, and ductile behaviour can either be distributed (cataclastic pore collapse) or localised (compaction bands). Understanding

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1974-661: The shallow soda lakes of the Eastern Rift Valley have crystallised salt turning the shores white and are famous for the large flocks of flamingo that feed on crustaceans . All the lakes in the Tanzanian section of this group are alkaline: The lakes of the Western or Albertine Rift , with Lake Victoria , include the largest, deepest, and oldest of the Rift Valley Lakes. They are also referred to as

2021-455: The surface of the Earth. Volcanic rocks are named according to both their chemical composition and texture. Basalt is a very common volcanic rock with low silica content. Rhyolite is a volcanic rock with high silica content. Rhyolite has silica content similar to that of granite while basalt is compositionally equal to gabbro . Intermediate volcanic rocks include andesite , dacite , trachyte , and latite . Pyroclastic rocks are

2068-474: The three lakes as remnants. Water levels in the Rift Valley Lakes have since varied considerably, with several of the lakes almost drying up several times. The water level for Lake Naivasha reached a low of 0.6 m depth in 1945, but the water level rose again, with minor drops, to reach a maximum depth of nearly 6 m in 1968. There was another major decline of the water level in 1987, when

2115-574: The vicinity of Lake Naivasha: Lake Oloiden and Lake Sonachi (a green crater lake ). The Crater Lake Game Sanctuary lies nearby, while the lake shore is known for its population of European immigrants and settlers. Between 1937 and 1950, the lake was used as a landing place for flying boats on the Imperial Airways passenger and mail route from Southampton in Britain to South Africa . It linked Kisumu and Nairobi . Joy Adamson ,

2162-453: Was going on while the mass was still creeping forward under the surface of the Earth, the latest formed minerals (in the ground-mass ) are commonly arranged in subparallel winding lines that follow the direction of movement (fluxion or fluidal structure)—and larger early minerals that previously crystallized may show the same arrangement. Most lavas fall considerably below their original temperatures before emitted. In their behavior, they present

2209-480: Was still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, and the steam and other gases, which they contained in great quantity were free to escape; many important modifications arise from this, the most striking being the frequent presence of numerous steam cavities ( vesicular structure) often drawn out to elongated shapes subsequently filled up with minerals by infiltration ( amygdaloidal structure). As crystallization

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