The geology of

Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Africa ...

comprises a variety of rocks of Precambrian age which make up the larger part of the east and centre of the island. They are intruded by

basalt 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 ...

s and rhyolites of

Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles, the Age of Conifers, and colloquially as the Age of the Dinosaurs is the second-to-last era of Earth's geological history, lasting from about , comprising the Triassic, Jurassic and Cretace ...

to Cenozoic age. In contrast, the western part of the island is formed from

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

rocks of Carboniferous to Quaternary age.

Archean The Archean Eon ( , also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history, representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic. The Earth during the Arc ...

rocks occur from the northeast portion of the island down to the south in the Ranotsara shear zone. Rocks in the northern portion of Madagascar are greenstone belts, from the Archean or Paleoproterozoic age.

Summary

The center and eastern portions of the island consist primarily of

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

and

igneous Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...

Precambrian basement

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

migmatite Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock th ...

s, and

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...

s, ranging in age from 3 billion to 550 million years old. They are mostly overlain by laterite

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

s. The

Itremo Itremo is a town and commune in Madagascar. It belongs to the district of Ambatofinandrahana, which is a part of Amoron'i Mania Region. The population of the commune was estimated to be approximately 7,000 in 2001 commune census. Only primary sch ...

Massif In geology, a massif ( or ) is a section of a planet's crust that is demarcated by faults or flexures. In the movement of the crust, a massif tends to retain its internal structure while being displaced as a whole. The term also refers to a ...

consists of

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...

and

marble Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Marble is typically not foliated (layered), although there are exceptions. In geology, the term ''marble'' refers to metamorphose ...

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. Features Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficia ...

s 630 million years in age. The

Morondava Morondava (, from mg, morona lava "long coast") is a city located in Menabe Region, of which it is the capital, in Madagascar. It is located in the delta of the Morandava River at . Its population as of the 2018 census, was 53,510. Population ...

, Mahajanga and Ambilobe

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subside ...

s occur along the western third of the island, and consist of terrestrial deposits that range in age from 350 million years ago to recent times. The western margins of Madagascar include

and

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

s. These outcrop in the Ankarana and Bemaraha Massifs, Namoroka, Analamerana, and Ankara Plateau (172-162 million years in age), and the Mahafaly Plateau (54-38 million years in age) respectively.

Flood basalt A flood basalt (or plateau basalt) is the result of a giant volcanic eruption or series of eruptions that covers large stretches of land or the ocean floor with basalt lava. Many flood basalts have been attributed to the onset of a hotspot reac ...

s that ring Madagascar include the Mahajanga

lava 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 ...

s (87.6 to 88.5 million years in age), the Toliara lavas and Ejeda-Bekily

dike swarm A dike swarm (American spelling) or dyke swarm (British spelling) is a large geological structure consisting of a major group of parallel, linear, or radially oriented magmatic dikes intruded within continental crust or central volcanoes ...

(84.5-84.8 million years old), and the Androy lava flow (84.4 million years in age). These igneous rocks were emplaced during the interval when India and Madagascar were together (91 million years ago), and when apart (84 million years ago). More recent volcanic activity includes the formation of the Ankaratra Massif over the past 5 million years, and Montagne d'Ambre, which erupted from 50 to 2 million years ago.

Tectonics

A NW-SE trending sinistral shear belt separates northern and southern crustal terranes. The Ampanihy ductile shear zone was created by flattening events associated with granulite metamorphism, isoclinal folding, flattened sheaths, steep to vertical foliations and sheath-like geometry of massif-type anorthosite bodies.

Paleontology

The

Maevarano Formation The Maevarano Formation is a Late Cretaceous sedimentary rock formation found in the Mahajanga Province of northwestern Madagascar. It is most likely Maastrichtian in age, and records a seasonal, semiarid environment with rivers that had greatly v ...

, dated to late

Maastrichtian The Maastrichtian () is, in the ICS geologic timescale, the latest age (uppermost stage) of the Late Cretaceous Epoch or Upper Cretaceous Series, the Cretaceous Period or System, and of the Mesozoic Era or Erathem. It spanned the interval ...

age, has provided a diverse faunal assemblage, including the giant frog ''

Beelzebufo ''Beelzebufo ampinga'' ( or ) was a particularly large species of prehistoric frog described in 2008. Common names assigned by the popular media include devil frog, devil toad, and the frog from hell. Fossils of ''Beelzebufo'' have been recover ...

'',

gondwanathere Gondwanatheria is an extinct group of mammaliaforms that lived in parts of Gondwana, including Madagascar, India, South America, Africa and Antarctica during the Upper Cretaceous through the Paleogene (and possibly much earlier, if '' Allostaffi ...

mammals,

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s, crocodylians and other groups. The Middle Jurassic Isalo III Formation contains fossil dinosaurs. Typical Gondwanan flora of ''

Glossopteris ''Glossopteris'' tymology: from Ancient Greek γλῶσσα (glôssa, " tongue ") + πτερίς (pterís, " fern ")is the largest and best-known genus of the extinct Permian order of seed ferns known as Glossopteridales (also known as Arberia ...

'' is found in the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last ...

formations in Madagascar.

Economic geology

Madagascar is the world's leading producer of many colored gemstones, including

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sa ...

rubies A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sap ...

, multi-coloured

tourmaline Tourmaline ( ) is a crystalline silicate mineral group in which boron is compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is a gemstone and can be found in a wide variety of colors. The te ...

s, emeralds, amethysts,

cordierite Cordierite (mineralogy) or iolite (gemology) is a magnesium iron aluminium cyclosilicate. Iron is almost always present and a solid solution exists between Mg-rich cordierite and Fe-rich sekaninaite with a series formula: to . A high-temperat ...

s, aquamarines and

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

s. Madagascar is also a major source of

graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on lar ...

, making it the second-largest producer in Africa. Additionally, Madagascar is Africa's third leading producer of chromite. Titanium oxide is found in significant quantities in beach sands located near Toalagnaro, however, 75% of the coastal forest zone in that area would have to be destroyed in order to exploit it, which has not been done. Huge oil fields lie to the west of Madagascar, however they would require steam injection to extract, which is both technologically difficult and expensive. The oil fields of

Tsimiroro Tsimiroro is a large oil field in the onshore Morondava Basin of Madagascar found south of the Bemolanga ultra heavy oil field and south of the town of Morafenobe. It is estimated to contain as many as of heavy oil with 14° to 16° API gra ...

and

Bemolanga Bemolanga is a large oil sands deposit in the onshore Morondava Basin of Madagascar. The deposit was discovered in the early 1900s but was known to locals for centuries. The field is located north of the Tsimiroro heavy oil field and east of th ...

are estimated to hold combined reserves of 11.5 billion barrels.

References

{{Africa topic, Geology of

Summary

Tectonics

Paleontology

Economic geology

See also

References