OR/14/005 Mt Meru case study - Revision history

Ajhil: /* Field study */

2021-10-20T08:35:05Z

Field study

← Older revision		Revision as of 09:35, 20 October 2021
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	Yellow triangles show the locations of samples collected by BGS on October 2013 as part of this study. Blue circles are sample locations from Roberts (2002)<ref name="Roberts"></ref>.		Yellow triangles show the locations of samples collected by BGS on October 2013 as part of this study. Blue circles are sample locations from Roberts (2002)<ref name="Roberts"></ref>.

	Roberts (2002)<ref name="Roberts 2002"></ref> proposed that the tephra fallout deposits form part of a large ‘climatic’ eruption, based on the similarities in field characteristics of the deposits, and the whole-rock geochemistry. When plotted on a TAS diagram the deposits reveal a clear fractionation trend, which Roberts (2002) interpreted as the emptying of a large, fractionated magma chamber. He calculated the volume of such an eruption, taking the combined thicknesses of the units, as 2.1 km<sup>3</sup>.		Roberts (2002)<ref name="Roberts 2002">ROBERTS, M A. 2002. The geochemical and volcanological evolution of the Mt Meru Region, Northern Tanzania. PhD Thesis, ''University of Cambridge''.</ref> proposed that the tephra fallout deposits form part of a large ‘climatic’ eruption, based on the similarities in field characteristics of the deposits, and the whole-rock geochemistry. When plotted on a TAS diagram the deposits reveal a clear fractionation trend, which Roberts (2002) interpreted as the emptying of a large, fractionated magma chamber. He calculated the volume of such an eruption, taking the combined thicknesses of the units, as 2.1 km<sup>3</sup>.

	===Tephra Sections===		===Tephra Sections===

Ajhil: /* Tephra Samples */

2021-10-20T08:33:48Z

Tephra Samples

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	\| 239314		\| 239314
	\| 9635172		\| 9635172
	\| ~~<p align="center">~~Block of consolidated pyroclastic flow material. Light grey colour with lithic and pumice fragments. Overlying TANZ_2a~~</p>~~		\| Block of consolidated pyroclastic flow material. Light grey colour with lithic and pumice fragments. Overlying TANZ_2a
	\|-		\|-
	\| TANZ_2c		\| TANZ_2c

Ajhil: /* Tephra Samples */

2021-10-20T08:33:19Z

Tephra Samples

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	<center>		<center>
	{\| class="wikitable"		{\| class="wikitable"
	\|+ Table 5 BGS sample descriptions.		\|+ Table 5    BGS sample descriptions.
	\| '''Sample'''		\| '''Sample'''
	\| '''Locality'''		\| '''Locality'''

Ajhil: /* Tephra Sections */

2021-10-20T08:32:35Z

Tephra Sections

← Older revision		Revision as of 09:32, 20 October 2021
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	The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.		The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.

	[[Image:14005 fig9.jpg\|thumb\|center\|~~600px~~\|'''Figure 9'''    Locality 5 stratigraphic log with field photographs. Sample descriptions are in Table 1.]]		[[Image:14005 fig9.jpg\|thumb\|center\|500px\|'''Figure 9'''    Locality 5 stratigraphic log with field photographs. Sample descriptions are in Table 1.]]

	===Tephra Samples===		===Tephra Samples===

Ajhil: /* Tephra Sections */

2021-10-20T08:32:12Z

Tephra Sections

← Older revision		Revision as of 09:32, 20 October 2021
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	The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.		The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.

	[[Image:14005 fig9.jpg\|thumb\|center\|~~300px~~\|'''Figure 9'''    Locality 5 stratigraphic log with field photographs. Sample descriptions are in Table 1.]]		[[Image:14005 fig9.jpg\|thumb\|center\|600px\|'''Figure 9'''    Locality 5 stratigraphic log with field photographs. Sample descriptions are in Table 1.]]

	===Tephra Samples===		===Tephra Samples===

Ajhil: /* Tephra Sections */

2021-10-20T08:31:42Z

Tephra Sections

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	Locality 2 is further up the same riverbed towards Mt Meru. The river valley is much deeper here (~5 m). Exposed near the top of the channel walls is a ~50 cm thick, unconsolidated ash-supported deposit that contains abundant rounded cream pumice fragments. Overlying this deposit is a ~1.5 m thick reworked deposit, possibly a lahar, with very large blocks (up to 1 m) of lithics and consolidated pyroclastic flow material (Figure 6).		Locality 2 is further up the same riverbed towards Mt Meru. The river valley is much deeper here (~5 m). Exposed near the top of the channel walls is a ~50 cm thick, unconsolidated ash-supported deposit that contains abundant rounded cream pumice fragments. Overlying this deposit is a ~1.5 m thick reworked deposit, possibly a lahar, with very large blocks (up to 1 m) of lithics and consolidated pyroclastic flow material (Figure 6).

	[[Image:14005 fig7.jpg\|thumb\|center\|600px\|'''Figure 7'''    Field photographs of Locality 3: the road-cut reveals numerous interbedded ash- rich reworked, and pumice fall deposits.]]		[[Image:14005 fig7.jpg\|thumb\|center\|600px\|'''Figure 7'''    Field photographs of Locality 3: the road-cut reveals numerous interbedded ash-rich reworked, and pumice fall deposits.]]

	Locality 3 is on the edge of the north-western slope of Mt Meru, through a Masaai village just south of the main Arusha-Nairobi road. A series of tephra fallout and reworked pyroclastic deposits are exposed in a road-cut through a topographic high (or mound) (Figure 7). It appears as though the pyroclastic material is draping a topographic feature such as a monogenetic cone, of which there are many surrounding and on the flanks of Mt Meru. At least three tephra fall deposits were identified, all of which are pumice-supported with abundant lithics (Figure 7). The pumice is very light grey/cream in colour, and up to 2 cm along the long-axis. These deposits are all between 10 and 15 cm thick. Interbedded between the pumice fall deposits are ash-supported, pyroclastic flow deposits with reworked pumice and lithic fragments. These deposits vary in thickness from <1 m to >2 m.		Locality 3 is on the edge of the north-western slope of Mt Meru, through a Masaai village just south of the main Arusha-Nairobi road. A series of tephra fallout and reworked pyroclastic deposits are exposed in a road-cut through a topographic high (or mound) (Figure 7). It appears as though the pyroclastic material is draping a topographic feature such as a monogenetic cone, of which there are many surrounding and on the flanks of Mt Meru. At least three tephra fall deposits were identified, all of which are pumice-supported with abundant lithics (Figure 7). The pumice is very light grey/cream in colour, and up to 2 cm along the long-axis. These deposits are all between 10 and 15 cm thick. Interbedded between the pumice fall deposits are ash-supported, pyroclastic flow deposits with reworked pumice and lithic fragments. These deposits vary in thickness from <1 m to >2 m.
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	Locality 4 is in a steep-sided river valley, ~50 m along the road from Locality 3. The gorge is ~20 m deep with clearly visible grey tephra fall deposits interbedded with yellow and pale cream ash-rich units (Figure 8). Near the top of the gorge is a rubbly a’a lava flow. The lava comprises a very dark glassy groundmass with abundant phenocrysts of feldspar, quartz and hornblende.		Locality 4 is in a steep-sided river valley, ~50 m along the road from Locality 3. The gorge is ~20 m deep with clearly visible grey tephra fall deposits interbedded with yellow and pale cream ash-rich units (Figure 8). Near the top of the gorge is a rubbly a’a lava flow. The lava comprises a very dark glassy groundmass with abundant phenocrysts of feldspar, quartz and hornblende.

	[[Image:14005 fig8.jpg\|thumb\|center\|~~600px~~\|'''Figure 8'''    Locality 4 stratigraphic section with field photographs. Sample descriptions are in Table 1.]]		[[Image:14005 fig8.jpg\|thumb\|center\|500px\|'''Figure 8'''    Locality 4 stratigraphic section with field photographs. Sample descriptions are in Table 1.]]

	The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.		The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.

Ajhil: /* Tephra Sections */

2021-10-20T08:31:17Z

Tephra Sections

← Older revision		Revision as of 09:31, 20 October 2021
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	As part of this study, tephra deposits were described and samples were collected at five localities to the west of Mt Meru (Figure 5). Locality 1 is in a dry river bed at the village of Kiushian, just off the main Arusha–Nairobi road. Within the dry riverbed, a ~1.5 m thick consolidated ash deposit with abundant pumice and lithics is exposed (Figure 6). Average clast sizes are between 1 and 5 cm, with rare larger clasts up to 30 cm. The deposit has internal structure with layers of more concentrated lithic and pumice fragments.		As part of this study, tephra deposits were described and samples were collected at five localities to the west of Mt Meru (Figure 5). Locality 1 is in a dry river bed at the village of Kiushian, just off the main Arusha–Nairobi road. Within the dry riverbed, a ~1.5 m thick consolidated ash deposit with abundant pumice and lithics is exposed (Figure 6). Average clast sizes are between 1 and 5 cm, with rare larger clasts up to 30 cm. The deposit has internal structure with layers of more concentrated lithic and pumice fragments.

	[[Image:14005 fig6.jpg\|thumb\|center\|~~500px~~\|'''Figure 6'''    Field photographs of Locality 1 and Locality 2.]]		[[Image:14005 fig6.jpg\|thumb\|center\|600px\|'''Figure 6'''    Field photographs of Locality 1 and Locality 2.]]

	Locality 2 is further up the same riverbed towards Mt Meru. The river valley is much deeper here (~5 m). Exposed near the top of the channel walls is a ~50 cm thick, unconsolidated ash-supported deposit that contains abundant rounded cream pumice fragments. Overlying this deposit is a ~1.5 m thick reworked deposit, possibly a lahar, with very large blocks (up to 1 m) of lithics and consolidated pyroclastic flow material (Figure 6).		Locality 2 is further up the same riverbed towards Mt Meru. The river valley is much deeper here (~5 m). Exposed near the top of the channel walls is a ~50 cm thick, unconsolidated ash-supported deposit that contains abundant rounded cream pumice fragments. Overlying this deposit is a ~1.5 m thick reworked deposit, possibly a lahar, with very large blocks (up to 1 m) of lithics and consolidated pyroclastic flow material (Figure 6).

	[[Image:14005 fig7.jpg\|thumb\|center\|~~500px~~\|'''Figure 7'''    Field photographs of Locality 3: the road-cut reveals numerous interbedded ash- rich reworked, and pumice fall deposits.]]		[[Image:14005 fig7.jpg\|thumb\|center\|600px\|'''Figure 7'''    Field photographs of Locality 3: the road-cut reveals numerous interbedded ash- rich reworked, and pumice fall deposits.]]

	Locality 3 is on the edge of the north-western slope of Mt Meru, through a Masaai village just south of the main Arusha-Nairobi road. A series of tephra fallout and reworked pyroclastic deposits are exposed in a road-cut through a topographic high (or mound) (Figure 7). It appears as though the pyroclastic material is draping a topographic feature such as a monogenetic cone, of which there are many surrounding and on the flanks of Mt Meru. At least three tephra fall deposits were identified, all of which are pumice-supported with abundant lithics (Figure 7). The pumice is very light grey/cream in colour, and up to 2 cm along the long-axis. These deposits are all between 10 and 15 cm thick. Interbedded between the pumice fall deposits are ash-supported, pyroclastic flow deposits with reworked pumice and lithic fragments. These deposits vary in thickness from <1 m to >2 m.		Locality 3 is on the edge of the north-western slope of Mt Meru, through a Masaai village just south of the main Arusha-Nairobi road. A series of tephra fallout and reworked pyroclastic deposits are exposed in a road-cut through a topographic high (or mound) (Figure 7). It appears as though the pyroclastic material is draping a topographic feature such as a monogenetic cone, of which there are many surrounding and on the flanks of Mt Meru. At least three tephra fall deposits were identified, all of which are pumice-supported with abundant lithics (Figure 7). The pumice is very light grey/cream in colour, and up to 2 cm along the long-axis. These deposits are all between 10 and 15 cm thick. Interbedded between the pumice fall deposits are ash-supported, pyroclastic flow deposits with reworked pumice and lithic fragments. These deposits vary in thickness from <1 m to >2 m.
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	Locality 4 is in a steep-sided river valley, ~50 m along the road from Locality 3. The gorge is ~20 m deep with clearly visible grey tephra fall deposits interbedded with yellow and pale cream ash-rich units (Figure 8). Near the top of the gorge is a rubbly a’a lava flow. The lava comprises a very dark glassy groundmass with abundant phenocrysts of feldspar, quartz and hornblende.		Locality 4 is in a steep-sided river valley, ~50 m along the road from Locality 3. The gorge is ~20 m deep with clearly visible grey tephra fall deposits interbedded with yellow and pale cream ash-rich units (Figure 8). Near the top of the gorge is a rubbly a’a lava flow. The lava comprises a very dark glassy groundmass with abundant phenocrysts of feldspar, quartz and hornblende.

	[[Image:14005 fig8.jpg\|thumb\|center\|~~300px~~\|'''Figure 8'''    Locality 4 stratigraphic section with field photographs. Sample descriptions are in Table 1.]]		[[Image:14005 fig8.jpg\|thumb\|center\|600px\|'''Figure 8'''    Locality 4 stratigraphic section with field photographs. Sample descriptions are in Table 1.]]

	The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.		The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.

Ajhil: /* Field study */

2021-10-20T08:30:34Z

Field study

Ajhil: /* Remote Sensing Image Data */

2021-10-20T08:27:53Z

Remote Sensing Image Data

← Older revision		Revision as of 09:27, 20 October 2021
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	<center>		<center>
	{\| class="wikitable"		{\| class="wikitable"
	\|+Table 4    Spectral and spatial characteristics of the Landsat TM and ETM+ sensors (from [http://landsat.usgs.gov/band_designations_landsat_satellites.php) http://landsat.usgs.gov/band_designations_landsat_satellites.php).]		\|+Table 4    Spectral and spatial characteristics of the Landsat TM and ETM+ sensors<br>(from [http://landsat.usgs.gov/band_designations_landsat_satellites.php) http://landsat.usgs.gov/band_designations_landsat_satellites.php).]
	\| Sensor		\| Sensor
	\| Band		\| Band

Ajhil: /* Remote Sensing Image Data */

2021-10-20T08:27:30Z

Remote Sensing Image Data

← Older revision		Revision as of 09:27, 20 October 2021
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	Remote Sensing image data were obtained from the USGS Global Visualisation Viewer ([http://glovis.usgs.gov/ http://glovis.usgs.gov]) when minimal cloud conditions prevailed over Mt Meru and the surrounding area. This data source provides global image data free-of-charge from specific satellite sensors, such as the TM (Thematic Mapper) onboard the Landsat-4/-5 satellite platforms and the ETM+ (Enhanced Thematic Mapper Plus) onboard the Landsat-7 satellite platform. These are spectral imaging sensors that measure the amount of solar radiation reflected by the ground surface across specific spectral wavebands. Data from both sensors were used in this study and their spectral and spatial characteristics are detailed in Table 4.		Remote Sensing image data were obtained from the USGS Global Visualisation Viewer ([http://glovis.usgs.gov/ http://glovis.usgs.gov]) when minimal cloud conditions prevailed over Mt Meru and the surrounding area. This data source provides global image data free-of-charge from specific satellite sensors, such as the TM (Thematic Mapper) onboard the Landsat-4/-5 satellite platforms and the ETM+ (Enhanced Thematic Mapper Plus) onboard the Landsat-7 satellite platform. These are spectral imaging sensors that measure the amount of solar radiation reflected by the ground surface across specific spectral wavebands. Data from both sensors were used in this study and their spectral and spatial characteristics are detailed in Table 4.

			<center>
	{\| class="wikitable"		{\| class="wikitable"
	\|+Table 4: Spectral and spatial characteristics of the Landsat TM and ETM+ sensors (from [http://landsat.usgs.gov/band_designations_landsat_satellites.php) http://landsat.usgs.gov/band_designations_landsat_satellites.php).]		\|+Table 4    Spectral and spatial characteristics of the Landsat TM and ETM+ sensors (from [http://landsat.usgs.gov/band_designations_landsat_satellites.php) http://landsat.usgs.gov/band_designations_landsat_satellites.php).]
	\| Sensor		\| Sensor
	\| Band		\| Band
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	\| <p align="center">30</p> <p align="center">30</p> <p align="center">30</p> <p align="center">30</p> <p align="center">30</p> <p align="center">60</p> <p align="center">30</p> <p align="center">15</p>		\| <p align="center">30</p> <p align="center">30</p> <p align="center">30</p> <p align="center">30</p> <p align="center">30</p> <p align="center">60</p> <p align="center">30</p> <p align="center">15</p>
	\|}		\|}
			</center>

	Level 1G terrain-corrected TM data were obtained for scene Path 168/Row 062 for 25 February 1987 (NASA, 1987a<ref name="NASA 1987a">NASA, 1987a. Landsat TM scene LT51680621987056XXX01, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 25/02/1987 </ref>), 1 June 1987 (NASA, 1987b<ref name="NASA 1987b">NASA, 1987b. Landsat TM scene LT51680621987152XXX01, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 01/06/1987 </ref>) and 17 February 1993 (NASA, 1993<ref name="NASA 1993">NASA, 1993. Landsat TM scene LT41680621993048XXX02, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 17/02/1993 </ref>). Level 1G terrain-corrected SLC-on (Scan Line Corrector-on) ETM+ data were obtained for the same scene for 14 September 1993 (NASA, 1999<ref name="NASA 1999">NASA, 1999. Landsat ETM+ scene LE71680621999257SGS03, SLC-on, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 14/09/1999 </ref>) and 21 February 2000 (NASA, 2000<ref name="NASA 2000">NASA, 2000. Landsat ETM+ scene LE71680622000052EDC00, SLC-on, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 21/02/2000 </ref>). A series of images were required in order that the peak and flanks of Mt Meru were fully exposed at some point during the time series as well as to minimise vegetation cover for mapping purposes (Figure 2).		Level 1G terrain-corrected TM data were obtained for scene Path 168/Row 062 for 25 February 1987 (NASA, 1987a<ref name="NASA 1987a">NASA, 1987a. Landsat TM scene LT51680621987056XXX01, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 25/02/1987 </ref>), 1 June 1987 (NASA, 1987b<ref name="NASA 1987b">NASA, 1987b. Landsat TM scene LT51680621987152XXX01, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 01/06/1987 </ref>) and 17 February 1993 (NASA, 1993<ref name="NASA 1993">NASA, 1993. Landsat TM scene LT41680621993048XXX02, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 17/02/1993 </ref>). Level 1G terrain-corrected SLC-on (Scan Line Corrector-on) ETM+ data were obtained for the same scene for 14 September 1993 (NASA, 1999<ref name="NASA 1999">NASA, 1999. Landsat ETM+ scene LE71680621999257SGS03, SLC-on, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 14/09/1999 </ref>) and 21 February 2000 (NASA, 2000<ref name="NASA 2000">NASA, 2000. Landsat ETM+ scene LE71680622000052EDC00, SLC-on, Orthorectified, Terrain-Corrected. Image courtesy of U.S. Geological Survey. 21/02/2000 </ref>). A series of images were required in order that the peak and flanks of Mt Meru were fully exposed at some point during the time series as well as to minimise vegetation cover for mapping purposes (Figure 2).

	[[Image:14005 fig2.jpg\|thumb\|center\|500px\|'''Figure 2''' Overview natural colour composite imagery for calibrated Landsat Path 198/ Row 062 scenes for a) 25/02/1987, b) 01/06/1987, c) 17/02/1993, d) 14/09/1999 and e) 21/02/2000. Data available from the U.S. Geological Survey.]]		[[Image:14005 fig2.jpg\|thumb\|center\|500px\|'''Figure 2'''    Overview natural colour composite imagery for calibrated Landsat Path 198/ Row 062 scenes for a) 25/02/1987, b) 01/06/1987, c) 17/02/1993, d) 14/09/1999 and e) 21/02/2000. Data available from the U.S. Geological Survey.]]

	These images were pre-georeferenced to UTM zone 37-South projection, with WGS84 horizontal datum. Each spectral band of TM and ETM+ data were converted from DN (digital number) to spectral radiance using the Landsat Calibration Pre-processing utility in ENVI Version 4.7 software (EXELIS Visual Information Solutions). This process uses published post-launch gains and offsets specific to the sensor for the user-defined date of image acquisition, in conjunction with parameters on sun elevation angle, band minimum DN value and band maximum DN values, each of which are specified in the associated image metadata file.		These images were pre-georeferenced to UTM zone 37-South projection, with WGS84 horizontal datum. Each spectral band of TM and ETM+ data were converted from DN (digital number) to spectral radiance using the Landsat Calibration Pre-processing utility in ENVI Version 4.7 software (EXELIS Visual Information Solutions). This process uses published post-launch gains and offsets specific to the sensor for the user-defined date of image acquisition, in conjunction with parameters on sun elevation angle, band minimum DN value and band maximum DN values, each of which are specified in the associated image metadata file.

	[[Image:14005 fig3.jpg\|thumb\|center\|500px\|'''Figure 3''' RGB band combinations for calibrated Landsat scene from 21 February 2000 a) 321, b) 432, c) 742, and d) 457.]]		[[Image:14005 fig3.jpg\|thumb\|center\|500px\|'''Figure 3'''    RGB band combinations for calibrated Landsat scene from 21 February 2000 a) 321, b) 432, c) 742, and d) 457.]]

	Digital image processing was performed on each calibrated scene to highlight particular surface features. A series of images were generated using specific RGB display channel band combinations with histogram manipulation in order to enhance visualisation of the study area. RGB band combinations visualised (Figure 3) were the natural colour composite 321 (similar colouring to that observed through a digital camera) and the false colour-composites for 432 (healthy vegetation appears red), 742 (exposed lithologies appear in blue hues), and 457 (enhances variations in exposed lithologies). All image enhancements were exported as GeoTIFF files and incorporated into the GIS for future analysis.		Digital image processing was performed on each calibrated scene to highlight particular surface features. A series of images were generated using specific RGB display channel band combinations with histogram manipulation in order to enhance visualisation of the study area. RGB band combinations visualised (Figure 3) were the natural colour composite 321 (similar colouring to that observed through a digital camera) and the false colour-composites for 432 (healthy vegetation appears red), 742 (exposed lithologies appear in blue hues), and 457 (enhances variations in exposed lithologies). All image enhancements were exported as GeoTIFF files and incorporated into the GIS for future analysis.

← Older revision		Revision as of 09:30, 20 October 2021
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	A day was spent in the field looking at the tephra fall deposits described by Roberts (2002)<ref name="Roberts"></ref>, to the west and northwest of Mt Meru. In northern Tanzania, the prevalent wind direction is from the east, therefore tephra fallout deposits occur on the western slopes of Mt Meru, and beyond. Roberts (2002)<ref name="Roberts"></ref> reported the locations of exposures of tephra fallout deposits (Figure 5). The aim of the day was to validate his observations, explore potential exposures along river gullies, and to sample pumice from fallout units to determine the ages of the eruptions. Dating of the pumice fallout deposits has been attempted; however, Ar-Ar dating on a sanidine-bearing pumice fall deposit yielded an age of 1 Ma, and was discarded (Roberts, 2002)<ref name="Roberts"></ref>. Through improved dating techniques, we hope to accurately determine the ages of these large explosive eruption.		A day was spent in the field looking at the tephra fall deposits described by Roberts (2002)<ref name="Roberts"></ref>, to the west and northwest of Mt Meru. In northern Tanzania, the prevalent wind direction is from the east, therefore tephra fallout deposits occur on the western slopes of Mt Meru, and beyond. Roberts (2002)<ref name="Roberts"></ref> reported the locations of exposures of tephra fallout deposits (Figure 5). The aim of the day was to validate his observations, explore potential exposures along river gullies, and to sample pumice from fallout units to determine the ages of the eruptions. Dating of the pumice fallout deposits has been attempted; however, Ar-Ar dating on a sanidine-bearing pumice fall deposit yielded an age of 1 Ma, and was discarded (Roberts, 2002)<ref name="Roberts"></ref>. Through improved dating techniques, we hope to accurately determine the ages of these large explosive eruption.

	[[Image:14005 fig5.jpg\|thumb\|center\|500px\|'''Figure 5''' Mt Meru sample location map.]]		[[Image:14005 fig5.jpg\|thumb\|center\|500px\|'''Figure 5'''    Mt Meru sample location map.]]

	Yellow triangles show the locations of samples collected by BGS on October 2013 as part of this study. Blue circles are sample locations from Roberts (2002)<ref name="Roberts"></ref>.		Yellow triangles show the locations of samples collected by BGS on October 2013 as part of this study. Blue circles are sample locations from Roberts (2002)<ref name="Roberts"></ref>.

	Roberts (2002) proposed that the tephra fallout deposits form part of a large ‘climatic’ eruption, based on the similarities in field characteristics of the deposits, and the whole-rock geochemistry. When plotted on a TAS diagram the deposits reveal a clear fractionation trend, which Roberts (2002) interpreted as the emptying of a large, fractionated magma chamber. He calculated the volume of such an eruption, taking the combined thicknesses of the units, as 2.1 ~~km3~~.		Roberts (2002)<ref name="Roberts 2002"></ref> proposed that the tephra fallout deposits form part of a large ‘climatic’ eruption, based on the similarities in field characteristics of the deposits, and the whole-rock geochemistry. When plotted on a TAS diagram the deposits reveal a clear fractionation trend, which Roberts (2002) interpreted as the emptying of a large, fractionated magma chamber. He calculated the volume of such an eruption, taking the combined thicknesses of the units, as 2.1 km<sup>3</sup>.

	===Tephra Sections===		===Tephra Sections===
	As part of this study, tephra deposits were described and samples were collected at five localities to the west of Mt Meru (Figure 5). Locality 1 is in a dry river bed at the village of Kiushian, just off the main Arusha–Nairobi road. Within the dry riverbed, a ~1.5 m thick consolidated ash deposit with abundant pumice and lithics is exposed (Figure 6). Average clast sizes are between 1 and 5 cm, with rare larger clasts up to 30 cm. The deposit has internal structure with layers of more concentrated lithic and pumice fragments.		As part of this study, tephra deposits were described and samples were collected at five localities to the west of Mt Meru (Figure 5). Locality 1 is in a dry river bed at the village of Kiushian, just off the main Arusha–Nairobi road. Within the dry riverbed, a ~1.5 m thick consolidated ash deposit with abundant pumice and lithics is exposed (Figure 6). Average clast sizes are between 1 and 5 cm, with rare larger clasts up to 30 cm. The deposit has internal structure with layers of more concentrated lithic and pumice fragments.

	[[Image:14005 fig6.jpg\|thumb\|center\|500px\|'''Figure 6''' Field photographs of Locality 1 and Locality 2.]]		[[Image:14005 fig6.jpg\|thumb\|center\|500px\|'''Figure 6'''    Field photographs of Locality 1 and Locality 2.]]

	Locality 2 is further up the same riverbed towards Mt Meru. The river valley is much deeper here (~5 m). Exposed near the top of the channel walls is a ~50 cm thick, unconsolidated ash-supported deposit that contains abundant rounded cream pumice fragments. Overlying this deposit is a ~1.5 m thick reworked deposit, possibly a lahar, with very large blocks (up to 1 m) of lithics and consolidated pyroclastic flow material (Figure 6).		Locality 2 is further up the same riverbed towards Mt Meru. The river valley is much deeper here (~5 m). Exposed near the top of the channel walls is a ~50 cm thick, unconsolidated ash-supported deposit that contains abundant rounded cream pumice fragments. Overlying this deposit is a ~1.5 m thick reworked deposit, possibly a lahar, with very large blocks (up to 1 m) of lithics and consolidated pyroclastic flow material (Figure 6).

	[[Image:14005 fig7.jpg\|thumb\|center\|500px\|'''Figure 7''' Field photographs of Locality 3: the road-cut reveals numerous interbedded ash- rich reworked, and pumice fall deposits.]]		[[Image:14005 fig7.jpg\|thumb\|center\|500px\|'''Figure 7'''    Field photographs of Locality 3: the road-cut reveals numerous interbedded ash- rich reworked, and pumice fall deposits.]]

	Locality 3 is on the edge of the north-western slope of Mt Meru, through a Masaai village just south of the main Arusha-Nairobi road. A series of tephra fallout and reworked pyroclastic deposits are exposed in a road-cut through a topographic high (or mound) (Figure 7). It appears as though the pyroclastic material is draping a topographic feature such as a monogenetic cone, of which there are many surrounding and on the flanks of Mt Meru. At least three tephra fall deposits were identified, all of which are pumice-supported with abundant lithics (Figure 7). The pumice is very light grey/cream in colour, and up to 2 cm along the long-axis. These deposits are all between 10 and 15 cm thick. Interbedded between the pumice fall deposits are ash-supported, pyroclastic flow deposits with reworked pumice and lithic fragments. These deposits vary in thickness from <1 m to > 2 m.		Locality 3 is on the edge of the north-western slope of Mt Meru, through a Masaai village just south of the main Arusha-Nairobi road. A series of tephra fallout and reworked pyroclastic deposits are exposed in a road-cut through a topographic high (or mound) (Figure 7). It appears as though the pyroclastic material is draping a topographic feature such as a monogenetic cone, of which there are many surrounding and on the flanks of Mt Meru. At least three tephra fall deposits were identified, all of which are pumice-supported with abundant lithics (Figure 7). The pumice is very light grey/cream in colour, and up to 2 cm along the long-axis. These deposits are all between 10 and 15 cm thick. Interbedded between the pumice fall deposits are ash-supported, pyroclastic flow deposits with reworked pumice and lithic fragments. These deposits vary in thickness from <1 m to >2 m.

	Locality 4 is in a steep-sided river valley, ~50 m along the road from Locality 3. The gorge is ~20 m deep with clearly visible grey tephra fall deposits interbedded with yellow and pale cream ash-rich units (Figure 8). Near the top of the gorge is a rubbly a’a lava flow. The lava comprises a very dark glassy groundmass with abundant phenocrysts of feldspar, quartz and hornblende.		Locality 4 is in a steep-sided river valley, ~50 m along the road from Locality 3. The gorge is ~20 m deep with clearly visible grey tephra fall deposits interbedded with yellow and pale cream ash-rich units (Figure 8). Near the top of the gorge is a rubbly a’a lava flow. The lava comprises a very dark glassy groundmass with abundant phenocrysts of feldspar, quartz and hornblende.

	[[Image:14005 fig8.jpg\|thumb\|center\|300px\|'''Figure 8''' Locality 4 stratigraphic section with field photographs. Sample descriptions are in Table 1.]]		[[Image:14005 fig8.jpg\|thumb\|center\|300px\|'''Figure 8'''    Locality 4 stratigraphic section with field photographs. Sample descriptions are in Table 1.]]

	The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.		The final locality, 5, is located on the main Arusha–Nairobi road in another river gorge. The gorge is ~10 m deep, and exposes a thick (>2.7 m) tephra fallout deposit, overlain by a series of tephra and ash fall layers (Figure 9). These are then overlain by a ~70 cm thick pumice-rich deposit. The thick fallout unit is clast-supported with predominantly light-grey pumices. The deposit is lithic-poor. Phenocrysts are clearly visible in the pumice clasts. Clasts are typically 3 cm, with common clasts up to 5 cm across.

	[[Image:14005 fig9.jpg\|thumb\|center\|300px\|'''Figure 9''' Locality 5 stratigraphic log with field photographs. Sample descriptions are in Table 1.]]		[[Image:14005 fig9.jpg\|thumb\|center\|300px\|'''Figure 9'''    Locality 5 stratigraphic log with field photographs. Sample descriptions are in Table 1.]]

	===Tephra Samples===		===Tephra Samples===
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	In total 12 samples were collected. The sample descriptions and locations are presented in Table 1. Nine of the samples comprise pumice clasts, while samples TANZ_2b and 2c are lithic fragments from a reworked lahar(?) deposit, and sample TANZ_4a is taken from the rubbly lava flow at Locality 4. Samples are labelled with the country (TANZ) followed by the locality and a letter (a, b, c) depending on the number of samples taken at a single locality.		In total 12 samples were collected. The sample descriptions and locations are presented in Table 1. Nine of the samples comprise pumice clasts, while samples TANZ_2b and 2c are lithic fragments from a reworked lahar(?) deposit, and sample TANZ_4a is taken from the rubbly lava flow at Locality 4. Samples are labelled with the country (TANZ) followed by the locality and a letter (a, b, c) depending on the number of samples taken at a single locality.

			<center>
	{\| class="wikitable"		{\| class="wikitable"
	\|+ Table 5 BGS sample descriptions.		\|+ Table 5 BGS sample descriptions.
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	\| Pumice from upper pumice-rick deposit		\| Pumice from upper pumice-rick deposit
	\|}		\|}
			</center>

	===Fieldwork recommendations===		===Fieldwork recommendations===