I ran across this topic courtesy of the Smithsonian GVP. Its home page has a rotating list of 9 volcanic systems that changes on an irregular basis. One system they listed was the Vakinankaratra Volcanic Field in the central Madagascar highlands. As I haven’t looked into that part of the world yet, it seemed to be a perfect suggestion. For her part, Granyia got regionally close with her post on Mayotte Island in the Mozambique Channel Dec 2018.
But when you go looking at things, you don’t necessarily find what you expect. So rather than a simple volcanic field in a part of the world I barely knew about, I find Vakinankaratra is the youngest of three neighboring monogenetic volcanic fields. Worse, it might not even be a separate field. Rather, it is the most recent volcanic activity from the largest one in the region, the Ankaratra volcanic complex. And all three volcanic fields are created by magma that pushes up through ancient bedrock, separate from far older volcanism triggered by the breakup of Gondwana some 135 Ma.
The region all this is taking place is Vakinankaratra, central Madagascar. It has an area of nearly 16,600 km with a population just over 2 million. This is called the central highlands with a relatively cool climate. The capital, Antsirabe has a population of nearly 260,000. It is known as the city of water in French and Malagasy, likely to the presence of multiple thermal springs in the area. The thermal springs in turn, argue for recent volcanism.
The town is at an altitude of 1,500 m, with a subtropical highland climate, similar to Brazilian high cities in the SE. Winter temperatures can fall below 0 C. The coldest and driest parts of the year are May – Sept, with only a couple days of precipitation per month. The wet part of the year is warmer and averages 12 – 17 wet days per month.
Farming (rice and grains) appears to be the largest industry, with some mining for precious minerals and a growing tourism sector.
There are three volcanic fields in the region, Ankaratra, Itasy and Vakinankaratra. They are all built on an ancient Precambrian magmatic basement. Recent activity in all three of these fields are essentially monogenetic, though initial activity in the largest, the Ankaratra was flood basalts.
Ankaratra Volcanic Complex
The largest of these is Ankaratra, lying N of Antsirabe stretching 100 km from Antsirabe N to Arivonimamo. Its highest peak is 2,644 m and is the most prominent and volcanically diverse field in Madagascar. There are several water-filled craters in the area around Antsirabe. There are hot springs used as hot baths in the city.
The Ankaratra Volcanic Field covers around 3,800 km2 in central Madagascar and includes a variety of lava flows, domes, scoria cones, tuff rings and maars. It was active for an extended period 17.5 – 8.5 Ma. Eruptive products are generally leucites, basalts, trachytes and rhyolites. Erupted magmas did not have much interaction with crustal materials. The source of these rocks is chemically similar to the currently active Comoro islands volcanoes.
The volcanic history of this field is divided into three phases. The first was dominated by flood basalts variously dated 28 – 3 Ma and around 7 Ma. The second phase emplaced basalt variants and trachytic extrusion. The third phase dated 1.7 – 1.4 Ma was dominated by andesitic flows, trachy-andesitic extrusions and leucocratic tephras.
Itasy Volcanic Field
The second field is Itasy to W of the northern portion of Ankaratra. It is about 110 km W of Antananarivo, covering some 1,600 km2. It also has a diverse array, but still monogenetic volcanic centers including scoria cones, lava domes and maars. There are around 131 scoria cones and domes. Activity began pre-Pleistocene and continued until 7.1 – 6.0 ka. Current activity is confined to warm springs and geysers. Juvenile magmas are chemically close to bimodal basanites and trachyphonolites, which may be due to partial melting of mantle rocks depressurized by regional uplift in an extensional regime underlying central Madagascar. The field is emplaced directly on the Precambrian basement. There are no underlying basalts between recent volcanism and the bedrock.
The youngest volcanic feature in the field may be a maar near Lake Itasy dated around 8.5 ka. This likely means the field is currently volcanically active. Early volcanism was trachyte surface eruptions with lava dome intrusion and lava flows at Kasigie. This was followed by eruptions of the oldest flows of basalts and then eruption and intrusion of trachytic dome and lavas of Andranonatoa. These were followed by trachyte-andesitic and tephrite flows. The final phase of volcanic activity produced maars.
This is one of the most seismically active regions of Madagascar with associated hot springs. There is a low velocity zone beneath this area believed to be asthenosphere upwelling from 65 km below which has uplifted the Itasy – Ankaratra regions over the last 10 – 15 Ma. This upwelling is either caused by a mantle plume or active rifting. Recent analysis leans more toward rifting due to the southward extension of the Somali – Africa plate boundary.
The Itasy zone is fragmented into multiple N-S trending fault blocks. Most of these are extension fractures. There are numerous indications of vent alignment along structural trends. Most of the field is bounded by fault lines. The geothermal zone is likewise bound. Localized stress-strain relations appear to allow upward movement of geothermal water heated by underlying magma.
Active tectonics, volcanic activity, hot springs and earthquakes in this field may all be related, with at least one paper referring to that relationship as obvious. Seismic activity and hot springs in the center of the island indicate the volcanic and tectonic reactivation that started in the Neogene is ongoing. There are local rumors of minor volcanic eruptions, “fire geysers” at the Kasigie cone along with some earthquakes. The concentration of earthquakes at the center of the island can be explained by an uplift bulge.
The Andranonatoa volcanic complex located 3.5 km SW of Analavory erupted in two phases. Initial eruptions were multiple trachyte extrusions creating an oval shape over 1 km long oriented parallel with the general N-S alignment of local faults. This dome was then fractured along the same line, creating a 300 m fracture 50 – 100 m wide.
Most cones in the Itasy are located 800 – 1,900 m above sea level. Semi-circular cinder cones with closed single craters are present in a wide range of sizes. These represent just over 8% of all identified cones. Kasigie (Kasigie Mountain), which was likely recently active, is a good example of this sort of cone. It is 1,000 in diameter, with a 300 m crater. These are typically surrounded by massive lava flows.
Horseshoe shaped cinder cones are typically breached. These are the most dominant type at nearly 75% of all volcanic cones. They are generally small, less than 750 m in diameter, around 100 m high. There are symmetric and asymmetric cones present. Fasia is an example of an asymmetric breached cone. A group of three small symmetric cones near Mananasy in the southern part of the field is an example of this type.
Fissure cones are represented by two parallel ridges with an elongated depression (fissure) between them. They are just over 5% of the field total. The Mandetika fissure cone in the northern tip of the field appears to be made of several eruptive vents aligned along a fissure.
Other less common types include coalesced breached cones where the openings seem to slightly face another. Parts of the cones are still connected. These are around 6% of the total number of cones. The final type is a single cone with multiple craters that seem to be generally aligned with the long axis of the cone base. There are around 4% of these in the field.
Vakinankaratra Volcanic Field
Activity at Vakinankaratra was initially considered to be the youngest, perhaps the fourth phase of volcanic activity at Ankaratra, but has over the years it has been treated as a separate field due to its limited geographic coverage and limited type of eruptions, scoria cones, lava flows, tuff rings and maars. These products overlie either the Precambrian basement or basalt flows of the neighboring Ankaratra volcanic field. Erupted products are primarily basanitic with some evolution toward basalts. There are hot springs in several locations for the Vakinankaratra field with temperatures 20 – 50 C.
Eruptive products at this field have only been indirectly dated, with paleosoils dated 36 – 35 Ka above and below a tephra layer near Lake Tritrivakely.
Volcanic landforms demonstrate explosive phreatomagmatic eruptive activity in the field where magma interacts with meteoric water or the groundwater table. These trigger a chain of eruptions with near instantaneous vaporization of large quantities of water as it combines with magma. The explosions produce distinct deposits ranging from proximal coarse-grained to blocky, chaotic explosion breccias, to finer surge and airfall deposits. Contribution of juvenile magmas to the erupted products varies with each locale.
The field is dominated by monogenetic scoria cones, associated lava flows, maar craters and tuff rings. Three of these are identified near the maar crater of Lake Andraikaba. This is the most prominent and the only permanent crater lake. There are two older unnamed tuff rings located 1 km E and 4 km SW of Lake Andraikaba. Lake Tritivakely is NE of Betafo. This is a small crater with an ephemeral marshy lake. The crater is described as a maar crater.
There may be other maar craters and tuff rings that have been erased by more recent volcanic activity such as scoria cones. There are widely spread fine-grained, large clast volcanoclastic sediments of basement rock. These layers extend far beyond the confined eruption range of scoria cones. They mostly overlie paleosoils and are overlain by tephras and their weathered products.
The Lake Andraikiba area is 7 km W of Antisirabe. It is a 1 km diameter maar crater with a permanent lake. While locally described as quite deep, no depth measurements of the lake have been done. The crater is encircled by a ring of volcanoclastic deposits with an average depth of 50 m. The ring has a steep internal slope and a shallow external one. The upper phreatomagmatic unit from this maar dates around 54 ka. The lower unit dates around 102 ka.
The lowest portion of the crater punches thru an older basaltic flow that appears to overly the basement rocks at the time of the eruption. This is in turn overlain with the first unit of volcanoclastic materials, a well-sorted homogeneous sandy material with no juvenile material. This unit is weakly layered. The thickest parts of this unit are generally toward the S and SW of the crater. The layer disappears within 5 km of the crater.
The thickest layer of breccias overlie the first unit. It is generally unsorted and consists of Precambrian basement rocks. Chunks of this up to a meter in diameter are present. Part of these rocks indicate elevated temperature prior to the eruption.
Along the crater rim, the explosion breccia is covered by a more tephra rich pyroclastic deposit measuring up to tens of meters thick. The prevalence of larger chunks of this deposit drop quickly with distance from the crater, disappearing completely less than 700 m. In the distance, these deposits are non-layered, homogeneous and represent a shift of the initial explosive activity to a less violent more strombolian type of eruption.
Lake Tritrivakely is a maar crater some 9 km NE of Betafo. It is a crater some 300 m with an ephemeral lake. Sedimentary infill is estimated at 50 – 90 m. Most of the crater is encircled by a 50 – 80 m basaltic tuff ring. The inner slopes are steep. There are numerous blocks up to a meter in diameter, volcanic bombs and basaltic blocks of similar size found on both the interior and exterior slopes of the maar. Juvenile material is present. The upper unit here dates around 19 ka. The lower unit dates around 32 ka.
There are two separate widespread units of predominantly sandy sized ejecta. The upper unit is 60 cm thick. The lower one is far less structured and is more than a meter thick. Source of these deposits has not yet been determined. The two widespread sandy layers date 40 – 38 and 22 – 17 ka respectively.
The Fizinana and Ampasamihaiky volcanic complexes are 6 km SE and 2 km E of Betafo. The twin scoria cones of the Ampasamihaiky volcanic complex are considered to be the oldest volcanic structures in the region around Betafo. The younger Ivaoko and Antsifotra volcanic complexes are significantly less eroded. Both complexes have basanitic lava flows, with the flow sourced from the main Iavoko scoria cone and extending 8 km mostly to the NW into the Betafo basin.
To the south, the Fizinana volcanic complex is dominated by a number of smaller cones, widely ranging in age. The younger northern one is similar in age to the Iavoko scoria cone. The area also has a number of smaller cones, ranging from far older cones to small secondary vents seemingly the same age as the main Fizinana cones. The complex is basaltic to basanitic, with generally effusive eruptions. Lava flows toward the north where they encounter the Iavoko flows. Fizinana dates around 65 ka. Ampasamihaiky dates around 68 ka.
Most outcrops of these volcanic complexes have at least two units similar to the sandy, basement volcaniclastic deposits described earlier. These units were erupted before the more recent lava flows of the Betafo region, indicating phreatomagmatic volcanic activity took place after an extended period with no volcanic activity. The geographic source of eruptions that produced the sandy basement material are unclear, though it may have been closer to the Fizinana complex than Ampasamihaiky.
Tectonics of Madagascar are complex and can be traced nearly 200 Ma back in the past to the breakup of Gondwana. Madagascar and India pulled away from Africa some 150 Ma. About 90 Ma, the island migrated over the Marion hotspot. This plume-delivered blast of heat was sufficient to trigger voluminous volcanic eruptions and separate Madagascar from India. This passage appears to have delaminated a chunk of underlying crust around 95 Ma which in turn led to uplift in the central portion of the island.
Today, there are three possible spreading centers impacting tectonics of Madagascar. These include the Davie Fracture Zone to the west connecting to the south end of the island which is an extension of the East African Rift System, and a combination of the Dhow Fracture Zone, the Very Large Crude Carrier Fracture Zone, and the Salvage Fracture Zone to the east all three connecting to the north end of the island. Note that these rifting zones are all generally parallel N-S and subject the entire island to generally an E-W extension.
Madagascar is a continental island with a central highland plateau around 1,200 m. The eastern two thirds of the island are metamorphic Precambrian basement. The remaining third mostly to the west is more recent sedimentary and volcanic cover rock. It has substantial seismic activity, with earthquakes greater than M 5.0. From 1983 – 2007, there were 20 quakes per year greater than M 4.0.
A major magmatic episode in late Cenozoic erupted large volcanic fields in central and northern Madagascar. There were local dike swarms near Ankiloaka in the SW part of the island. This magmatism was associated with intracontinental rifting considered to be an extension of the Somalian – African plate boundary migration. Since mid-Miocene, the entire SW edge of the Indian Ocean has undergone an overall E-W extensional regime. This has reactivated the Davie Ridge and created major N-S trending rift graben basins in the central highlands of Madagascar.
There is scattered, frequent seismicity under the island with quakes up to M 5.8. Earthquake focal mechanisms are generally normal faulting. This seismicity is concentrated in the region of the Ankaratra and Itasy volcanic fields and in the immediate vicinity of the Ankay – Alaotra graben system.
The volcanic fields of central Madagascar are examples of monogenetic volcanism connected to tectonic rifting, mantle depressurization and resulting uplift as the island tectonically splits along its long axis. The rifting has allowed mantle fluids to rise though a faulted bedrock to the surface. The oldest activity was flood basalts. Most recent activity has been monogenetic cinder cones, tuff rings and maar formation. There are multiple hot springs in the region indicating reasonably close proximity to magma. While there are no recent observed eruptions, there are oral traditions of recent activity among the native population. These fields are not finished quite yet.
Relation between regional lineament systems and geological structures: implications for understanding structural controls of geothermal system in the volcanic area of Itasy, Madagascar, Andrianaivo and Ramasiarinoro, Apr 2010
The magmatic evolution and genesis of the quaternary basanite – trachyphonolite suite of Itasy (Madagascar) as inferred by geochemisty, Sr-Nb-Pb isotopes and trace element distribution in coexisting phases, Melluso, et al, Jun 2018
From olivine nephelinite, basanite and basalt to peralkaline trachyphonolite and comendite in the Ankaratra volcanic complex, Madagascar: Argon isotope ages, phase compositions and bulk rock geochemical and isotopic evolution, Cucciniello, et al, Mar 2017