**3.5 The volcanological evolution of the caldera of mount Bambouto**

Mount Bambouto is a Hawaiian shield volcano [18]. Its history has been ruled by volcanic and tectonic events that led to the formation of a huge caldera on the Pan-African granitoid basement [35–37]. The Mount Bambouto Caldera formation (**Figure 11**) included three main stages [38] as follow:

The *Precaldera Stage* (Over 19 Ma) is characterized by the tumescence of the volcanic shield due to magma injection giving rise to several annular fissures observed in the whole volcano.

The *Syncaldera stage* (18–15.28 Ma) is materialized by two features: firstly, explosive eruptions are responsible for scoria, ignimbrites, trachytes and rhyolites; secondly, piecemeal intravolcanic collapse of the magmatic chamber roof is followed by the protrusion of trachytic domes and some basaltic supplies.

The *Postcaldera stage* (15–0.5 Ma) is typified by some trachytic and basaltic supplies and the protrusion of phonolitic domes. Activity ends with the explosive eruptions on the northeastern flank of the volcano where is built the multiple scoria cones.

### **3.6 Classification of calderas**

### *3.6.1 Location*

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

majority. In ignimbrites, the composition of alkali feldspars is between Or33 and

The lavas in the caldera of Mount Bambouto are alkaline in nature as shown in the following diagrams in **Figure 10**. The data used to make these diagrams have

Or37 and are therefore exclusively anorthoses.

*Evolution of the anorthite content in lavas of the Mount Bambouto caldera.*

**3.4 Classification of lava in the study areas**

been supplemented by the data in [20, 22].

*Chemical nature of lavas in the Mount Bambouto caldera.*

**288**

**Figure 10.**

**Figure 9.**

To assign the code of a given caldera, one must use the numbering system developed in the Catalog of Active Volcanoes of the World. In fact, the world is divided in 19 main regions that are subdivided, in turn, in several subregions. Hence, the study area is located in the African Region with the corresponding database code 2. In addition, these calderas are located in the Central African Sub-Region with the corresponding database code 203.

**Figure 11.** *Sketch highlighting the stages of the formation of the Mount Bambouto caldera.*

### *3.6.2 Ramparts and type/geometry of collapse*

In the caldera ramparts are almost vertical at some levels (**Figure 3**); but on the whole these ramparts seem to merge with the floor.

At the level of the Mount Bambouto, the floor of the caldera is very dissected and presents in places a stepped structure (**Figure 3**), which indicates a piecemeal collapse.

### *3.6.3 Petrographic types*

Several petrographic types are observed in the study area. These petrographic types are dominated by basalts, intermediate rocks, trachytes, phonolites and ignimbrites. Thus, the caldera of the Mount Bambouto is assigned the code B, I, T, P and Ig.

### *3.6.4 Magmatic series*

The study area is characterized by an alkaline magmatic series as they are dominated by mafic, intermediate, and felsic terms. They are assigned the codes ALKAf (Alkaline felsic), ALKAi (Alkaline intermediate) and ALKAm (Alkaline mafic).

### *3.6.5 Crust type and tectonic setting*

Mount Bambouto rests on a granito-gneissic bedrock with a thickness (hc) of about 35.5 km [39]. According to the Database [14], these crustal thicknesses in the study areas are greater than the 30–35 km interval; hence the code is C.

From the internal geodynamic point of view, the Mount Bambouto Caldera is located in the Cameroon Volcanic Line which originates, according to some authors [40–43], from a Continental Rift; Its code is be RC. This nascent rift [44, 45], at the origin of the Cameroon Volcanic Line in general and of Mount Bambouto and its respective caldera system in particular, is of the extensional type and their code is EXT.

### *3.6.6 Pre-caldera volcanism*

A Pre-caldera regional dome occurred through a tumescence that created numerous concentric faults. These fissures favored a pre-caldera magmatic activity that further contributed to the building of the Bambouto stratovolcano. Its code is therefore STR.

### *3.6.7 Caldera collapse period and post-caldera volcanic activity*

The collapse of the Caldera of Mount Bambouto occurred at the beginning of the sequence of eruptions that contributed to their formation. Their code is A.

In the Mount Bambouto, this volcanic activity is dominated by the presence of several eruptive vents, notably on the ramparts, the eastern floor of the caldera and the NE slope of the volcano. Thus, the Mount Bambouto is classified as Type-S and Type-MS.

### *3.6.8 Preservation of the caldera*

On the other hand, the ramparts of the Mount Bambouto Caldera are threatened by growing urbanization and agro-pastoral activity, particularly to the south and east of the caldera. Its boundaries are therefore slightly destroyed. Its code is PD.

**291**

**4. Discussion**

*Classification of the Mount Bambouto caldera in the CCDB of [10].*

**Table 1.**

*The Caldera of Mount Bambouto: Volcanological Characterization and Classification*

**Collapse caldera database Criteria Data**

Surface (km2

Caldera volume (km3

Volume of deposits (km3

Total volume of lavas (km3

Latitude 05°37′–05°44′N Longitude 09°57′–10°07′E

Maximum Caldera diameter Not Applicable Minimum Caldera diameter Not Applicable

) 155.1

) —

) —

) —

Magmatic series ALKAm, ALKAi, ALKAf

Region 2 Subregion 203 Age (Ma) 15

Subsidence —

Thickness of deposits —

Type of collapse Piecemeal Name linked to the deposits Ignimbritic

Petrographic types B, I, T, P

Periods of pre-caldera doming (PCD) Over 19 Ma Type of pre-caldera volcanism (PCV) STR Timing of caldera onset (TCO) A Post-caldera volcanic activity (PCVA) S, MS Post-caldera resurgence (PCR) Absence Caldera preservation (CPR) PD

Magmatic chamber depth (km) 35.5 Ratio depth/width of magmatic chamber — Plate tectonic setting (PTS) RC Crustal type (CT) C Type of tectonic faulting (TF) Ext

Through the mode of outcropping of different rocks in the caldera of Mount Bambouto, all types of dynamism (extrusive, effusive, explosive) exist in the caldera. These are therefore polygenic volcanoes marked by long periods of activity and varied dynamisms, resting and erosion phases during different tectonic episodes [46]. In addition, the diversity of rocks is indicative of the high degree of magma differentiation induced here by the fractional crystallization process [21, 47, 48]. The presence of the trachytes in ignimbrites of the study area

*DOI: http://dx.doi.org/10.5772/intechopen.93694*

The overall results have been used to fill the CCDB table (**Table 1**).


*The Caldera of Mount Bambouto: Volcanological Characterization and Classification DOI: http://dx.doi.org/10.5772/intechopen.93694*

### **Table 1.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

In the caldera ramparts are almost vertical at some levels (**Figure 3**); but on the

At the level of the Mount Bambouto, the floor of the caldera is very dissected and presents in places a stepped structure (**Figure 3**), which indicates a piecemeal collapse.

Several petrographic types are observed in the study area. These petrographic types are dominated by basalts, intermediate rocks, trachytes, phonolites and ignimbrites. Thus, the caldera of the Mount Bambouto is assigned the code B, I, T, P and Ig.

The study area is characterized by an alkaline magmatic series as they are dominated by mafic, intermediate, and felsic terms. They are assigned the codes ALKAf (Alkaline felsic), ALKAi (Alkaline intermediate) and ALKAm (Alkaline

Mount Bambouto rests on a granito-gneissic bedrock with a thickness (hc) of about 35.5 km [39]. According to the Database [14], these crustal thicknesses in the

From the internal geodynamic point of view, the Mount Bambouto Caldera is located in the Cameroon Volcanic Line which originates, according to some authors [40–43], from a Continental Rift; Its code is be RC. This nascent rift [44, 45], at the origin of the Cameroon Volcanic Line in general and of Mount Bambouto and its respective caldera system in particular, is of the extensional type and their

A Pre-caldera regional dome occurred through a tumescence that created numerous concentric faults. These fissures favored a pre-caldera magmatic activity that further contributed to the building of the Bambouto stratovolcano. Its code is

The collapse of the Caldera of Mount Bambouto occurred at the beginning of the

In the Mount Bambouto, this volcanic activity is dominated by the presence of several eruptive vents, notably on the ramparts, the eastern floor of the caldera and the NE slope of the volcano. Thus, the Mount Bambouto is classified as Type-S and Type-MS.

On the other hand, the ramparts of the Mount Bambouto Caldera are threatened by growing urbanization and agro-pastoral activity, particularly to the south and east of the caldera. Its boundaries are therefore slightly destroyed. Its code is PD.

sequence of eruptions that contributed to their formation. Their code is A.

The overall results have been used to fill the CCDB table (**Table 1**).

*3.6.7 Caldera collapse period and post-caldera volcanic activity*

study areas are greater than the 30–35 km interval; hence the code is C.

*3.6.2 Ramparts and type/geometry of collapse*

*3.6.3 Petrographic types*

*3.6.4 Magmatic series*

*3.6.5 Crust type and tectonic setting*

mafic).

code is EXT.

therefore STR.

*3.6.6 Pre-caldera volcanism*

*3.6.8 Preservation of the caldera*

whole these ramparts seem to merge with the floor.

**290**

*Classification of the Mount Bambouto caldera in the CCDB of [10].*

## **4. Discussion**

Through the mode of outcropping of different rocks in the caldera of Mount Bambouto, all types of dynamism (extrusive, effusive, explosive) exist in the caldera. These are therefore polygenic volcanoes marked by long periods of activity and varied dynamisms, resting and erosion phases during different tectonic episodes [46]. In addition, the diversity of rocks is indicative of the high degree of magma differentiation induced here by the fractional crystallization process [21, 47, 48]. The presence of the trachytes in ignimbrites of the study area is an indicator of a relative chronology of the rocks. Indeed, there was an anteignimbritic trachytic volcanic phase. This means that there has been in the course of the evolution of the Bambouto volcano, the eruption of trachytic rocks before that of ignimbritic materials [49, 50].

The caldera of Mount Bambouto was formed at a well-defined time. The stages of formation of these calderas correspond globally to the model of [9]: a regional tumescence, a volcanic eruption, a collapse of the caldera, volcanism on the annular fractures and sedimentation. The present structure of the caldera floor shows that the roof of the magma chamber collapsed piecemeal during its formation. The border faults generally observed on calderas in certain volcanic environments in Cameroon, which are evidence of the different phases of caldera collapse, are difficult to observe in the caldera of Mount Bambouto. These faults, when identifiable on certain ramparts, present a some stages of collapse (**Figure 3**). In this caldera, the ramparts are often confused with the floor. The latter constitutes the most dissected floor of all the caldera units studied along the Cameroon Volcanic Line and their arrangement in decreasing steps from west to east, would testify to the multiple collapses that marked its formation [51, 52]. On the other hand, in the Eboga and Lefo calderas, where the ramparts are clearly visible from the caldera floor, there are boundary faults marked by about 2–4 stages of collapse [29]. Post-caldera volcanism has manifested itself on the Mount Bambouto. This has been observed in other caldera environments on the Cameroon Volcanic Line, notably the Santa-Mbu and Lefo caldera in the Bamenda Mountains, the Eboga and Elengoum calderas in Mount Manengouba and the Bangou caldera in Mount Bangou. It is at the origin of numerous doleritic, phonolitic and trachytic protrusions and, cones and maars found on the floor and external slopes of these calderas [33, 51–54]. These post-caldera geomorphological units give the caldera of Mount Bambouto the S and MS types according to [14]. Mount Bambouto constitutes a stratovolcano [20, 33]. The shape of this caldera is comparable to the elliptical shape of the calderas of Suswa, Kenya [55] and Chã das of Fogo Island in Cape Verde [56] and the calderas of the basaltic shields described by [9, 57]. This shape results from the geometry of the magma chamber which is the main factor controlling the final morphology of the calderas [58]. The presence of ignimbrites, tuffs, trachytes and rhyolites in the caldera of the Mount Bambouto qualifies it as an ignimbrite caldera. Ignimbrite calderas are usually over 10 km in diameter and over 1 km in depth, formed after the voluminous deposition of silicic ignimbrites [9, 11, 59]. We can list the example of Batur Caldera in Bali, New Zealand [60]. However, the term ignimbrite caldera is clearly used by (2015) [61] to qualify the calderas of the Southern Rocky Mountain Volcanic Field in Colorado (USA) notably Bonanza, Bachelor, Cochetopa Park, Creede, and Platoro calderas. Their presence in Mount Bambouto is explained by the fact that, considering the ages, this massif is sufficiently old compared to the other massifs, especially Mount Manengouba, because these acid magmas, according to [62], require a significant period of time for their formation to be elaborated.

Calderas are places where several natural hazards occur, including volcanic eruptions and mass movements [63, 64]. According to [65], calderas are destructive volcanic forms because they cause pre-existing reliefs to collapse, unlike postcaldera cones and domes, which are constructive because pre-existing reliefs are put in place. Moreover, the volcanic formations that cover them favor the formation of fertile soils and the development of a plant cover of various species conducive to an agropastoral activity [16]. These are environments where hydrothermal activities and mineralization processes generally occur [57, 66, 67]. In this respect, it is clear that calderas have a strong educational value as they allow us to understand the complexity of certain craters in volcanic environments around the world. As

**293**

**Author details**

**5. Conclusion**

Ghislain Zangmo Tefogoum1

of Maroua, Maroua, Cameroon

provided the original work is properly cited.

Armand Kagou Dongmo2

Dschang, Cameroon

*The Caldera of Mount Bambouto: Volcanological Characterization and Classification*

such, they allow us to understand the degree of fracturing of the ante-caldera substratum, the superposition of eruptive products and the slices of the flows at the ramparts and the post-eruptive geological processes. For this reason, calderas have been the subject of several studies in the field of geological heritage, notably the Mount Teide caldera in Spain, Aso caldera in Japan; Santorini caldera in Greece; Erta Alé and Fentale caldera in Ethiopia; Cha Das caldera in Cape Verde; Eboga, Santa Mbu, Lefo and Bambouto caldera in Cameroon [17, 68–72]. Thus, calderas are often the seat of later volcanic activities that leave exceptional geomorphologi-

cal units with several values suitable for geotourism [33, 51, 52, 56, 73–75].

The Caldera of Mount Bambouto is a volcanic unit that formed at a period between 18.68 and 22 Ma. Its emplacement model is comparable to that of Cole et al. 2005. Its formation and evolution gave it a rather varied petography and a characteristic structure. Its classification according to the Caldeira DataBase of Geyer and Marti (2008) allows us to conclude that its type of collapse is piecemeal. Chemically, the caldera is alkaline with codes ALKAf, ALKAi, and ALKAm. Furthermore, this caldera was formed through a continental rifting of extensional type, and their postcaldera protrusions give them Type-S and Type-MS. Moreover, it is a well-preserved caldera because its ridge lines are well observable. The classification of the caldera of Mount Bambouto made within the framework of this work makes it possible to understand the similarities of this caldera with other calderas around the world on the one hand and to understand part of the global dynamics of the functioning of the Cameroon Volcanic Line on the other hand. Furthermore, this study contributes to elucidate the origin of the Cameroon Volcanic Line, which is still a subject of discussion among Cameroonian and foreign researchers today. Moreover, through this work, the Mount Bambouto Caldera is promoted next to the world scientific community that is still ignoring his existence.

\*, David Guimolaire Nkouathio2

and Merlin Gountié Dedzo3

1 Department of Earth Sciences, University of Maroua, Maroua, Cameroon

2 Department of Earth Sciences, Faculty of Sciences, University of Dschang,

\*Address all correspondence to: zangmotefogoum@gmail.com

3 Department of Life and Earth Science, High Teacher Training College, University

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

,

*DOI: http://dx.doi.org/10.5772/intechopen.93694*

*The Caldera of Mount Bambouto: Volcanological Characterization and Classification DOI: http://dx.doi.org/10.5772/intechopen.93694*

such, they allow us to understand the degree of fracturing of the ante-caldera substratum, the superposition of eruptive products and the slices of the flows at the ramparts and the post-eruptive geological processes. For this reason, calderas have been the subject of several studies in the field of geological heritage, notably the Mount Teide caldera in Spain, Aso caldera in Japan; Santorini caldera in Greece; Erta Alé and Fentale caldera in Ethiopia; Cha Das caldera in Cape Verde; Eboga, Santa Mbu, Lefo and Bambouto caldera in Cameroon [17, 68–72]. Thus, calderas are often the seat of later volcanic activities that leave exceptional geomorphological units with several values suitable for geotourism [33, 51, 52, 56, 73–75].
