Kelvin S. Rodolfo | University of Illinois at Chicago (original) (raw)
Papers by Kelvin S. Rodolfo
Journal of Asian Earth Sciences, Jun 1, 2000
U.S. Government Printing Office eBooks, 1981
An academic directory and search engine.
Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer ad... more Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer additional information about the timing and spatial characteristics of arc volcanism and the evolution of island arcs. Explosive volcanism along the Izu-Bonin Arc, with maxima just before rifting of the arc at ~40 and 5-0 Ma, produced black and white tephras of variable grain sizes and chemical compositions. Most of the tephras belong chemically to low-K and low-alkali tholeiitic rock series with a few tephra of the high-K and alkalic rock series. Most of the tephras (low-K series) were derived from the Izu-Bonin Arc, although a few were produced far to the west of the Izu-Bonin Arc (e.g., from the Ryukyu Arc). Black tephras may have come from nearby sources, such as Aogashima, Sumisu, and Torishima islands. The high-K series of tephras, within the sediments younger than 3 Ma, may reflect thickening of the island-arc crust.
To examine the processes and histories of arc and of volcanism associated with backarc rifting, 1... more To examine the processes and histories of arc and of volcanism associated with backarc rifting, 130 samples containing igneous glass shards were taken from the Pliocene-Quaternary succession on the rift flank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses that result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacites to basalts) are no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to the seafloor) generally have higher values than those from the rift-basin fill. How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre-and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolite explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andesitic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre-and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkalic products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic; the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts erupted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, are compositionally similar to those erupted from mature backarc basins elsewhere.
Proceedings of the Ocean Drilling Program, Aug 1, 1992
To examine the processes and histories of arc and of volcanism associated with backarc rifting, 1... more To examine the processes and histories of arc and of volcanism associated with backarc rifting, 130 samples containing igneous glass shards were taken from the Pliocene-Quaternary succession on the rift flank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses that result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacites to basalts) are no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to the seafloor) generally have higher values than those from the rift-basin fill. How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre-and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolite explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andesitic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre-and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkalic products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic; the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts erupted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, are compositionally similar to those erupted from mature backarc basins elsewhere.
Limnology and Oceanography, Apr 1, 1962
Waters at the sill depths and near the bottoms of two basins off southern California were sampled... more Waters at the sill depths and near the bottoms of two basins off southern California were sampled in a grid pattern and analyzed for possible anomalies in temperature, salinity, and contents of oxygen, nutrients and suspended sediment due to turbidity currents. The only significantly anomalous patterns revealed were those of low salinity and high suspended sediment, atop a turbidity-current apron opposite the mouths of two submarine canyons. This water was probably emplaced by turbidity currents, and its presence here indicates that areal variations of bottom-water properties elsewhere warrant detailed studies.
AGU Fall Meeting Abstracts, Dec 1, 2018
AGU Fall Meeting Abstracts, Dec 1, 2017
Natural Hazards and Earth System Sciences, Dec 15, 2016
Category 5 Super Typhoon Bopha, the world's worst storm of 2012, formed abnormally close to the E... more Category 5 Super Typhoon Bopha, the world's worst storm of 2012, formed abnormally close to the Equator, and its landfall on Mindanao set the record proximity to the Equator for its category. Its torrential rains generated an enormous debris flow in the Mayo River watershed that swept away much of the village Andap in the New Bataan municipality, burying areas under rubble as thick as 9 m and killing 566 people. Established in 1968, New Bataan had never experienced super typhoons and debris flows. This unfamiliarity compounded the death and damage. We describe Bopha's history, debris flows and the Mayo River disaster, and then we discuss how population growth contributed to the catastrophe, as well as the possibility that climate change may render other near-Equatorial areas vulnerable to hazards brought on by similar typhoons. Finally, we recommend measures to minimize the loss of life and damage to property from similar future events.
Geological Society of America Bulletin, Mar 1, 1989
Mabinit Channel, 5 km long, IS to 70 m wide, and 2 to 22 m deep, was formed by lahars on the sout... more Mabinit Channel, 5 km long, IS to 70 m wide, and 2 to 22 m deep, was formed by lahars on the southeastern slope of Mayon Volcano during its 1984 eruption. In 1985, the channel was drastically modified by a lahar triggered by a typhoon. Sediment-budget calculations from surveys conducted in 1985 and 1986 corroborate inferences from other volcanoes that lahars can grow significantly in volume by eroding their channels. The 1984 eruption produced a deep summit ravine and, at its base, a fan of avalanche and pyroclastic-flow deposits 2 km 2 in area and 40 x 10 6 m 3 in volume. Lahars initiated in the ravine by heavy rains from eruption updrafts carved out 5-km-long Mabinit Channel along the west margin of the pyroclastic-avalanche field, a position that could not have been predicted from pre-eruption topography. By eroding the channel, the lahars generated more than half of their own solid contents. Below 240 m, where the volcano slope decreases from 5° to 3°, the first eruption debris flows were unconfined and spread out over a 512,000-m 2 area, depositing a layer locally more than 4 m thick with a volume of 1.25 x 10 6 m 3. The thin margins of these unconfined flows were non-erosional; however, as they stopped, their thick central portions continued flowing, extending the new channel downslope through the depositional field. Subsequent eruption lahars modified the channel and overflowed at its bends. A moderately intense typhoon in October 1985 triggered a single 9-hr lahar event that widened the channel by an average of 25 m and caused as much as 66 m of lateral bank erosion. Debris flows overtopping the channel at four sites coalesced to cover a 2-km-long, 200,000m 2 area with bouldery sediment. In wider channel stretches, debris-flow margins left prominent paired levees, while their central portions continued moving down-channel as rigid plugs. The lahar plugged the lower 0.5-km stretch of the channel and replaced it by eroding a new channel of comparable size. This demonstrated how easily lahars can change the courses of channels on the upwardly convex debris apron of a stratovolcano, with serious consequences for farmland and communities.
AGU Fall Meeting Abstracts, Dec 1, 2016
Journal of Asian Earth Sciences, Nov 1, 2005
Subic Bay sediments and faults identified in seismic-reflection profiles were dated using sea-lev... more Subic Bay sediments and faults identified in seismic-reflection profiles were dated using sea-level curves. The oldest sedimentary packages are marine sediments subaerially exposed and eroded 20 ka. Fluvio-marine to wholly marine sediments were deposited during the ensuing transgression, and prograding units were deposited during stillstands or minor sea-level falls. Faults within the bay have three age ranges. The oldest set cuts through the pre-d 18 O Stage 2 rock units, O18 ka; a second disrupts 10.2-11.3 ka sediments; and the youngest, which cut the uppermost sedimentary package, show that movements occurred about every 2 ky, most recently about 3 ka. Northwest-southeast faults that parallel onshore structures associated with Paleogene emplacement of the Zambales Ophiolite Complex to the west and north likely represent rejuvenated tectonism. The northern coastline and north-south-trending axial bay islands appear related to a lineament that dissects Mt Pinatubo farther northeast. A breach in the caldera of Mt Natib is the most likely source of a presumed pyroclastic deposit in the eastern bay that is associated with sediments about 11.3-18 ka, indicating that a Natib eruption occurred much more recently than previously documented for this volcano.
Journal of Asian Earth Sciences, Jun 1, 2000
U.S. Government Printing Office eBooks, 1981
An academic directory and search engine.
Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer ad... more Numerous marine tephra layers cored at Sites 792 and 793 in the Izu-Bonin forearc region offer additional information about the timing and spatial characteristics of arc volcanism and the evolution of island arcs. Explosive volcanism along the Izu-Bonin Arc, with maxima just before rifting of the arc at ~40 and 5-0 Ma, produced black and white tephras of variable grain sizes and chemical compositions. Most of the tephras belong chemically to low-K and low-alkali tholeiitic rock series with a few tephra of the high-K and alkalic rock series. Most of the tephras (low-K series) were derived from the Izu-Bonin Arc, although a few were produced far to the west of the Izu-Bonin Arc (e.g., from the Ryukyu Arc). Black tephras may have come from nearby sources, such as Aogashima, Sumisu, and Torishima islands. The high-K series of tephras, within the sediments younger than 3 Ma, may reflect thickening of the island-arc crust.
To examine the processes and histories of arc and of volcanism associated with backarc rifting, 1... more To examine the processes and histories of arc and of volcanism associated with backarc rifting, 130 samples containing igneous glass shards were taken from the Pliocene-Quaternary succession on the rift flank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses that result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacites to basalts) are no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to the seafloor) generally have higher values than those from the rift-basin fill. How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre-and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolite explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andesitic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre-and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkalic products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic; the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts erupted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, are compositionally similar to those erupted from mature backarc basins elsewhere.
Proceedings of the Ocean Drilling Program, Aug 1, 1992
To examine the processes and histories of arc and of volcanism associated with backarc rifting, 1... more To examine the processes and histories of arc and of volcanism associated with backarc rifting, 130 samples containing igneous glass shards were taken from the Pliocene-Quaternary succession on the rift flank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses that result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacites to basalts) are no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to the seafloor) generally have higher values than those from the rift-basin fill. How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre-and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolite explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andesitic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre-and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkalic products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic; the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts erupted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, are compositionally similar to those erupted from mature backarc basins elsewhere.
Limnology and Oceanography, Apr 1, 1962
Waters at the sill depths and near the bottoms of two basins off southern California were sampled... more Waters at the sill depths and near the bottoms of two basins off southern California were sampled in a grid pattern and analyzed for possible anomalies in temperature, salinity, and contents of oxygen, nutrients and suspended sediment due to turbidity currents. The only significantly anomalous patterns revealed were those of low salinity and high suspended sediment, atop a turbidity-current apron opposite the mouths of two submarine canyons. This water was probably emplaced by turbidity currents, and its presence here indicates that areal variations of bottom-water properties elsewhere warrant detailed studies.
AGU Fall Meeting Abstracts, Dec 1, 2018
AGU Fall Meeting Abstracts, Dec 1, 2017
Natural Hazards and Earth System Sciences, Dec 15, 2016
Category 5 Super Typhoon Bopha, the world's worst storm of 2012, formed abnormally close to the E... more Category 5 Super Typhoon Bopha, the world's worst storm of 2012, formed abnormally close to the Equator, and its landfall on Mindanao set the record proximity to the Equator for its category. Its torrential rains generated an enormous debris flow in the Mayo River watershed that swept away much of the village Andap in the New Bataan municipality, burying areas under rubble as thick as 9 m and killing 566 people. Established in 1968, New Bataan had never experienced super typhoons and debris flows. This unfamiliarity compounded the death and damage. We describe Bopha's history, debris flows and the Mayo River disaster, and then we discuss how population growth contributed to the catastrophe, as well as the possibility that climate change may render other near-Equatorial areas vulnerable to hazards brought on by similar typhoons. Finally, we recommend measures to minimize the loss of life and damage to property from similar future events.
Geological Society of America Bulletin, Mar 1, 1989
Mabinit Channel, 5 km long, IS to 70 m wide, and 2 to 22 m deep, was formed by lahars on the sout... more Mabinit Channel, 5 km long, IS to 70 m wide, and 2 to 22 m deep, was formed by lahars on the southeastern slope of Mayon Volcano during its 1984 eruption. In 1985, the channel was drastically modified by a lahar triggered by a typhoon. Sediment-budget calculations from surveys conducted in 1985 and 1986 corroborate inferences from other volcanoes that lahars can grow significantly in volume by eroding their channels. The 1984 eruption produced a deep summit ravine and, at its base, a fan of avalanche and pyroclastic-flow deposits 2 km 2 in area and 40 x 10 6 m 3 in volume. Lahars initiated in the ravine by heavy rains from eruption updrafts carved out 5-km-long Mabinit Channel along the west margin of the pyroclastic-avalanche field, a position that could not have been predicted from pre-eruption topography. By eroding the channel, the lahars generated more than half of their own solid contents. Below 240 m, where the volcano slope decreases from 5° to 3°, the first eruption debris flows were unconfined and spread out over a 512,000-m 2 area, depositing a layer locally more than 4 m thick with a volume of 1.25 x 10 6 m 3. The thin margins of these unconfined flows were non-erosional; however, as they stopped, their thick central portions continued flowing, extending the new channel downslope through the depositional field. Subsequent eruption lahars modified the channel and overflowed at its bends. A moderately intense typhoon in October 1985 triggered a single 9-hr lahar event that widened the channel by an average of 25 m and caused as much as 66 m of lateral bank erosion. Debris flows overtopping the channel at four sites coalesced to cover a 2-km-long, 200,000m 2 area with bouldery sediment. In wider channel stretches, debris-flow margins left prominent paired levees, while their central portions continued moving down-channel as rigid plugs. The lahar plugged the lower 0.5-km stretch of the channel and replaced it by eroding a new channel of comparable size. This demonstrated how easily lahars can change the courses of channels on the upwardly convex debris apron of a stratovolcano, with serious consequences for farmland and communities.
AGU Fall Meeting Abstracts, Dec 1, 2016
Journal of Asian Earth Sciences, Nov 1, 2005
Subic Bay sediments and faults identified in seismic-reflection profiles were dated using sea-lev... more Subic Bay sediments and faults identified in seismic-reflection profiles were dated using sea-level curves. The oldest sedimentary packages are marine sediments subaerially exposed and eroded 20 ka. Fluvio-marine to wholly marine sediments were deposited during the ensuing transgression, and prograding units were deposited during stillstands or minor sea-level falls. Faults within the bay have three age ranges. The oldest set cuts through the pre-d 18 O Stage 2 rock units, O18 ka; a second disrupts 10.2-11.3 ka sediments; and the youngest, which cut the uppermost sedimentary package, show that movements occurred about every 2 ky, most recently about 3 ka. Northwest-southeast faults that parallel onshore structures associated with Paleogene emplacement of the Zambales Ophiolite Complex to the west and north likely represent rejuvenated tectonism. The northern coastline and north-south-trending axial bay islands appear related to a lineament that dissects Mt Pinatubo farther northeast. A breach in the caldera of Mt Natib is the most likely source of a presumed pyroclastic deposit in the eastern bay that is associated with sediments about 11.3-18 ka, indicating that a Natib eruption occurred much more recently than previously documented for this volcano.