**3. Geological analysis of Mazo volcano and deposits**

Mazo is a monogenetic volcano with a relative height of 179 m, resting on a leaning volcanic substrate with a difference in height of 30 m between the highest and the lowest point of its external base. The cone and deposits are partially covered by lavas from historical eruptions, leaving exposed only the highest parts of Mazo deposits. The cone has an irregular shape and a crater with two open depressions aligned in the ENE-WSW direction, with a maximum diameter of 493 m. The main crater, located to the SW, has a funnel shape, 178 m deep inside, with an internal platform on its northern slope elevated 18 m over the bottom (**Figure 2**). The other depression is of bowl type, with an interior depth of 120 m. The rim of this double depression is higher in its southern part (429 m asl), just at the contact between both depressions. From this point, the rim appears lobed towards the NE and SW, gradually decreasing in altitude until reaching a minimum height of 280 m in its NW sector.

**Figure 2.** *Geomorphological (A); slope (B); and roughness maps (C) of Mazo.*

The original cone consisted of welded pyroclastics, lapilli and bombs and some interbedded clastogenic lavas that can be identified in the SSW flank, affected by small fractures. However, most of the cone is formed by a debris avalanche deposit (DAD) that extends towards the NNW and ENE covering an area of 1218 km2 and reaching a maximum distance from the vent of 1.6 km (**Figures 2** and **3**). The thickness of the deposit is difficult to estimate but minimum values of 35 m and 5 m can be assumed for the proximal and the distal area, respectively. The DAD is made of an unconsolidated breccia without stratification. Two main facies are identified: block and mixed facies.

Most of the cone, as well as proximal areas, are composed of block facies characterized by the presence of toreva blocks (**Figures 2** and **3A**–**C**), that are fractured and backtilting blocks that slumped in an almost completely coherent manner [22, 23].

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pyroclastic deposits.

**Figure 3.**

*T: Toreva blocks.*

temperature towards the surface (**Figure 4F**).

*Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano…*

This facies consists of broken, slightly unstructured and staggered pieces of the cone (pyroclastics and clastogenic lavas), attached to the remnant cone, separated by inter-toreva depressions, normal faults and small graben structures (**Figure 2**). At the southeastern flank a set of conjugated faults, which are inserted in a listric fault plane, individualize several toreva blocks (**Figure 3B**). The eastern and northern flanks are also formed by several big blocks made of lapilli and scoria, as well as

*(A) General view of Mazo volcano showing the block facies of the proximal area with toreva blocks in the background, and the distal mixed facies in the foreground. (B) Listric faults and horst-graben structure on the flank. (C) Mixed facies with hummocky topography in between toreva blocks. B: Blast deposit; H: Hummocks;* 

The mixed facies is composed of a poorly sorted deposit, with milimetric to hectometric clasts and megablocks (**Figures 3A, C** and **4**). Most outcrops show a grain supported deposit, but matrix supported is also found (**Figure 4A** and **B**). Clast are mostly polyhedral and polymictic with abundant dense lavas and minor clast of vesicular lavas, welded scoria, weathered hydrovolcanic deposits, calcrete and paleosoils. Blocks and clasts are usually fractured, with frequent jigsaw cracks and slickensides (**Figure 4C**–**E**) or linked to deformation structures in the deposit (**Figure 4G**). Some of them show evidences of thermal alteration displaying a banded sequence of wine, reddish and yellowish colors suggesting decreasing of

From inter-toreva depressions towards the base of the cone, flows characterized by a hummocky surface topography were emplaced (**Figures 2** and **3C**). These flows were formed as toreva blocks break into smaller blocks. To the N and E of the volcanic edifice, lying on a steep slope area, these avalanche deposits consist of several flows with well-defined lobes and steep fronts. They have the highest concentration of hummocks, mostly elongated trending parallel to the flow direction. To the south of the cone there is also a hummocky surface completely covered by Mazo fallout

deposits so it cannot be clearly assigned to this eruption (**Figure 2**).

To the NNW of the volcanic edifice the DAD spreads gently dipping with a roughly surface characterized by small and dispersed hummocks (**Figures 2** and **3A**).

*DOI: http://dx.doi.org/10.5772/intechopen.93882*

*Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano… DOI: http://dx.doi.org/10.5772/intechopen.93882*

### **Figure 3.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

The original cone consisted of welded pyroclastics, lapilli and bombs and some interbedded clastogenic lavas that can be identified in the SSW flank, affected by small fractures. However, most of the cone is formed by a debris avalanche deposit (DAD) that extends towards the NNW and ENE covering an area of 1218 km2 and reaching a maximum distance from the vent of 1.6 km (**Figures 2** and **3**). The thickness of the deposit is difficult to estimate but minimum values of 35 m and 5 m can be assumed for the proximal and the distal area, respectively. The DAD is made of an unconsolidated breccia without stratification. Two main facies are identified:

*Geomorphological (A); slope (B); and roughness maps (C) of Mazo.*

Most of the cone, as well as proximal areas, are composed of block facies characterized by the presence of toreva blocks (**Figures 2** and **3A**–**C**), that are fractured and backtilting blocks that slumped in an almost completely coherent manner [22, 23].

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**Figure 2.**

block and mixed facies.

*(A) General view of Mazo volcano showing the block facies of the proximal area with toreva blocks in the background, and the distal mixed facies in the foreground. (B) Listric faults and horst-graben structure on the flank. (C) Mixed facies with hummocky topography in between toreva blocks. B: Blast deposit; H: Hummocks; T: Toreva blocks.*

This facies consists of broken, slightly unstructured and staggered pieces of the cone (pyroclastics and clastogenic lavas), attached to the remnant cone, separated by inter-toreva depressions, normal faults and small graben structures (**Figure 2**). At the southeastern flank a set of conjugated faults, which are inserted in a listric fault plane, individualize several toreva blocks (**Figure 3B**). The eastern and northern flanks are also formed by several big blocks made of lapilli and scoria, as well as pyroclastic deposits.

The mixed facies is composed of a poorly sorted deposit, with milimetric to hectometric clasts and megablocks (**Figures 3A, C** and **4**). Most outcrops show a grain supported deposit, but matrix supported is also found (**Figure 4A** and **B**). Clast are mostly polyhedral and polymictic with abundant dense lavas and minor clast of vesicular lavas, welded scoria, weathered hydrovolcanic deposits, calcrete and paleosoils. Blocks and clasts are usually fractured, with frequent jigsaw cracks and slickensides (**Figure 4C**–**E**) or linked to deformation structures in the deposit (**Figure 4G**). Some of them show evidences of thermal alteration displaying a banded sequence of wine, reddish and yellowish colors suggesting decreasing of temperature towards the surface (**Figure 4F**).

From inter-toreva depressions towards the base of the cone, flows characterized by a hummocky surface topography were emplaced (**Figures 2** and **3C**). These flows were formed as toreva blocks break into smaller blocks. To the N and E of the volcanic edifice, lying on a steep slope area, these avalanche deposits consist of several flows with well-defined lobes and steep fronts. They have the highest concentration of hummocks, mostly elongated trending parallel to the flow direction. To the south of the cone there is also a hummocky surface completely covered by Mazo fallout deposits so it cannot be clearly assigned to this eruption (**Figure 2**).

To the NNW of the volcanic edifice the DAD spreads gently dipping with a roughly surface characterized by small and dispersed hummocks (**Figures 2** and **3A**).

### **Figure 4.**

*Mixed facies. (A) Grain supported; (B) matrix supported; (C) jig-saw fit cracks; (D) fractured block; (E) slickensides; (F) alteration bands and (G) deformation structures under a block.*

Here the mixed facies are not related to a hummocky terrain being in turn dominated by ridges and a blocky surface. Lateral and frontal levees are also common, being the best defined those surrounding a single isolated mega-block, 120 m in diameter and 43 m high, located 472 m far from the vent (**Figures 2** and **5**). Decametric blocks (< 90 m3 ) outcrop mainly on the distal area.

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**Figure 5.**

*high). LC: Lava cover; SL: Substrate lava.*

*Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano…*

The single isolated mega-block (**Figure 5**) outcropping in this area consists of a stratigraphic sequence of several piled lava flows, hydromagmatic deposits with a paleosoil and a calcrete at the top with terrestrial gastropods, and finally a volcanic spatter deposit. The block is fractured and broken in the distal area and shows an injection of deformed hydromagmatic deposits into the overlying spatter. All these features lead us to interpret it as a substratum block. Part of the block was covered

Several squeeze-up structures (**Figures 2A** and **6**) have also been identified in the distal mixed facies indicating the presence of molten lava during the debris avalanche emplacement. They are made up of massive lava sheets tens of centimeters thick that make thinner and curve towards the top, constituting authentic spines with fluted and wavy surfaces. Squeeze-up are arranged in bands more or less parallel to each other with a curved longitudinal layout, and the convex side arranged in the direction of flow. Trapped between the fingerings of the intrusions there are clasts of the deposit; slickensides are common in the margins of these lava intrusions; and lava fingers have also been injected in between clasts. These structures are located along a zone of slope break suggesting they were formed due to compression processes in the avalanche when adapting to the slope during the

*Isolated substrate megablock to the NNW of Mazo: (A) general view of the megablock inserted in a hummocky topography, indicating the location of the lava cover and B; (B) unstructured part of the megablock with clay injections into a scoria deposit; (C) layered deposits forming the megablock (for scale, the scar is around 10 m* 

*DOI: http://dx.doi.org/10.5772/intechopen.93882*

emplacement.

by molten lava during the debris avalanche emplacement.

### *Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano… DOI: http://dx.doi.org/10.5772/intechopen.93882*

The single isolated mega-block (**Figure 5**) outcropping in this area consists of a stratigraphic sequence of several piled lava flows, hydromagmatic deposits with a paleosoil and a calcrete at the top with terrestrial gastropods, and finally a volcanic spatter deposit. The block is fractured and broken in the distal area and shows an injection of deformed hydromagmatic deposits into the overlying spatter. All these features lead us to interpret it as a substratum block. Part of the block was covered by molten lava during the debris avalanche emplacement.

Several squeeze-up structures (**Figures 2A** and **6**) have also been identified in the distal mixed facies indicating the presence of molten lava during the debris avalanche emplacement. They are made up of massive lava sheets tens of centimeters thick that make thinner and curve towards the top, constituting authentic spines with fluted and wavy surfaces. Squeeze-up are arranged in bands more or less parallel to each other with a curved longitudinal layout, and the convex side arranged in the direction of flow. Trapped between the fingerings of the intrusions there are clasts of the deposit; slickensides are common in the margins of these lava intrusions; and lava fingers have also been injected in between clasts. These structures are located along a zone of slope break suggesting they were formed due to compression processes in the avalanche when adapting to the slope during the emplacement.

### **Figure 5.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

Here the mixed facies are not related to a hummocky terrain being in turn dominated by ridges and a blocky surface. Lateral and frontal levees are also common, being the best defined those surrounding a single isolated mega-block, 120 m in diameter and 43 m high, located 472 m far from the vent (**Figures 2** and **5**). Decametric blocks

*Mixed facies. (A) Grain supported; (B) matrix supported; (C) jig-saw fit cracks; (D) fractured block; (E)* 

) outcrop mainly on the distal area.

*slickensides; (F) alteration bands and (G) deformation structures under a block.*

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(< 90 m3

**Figure 4.**

*Isolated substrate megablock to the NNW of Mazo: (A) general view of the megablock inserted in a hummocky topography, indicating the location of the lava cover and B; (B) unstructured part of the megablock with clay injections into a scoria deposit; (C) layered deposits forming the megablock (for scale, the scar is around 10 m high). LC: Lava cover; SL: Substrate lava.*

### **Figure 6.**

*Squeeze-up structures in the DAD of Mazo: (A) curving pressure ridge; (B) DAD between the fingers of the squeeze-ups; (C) slickensides in a squeeze-up margin; and (D) injection of lava between DAD lithics.*

The DAD is overlaid with a blast deposit in which three main layers have been identified (**Figure 3B** and **7**). The first layer consists of big bombs and blocks up to 40 m3 , composed of fragments of DAD or mafic dense blocks that appear scattered in proximal areas up to a distance of 500 m from the vent. Some of them are broken, split and wrapped in a fine layer of lava. Juvenile breadcrust bombs are also present. The second layer consist of a gray, clast-supported, well sorted and normal grading deposit (gravel to fine sand size) of clasts with parallel lamination up to 84 cm. Content of juvenile fragments is low. Finally, covering all previous deposits and adapting to the topography, a gray to yellow sand, matrix-supported and wavy laminated deposit is observed, being formed by hydromagmatic surges. It is better exposed in proximal areas with a thickness of around 40 cm decreasing to few centimeters in distal areas.

A blast deposit can be easily identified by a light gray layer below the strombolian fallout deposit. This layer can be observed over several cinder cones, at a distance larger than 7 km away from Mazo volcano. Although the blast deposit is distributed in patches, it should have covered the entire area, being better preserved in areas with thermal alteration like those close to the crater and in most of the

**181**

included in **Figure 8A**.

**Figure 7.**

*Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano…*

megablocks and hummocks. The alteration affects both the DAD and the blast deposit that cover them, giving place to yellowish-colored crusts which are broken into sheets in the steepest sectors. Degassing structures are also observed affecting these deposits. At the top of the sequence there is a strombolian fallout deposit, thicker in proximal areas (**Figure 3B**). A simplified stratigraphic column has been

*of native sulfur, gypsum, jarosite, and minor anhydrite (yellow in color).*

*Blast deposit. (A) Broken bomb 500 m far from the vent; (B) core of bomb in* **A** *consisting of DAD; (C) layer two; (D) layer three overlaying an oncoids mound; (E) hydrothermal fluids escape pipes in layer 3; (F) Oncolite structure made up of a lithic nuclei covered by hematite (black in color) and several layers composed* 

In the northern sector of the cone and along the graben fractures near to the crater, there are clear evidences of hydrothermal alteration and fumaroles activity. At this site a mound-type deposit with an external structure similar to a cauliflower covered by hydromagmatic surges is found. This mound is formed by soldered centimetric to decimetric oncoids (**Figure 7F** and **H**), yellow to cream in color; all of them have concentric build-ups around a lithic nucleus. Mineralogy of different laminae are determined by X-ray diffraction method and major chemical analyses were made on the IGME laboratories being their general structure and composition as follows: 1) single or composed subrounded lithic nuclei that have a thin black Fe-hydroxides coating; 2) several (three to six) concentric yellow laminae of native

*DOI: http://dx.doi.org/10.5772/intechopen.93882*

*Syn-Eruptive Lateral Collapse of Monogenetic Volcanoes: The Case of Mazo Volcano… DOI: http://dx.doi.org/10.5772/intechopen.93882*

### **Figure 7.**

*Updates in Volcanology – Transdisciplinary Nature of Volcano Science*

The DAD is overlaid with a blast deposit in which three main layers have been identified (**Figure 3B** and **7**). The first layer consists of big bombs and blocks up

*Squeeze-up structures in the DAD of Mazo: (A) curving pressure ridge; (B) DAD between the fingers of the squeeze-ups; (C) slickensides in a squeeze-up margin; and (D) injection of lava between DAD lithics.*

A blast deposit can be easily identified by a light gray layer below the strombolian fallout deposit. This layer can be observed over several cinder cones, at a distance larger than 7 km away from Mazo volcano. Although the blast deposit is distributed in patches, it should have covered the entire area, being better preserved in areas with thermal alteration like those close to the crater and in most of the

, composed of fragments of DAD or mafic dense blocks that appear scattered in proximal areas up to a distance of 500 m from the vent. Some of them are broken, split and wrapped in a fine layer of lava. Juvenile breadcrust bombs are also present. The second layer consist of a gray, clast-supported, well sorted and normal grading deposit (gravel to fine sand size) of clasts with parallel lamination up to 84 cm. Content of juvenile fragments is low. Finally, covering all previous deposits and adapting to the topography, a gray to yellow sand, matrix-supported and wavy laminated deposit is observed, being formed by hydromagmatic surges. It is better exposed in proximal areas with a thickness of around 40 cm decreasing to few

**180**

to 40 m3

**Figure 6.**

centimeters in distal areas.

*Blast deposit. (A) Broken bomb 500 m far from the vent; (B) core of bomb in* **A** *consisting of DAD; (C) layer two; (D) layer three overlaying an oncoids mound; (E) hydrothermal fluids escape pipes in layer 3; (F) Oncolite structure made up of a lithic nuclei covered by hematite (black in color) and several layers composed of native sulfur, gypsum, jarosite, and minor anhydrite (yellow in color).*

megablocks and hummocks. The alteration affects both the DAD and the blast deposit that cover them, giving place to yellowish-colored crusts which are broken into sheets in the steepest sectors. Degassing structures are also observed affecting these deposits. At the top of the sequence there is a strombolian fallout deposit, thicker in proximal areas (**Figure 3B**). A simplified stratigraphic column has been included in **Figure 8A**.

In the northern sector of the cone and along the graben fractures near to the crater, there are clear evidences of hydrothermal alteration and fumaroles activity. At this site a mound-type deposit with an external structure similar to a cauliflower covered by hydromagmatic surges is found. This mound is formed by soldered centimetric to decimetric oncoids (**Figure 7F** and **H**), yellow to cream in color; all of them have concentric build-ups around a lithic nucleus. Mineralogy of different laminae are determined by X-ray diffraction method and major chemical analyses were made on the IGME laboratories being their general structure and composition as follows: 1) single or composed subrounded lithic nuclei that have a thin black Fe-hydroxides coating; 2) several (three to six) concentric yellow laminae of native

### **Figure 8.**

*(A) Schematic stratigraphic column of Mazo. (B) Minimum area affected by ash dispersion (oval in gray) from Mazo volcano (in green) based on the location of affected villages (red dots). (C) Cartoons showing the main phases of Mazo eruption (see text for explanation).*

sulfur, jarosite, gypsum and anhydrite; 3) a white laminae made of amorphous silica and opal-A. Occasionally, oncoids are composed of Fe-hydroxide coatings, opal-A white laminae and yellow laminae.
