North Crater occupies the north-western quadrant of the Tongariro Volcanic Centre and represents one of at least eleven vents which have been active on Tongariro since the last glacial maximum. The most recent cone-forming activity at North Crater is thought to have occurred between 14-12 ka ago to produce the distinct, wide, flattopped andesite cone. This project focused mainly on the cone-building eruptions at North Crater, including stratigraphic correlations with distal tephra, interpreting eruptive processes, and establishing the sequence of events during cone construction. Detailed field work identified key stratigraphic sections and facies in the proximal, medial and distal environments. These sections allowed stratigraphic correlations to be made between proximal cone-building facies and distal sheet-forming facies at North Crater, and established a complete North Crater eruption stratigraphy. In the proximal environment, welded and non to poorly welded facies formed from fallout of a lava-fountain, pyroclastic flow or as fallout from a convecting plume. In the medial and distal environment, the lithofacies consist of fallout from a convecting plume and minor pyroclastic flow. Convective fall and non to poorly welded pyroclastic flow deposits dominate the lower eruption stratigraphy suggesting explosive eruptions involving a gas-rich magma. A change to welded deposits produced from lava-fountaining occurs later in the cone-building sequence and suggest a change to lower explosively and eruption of gas-poor magma. Grain size, componentry data, density, petrography and SEM analysis were carried out on representative samples to characterise the different facies, and reveal information about eruption processes. The non to poorly welded deposits are typically made up of vesicular pumice, scoria and mingled clasts of sub-rounded bombs and lapilli. The welded facies are relatively dense and clast outlines are often difficult to distinguish. The eruptives are porphyritic with abundant plagioclase gt clinopyroxene gt orthopyroxene gt opaques. Quartzofeldspathic crustal xenoliths are common and indicate crustal assimilation. Mingled clasts of light and dark glass were found to have microlites present in the dark glass, but were absent in the light glass. Electron microprobe analyses found that the dark and light glass components in a single clast had similar compositions, showing that the contrasting physical appearance of the glass is not due to a different chemical composition. Forty three whole rock XRF analyses showed that the magmas ranged from basaltic andesite to andesite, and Harker variation plots display linear trends typical of magma mixing. Magma mixing as the most important magmatic process is supported by disequilibrium of phenocryst compositions and phenocryst textures. Magma viscosity, bulk density and temperature was determined using MAGMA (Kware), and indicate that they fall into the range of typical andesites. Eruptive activity involved vigorous lava-fountaining, minor convecting eruption plumes and dominant collapsing eruption plumes. This activity has produced welded and non-welded pyroclastic flow and fall deposits to form the large cone seen today. There are significant volcanic hazards associated with this style of activity at North Crater, characterised by lava-fountaining, eruption plume fallout, and widespread pyroclastic flows and lahars extending beyond the ring plain. These could all be potentially devastating to the central North Island of New Zealand.

A relatively new fracture toughness testing method called Straight Notched Disc Bending (SNDB) was used before for fracture testing of Ankara Andesite and Afyon Marble cores. In this work to investigate the applicability of the new method to other rock types. With a preliminary notch of 10 mm, straight notched disc type specimens with a diameter of 75 mm were loaded by three-point bending loads. Investigation of effect of specimen height on the stress intensity factor and fracture toughness was carried out. Specimen heights (B) between 18 &ndash / 67 mm were tried for andesite and marble cylindrical specimens. Loading span, that is span/radius (S/R) ratio was changed between 0.6 - 0.9 for andesite specimens. Stress intensity factor for specimens was computed with ABAQUS program. Stress intensity factor was found to increase with increasing specimen diameter for a fixed span/radius ratio. Stress intensity factor decreased with increasing specimen height. Changing span was found to have no significant effect on fracture toughness of andesite. Fracture toughness was significantly lower for specimens with smaller height. The suggested testing height interval for this type of specimens was between height/diameter ratios of 0.49 &ndash / 0.64. Results were compared to the results obtained by a well-known specimen geometry named semi-circular bend specimens (SCB) under three-point bending. SCB tests produced lower values for fracture toughness for both rock types. Fracture toughness was 0.99 MPa&amp / #8730 / m for Ankara Andesite and 0.70 MPa&amp / #8730 / m for Afyon Marble.

published_or_final_version / abstract / toc / Earth Sciences / Doctoral / Doctor of Philosophy

Στα πλαίσια της παρούσας διπλωματικής εργασίας εκτελέστηκαν δοκιμές εντός του εργαστηρίου σε δοκίμια βραχωδών σχηματισμών. Η εργασία εστιάζει σε δύο περιοχές εντός του Ελλαδικού χώρου, στα Μέθανα και στο Καλλίδρομο. Τα πετρώματα της περιοχής των Μεθάνων αντιστοιχούν σε ανδεσίτες και δακίτες, ενώ εκείνα του Καλλιδρόμου αντιστοιχούν σε περιδοτίτες. Βασικός σκοπός ήταν η μελέτη και ο καθορισμός των τεχνικογεωλογικών χαρακτηριστικών των πετρωμάτων από τις δύο προαναφερθείσες περιοχές. Προς την κατεύθυνση αυτή, έγιναν δοκιμές με στόχο τη διερεύνηση των φυσικών, δυναμικών και μηχανικών χαρακτηριστικών τους καθώς και συσχετίσεις μεταξύ τους. / --

Subduction is a dynamic and time-dependent process which requires time-dependent models for its study. In addition, due to the very high pressures within the Earth's interior, an evaluation of the role of compressibility in subduction studies should be undertaken. However, most subduction studies have been conducted by using kinematic, steady-state, and/or incompressible mantle convection models; these simplifications may miss important elements of the subduction process. In this dissertation, I evaluate the effects of time-dependence and compressibility on the evolution of subduction by using 2-D Cartesian numerical models. The effect of compressibility on the thermal and flow structures of subduction zones is evaluated by using kinematically prescribed slab and steady-state models. The effect of compressibility is primarily expressed as an additional heat source created by viscous dissipation. The heat results in thinner thermal boundary layer on the subducting slab and increases slab temperatures. With that exception, the effect of compressibility is relatively small compared with, for example, the effect of the mantle rheology on the thermal and flow structures of the mantle wedge. Plate reconstruction models show that the convergence rate and age of the incoming plate to trench vary with time, which poses a problem for steady-state subduction models. Thus, I consider the time-dependent convergence rate and age of the incoming plate in the kinematic-dynamic subduction models in order to understand the localization of high-Mg# andesites in the western Aleutians. The results show that the localization of high-Mg# andesites is a consequence of the time-dependent convergence rate and slab age along the Aleutian arc. The influence of mantle and slab parameters as well as compressibility on the slab dynamics is evaluated by using 2-D dynamic subduction models. The results demonstrate that periodic slab buckling in the mantle results in periodic convergence rate and dip of the subducting slab; time-dependence is a natural expression of subduction. The effect of compressibility on the slab dynamics is not significant. The periodic convergence rate and dip of the subducting slab explain time-dependent seafloor spreading at the mid-ocean ridge, convergence rate of the oceanic plate at trench and arc-normal migration of arc volcanoes. / Ph. D.

The assemblage of ca. 6–4 Ma volcanic rocks exposed at the Sagehen Research station in the Truckee-Tahoe region of the northern Sierra Nevada, United States, is interpreted to be, within the Ancestral Cascades volcanic arc, a Lassen-type stratovolcano complex. Sagehen is of particular importance because it is one of the few Tertiary arc volcanic centres in California which has not been heavily glaciated during the Pleistocene. The volcanic rocks are variably porphyritic or aphanitic, including abundant plagioclase with clinopyroxene and amphibole. The rocks range from basalt to basaltic-andesite to andesite in composition. Basalts are olivineand clinopyroxene-bearing with minor phenocrysts of plagioclase. The basaltic-andesites are primarily pyroxene bearing while the andesites contain pyroxene-, plagioclase- and hornblende porphyritic phases. Sagehen arc lavas are calc-alkaline and enriched in the large ion lithophile elements and depleted in High Field Strength Elements. The basalts are depleted in Zr and Hf while the andesites are enriched with Zr and Hf relative to the middle rare earth elements. Compared to previously studied Ancestral Cascade arc samples, Sagehen region basalts have lower 143Nd/144Nd isotopic values that do not correspond to proposed mantle-lithosphere mixing lines, while the andesite samples appear to represent the interplay of these two components on a 87Sr/86Sr vs. 143Nd/144Nd. The trace element data and isotopic plots suggest that the melts that produced the basalts are from subduction modified mantle wedge peridotites that ponded near the base of the lithosphere similar to the generation of other subduction related calc-alkaline lavas along convergent continental margins. The andesitic samples appear to be the result of further modification through crustal assimilation as seen in the higher isotopic Sr contents in the andesites and Ce/Smpmn vs. Tb/Ybpmn plots. Finally, the proposed map units from Sylvester & Raines (2007) were found to contain various geochemical facies based on the samples collected indicating that some map units may have to be redefined or sub-divided.

Throughout its eruptive history, Mount Hood has produced compositionally similar calc-alkaline andesite as lava flows and domes near the summit and basaltic andesitic flows from flank vents. Found within the andesite are slightly more mafic inclusions that are compositionally similar to the host andesite (or host lavas); no inclusions were found in the flank lavas. Host lavas and inclusions have the following mineral assemblage: plag + opx ± cpx ± amp + oxides. Flank lava mineralogy is similar to the inclusions and host lavas, but since they are more mafic they contain olivine instead of amphibole. Average silica content among samples analyzed ranges from 57.6 to 62.7 weight percent; however the incompatible trace element composition is more variable at lower silica contents and becomes less variable at higher silica contents. In terms of incompatible trace element composition, the host lavas and inclusions are either depleted (no amp) or enriched (amp± cpx).

The assemblage of ca. 6–4 Ma volcanic rocks exposed at the Sagehen Research station in the Truckee-Tahoe region of the northern Sierra Nevada, United States, is interpreted to be, within the Ancestral Cascades volcanic arc, a Lassen-type stratovolcano complex. Sagehen is of particular importance because it is one of the few Tertiary arc volcanic centres in California which has not been heavily glaciated during the Pleistocene. The volcanic rocks are variably porphyritic or aphanitic, including abundant plagioclase with clinopyroxene and amphibole. The rocks range from basalt to basaltic-andesite to andesite in composition. Basalts are olivineand clinopyroxene-bearing with minor phenocrysts of plagioclase. The basaltic-andesites are primarily pyroxene bearing while the andesites contain pyroxene-, plagioclase- and hornblende porphyritic phases. Sagehen arc lavas are calc-alkaline and enriched in the large ion lithophile elements and depleted in High Field Strength Elements. The basalts are depleted in Zr and Hf while the andesites are enriched with Zr and Hf relative to the middle rare earth elements. Compared to previously studied Ancestral Cascade arc samples, Sagehen region basalts have lower 143Nd/144Nd isotopic values that do not correspond to proposed mantle-lithosphere mixing lines, while the andesite samples appear to represent the interplay of these two components on a 87Sr/86Sr vs. 143Nd/144Nd. The trace element data and isotopic plots suggest that the melts that produced the basalts are from subduction modified mantle wedge peridotites that ponded near the base of the lithosphere similar to the generation of other subduction related calc-alkaline lavas along convergent continental margins. The andesitic samples appear to be the result of further modification through crustal assimilation as seen in the higher isotopic Sr contents in the andesites and Ce/Smpmn vs. Tb/Ybpmn plots. Finally, the proposed map units from Sylvester & Raines (2007) were found to contain various geochemical facies based on the samples collected indicating that some map units may have to be redefined or sub-divided.

The assemblage of ca. 6–4 Ma volcanic rocks exposed at the Sagehen Research station in the Truckee-Tahoe region of the northern Sierra Nevada, United States, is interpreted to be, within the Ancestral Cascades volcanic arc, a Lassen-type stratovolcano complex. Sagehen is of particular importance because it is one of the few Tertiary arc volcanic centres in California which has not been heavily glaciated during the Pleistocene. The volcanic rocks are variably porphyritic or aphanitic, including abundant plagioclase with clinopyroxene and amphibole. The rocks range from basalt to basaltic-andesite to andesite in composition. Basalts are olivineand clinopyroxene-bearing with minor phenocrysts of plagioclase. The basaltic-andesites are primarily pyroxene bearing while the andesites contain pyroxene-, plagioclase- and hornblende porphyritic phases. Sagehen arc lavas are calc-alkaline and enriched in the large ion lithophile elements and depleted in High Field Strength Elements. The basalts are depleted in Zr and Hf while the andesites are enriched with Zr and Hf relative to the middle rare earth elements. Compared to previously studied Ancestral Cascade arc samples, Sagehen region basalts have lower 143Nd/144Nd isotopic values that do not correspond to proposed mantle-lithosphere mixing lines, while the andesite samples appear to represent the interplay of these two components on a 87Sr/86Sr vs. 143Nd/144Nd. The trace element data and isotopic plots suggest that the melts that produced the basalts are from subduction modified mantle wedge peridotites that ponded near the base of the lithosphere similar to the generation of other subduction related calc-alkaline lavas along convergent continental margins. The andesitic samples appear to be the result of further modification through crustal assimilation as seen in the higher isotopic Sr contents in the andesites and Ce/Smpmn vs. Tb/Ybpmn plots. Finally, the proposed map units from Sylvester & Raines (2007) were found to contain various geochemical facies based on the samples collected indicating that some map units may have to be redefined or sub-divided.

The interaction of magma with continental crust at convergent margins is fundamental to understanding if and how continents grow. Isotopic and elemental data constrain the progressive stages of development of the magmatic underpinnings of the long-lived Aucanquilcha Volcanic Cluster (AVC), situated atop the thick continental crust of the central Andes in northern Chile. Whole rock data are used in conjunction with mineral compositions to infer processes that gave rise to eleven million years of intermediate, dominantly dacite, arc volcanism. A pulse of volcanic activity at the AVC between ~5 and 2 Ma is bracketed by more sluggish rates. We document chemical changes in the lavas that accompany this eruptive evolution. Trace element data suggest that crystal fractionation and magma mixing were the dominant mechanisms generating the diversity observed in the AVC whole rock data. Fractionation was dominant during early and waning stages of magmatism, and magma mixing was an important process during the high flux period. Peak thermal maturity of the AVC underpinnings coincided with the high magma flux and likely promoted open system processes during this time. Mineral compositions from zircon, amphibole, pyroxene, and Fe-Ti oxides confirm the importance of material recycling in the production of evolved AVC rocks. Various geothermometers were employed to calculate the pre-eruptive conditions of AVC magma using mineral compositions. Pressure estimates from amphibole and two-pyroxene barometry indicate crystallization depths of 1 ��� 5 kb and 4 ��� 6 kb, respectively. Temperature estimates from zircon, Fe-Ti oxides, amphiboles, and pyroxenes indicate temperatures ranging from ~700��C to 1100��C. Zircon temperatures are always the lowest (700��C - 950��C), and pyroxene temperatures are always the highest (1000��C - 1100��C), with Fe-Ti oxide and amphiboles temperatures falling in between. U-Pb ages from zircons and thermometry from individual samples evidence the thermal maturation and consolidation of the underpinnings below the AVC, presumably culminating in a large, crystal-rich mush zone where magmas were trapped and processed. It is in these middle to upper crustal zones where magmatic diversity is attenuated and giant, relatively homogeneous batholiths are formed. Isotopes of AVC lavas are similar to values observed from other central Andes volcanic centers. Lead isotopes are consistent with the AVC's location within a Pb isotope transition zone between the Antofalla and Arequipa basement terranes. Oxygen and Sr isotopic ratios are high and Nd isotopic ratios low with respect to a depleted mantle. Through time, ������Sr/������Sr values of AVC lavas progressively increase from lows of ~0.70507 to ~0.70579 (upper values of 0.70526 to 0.70680), and ��Nd values decrease from highs of -1.0 to -4.6 (lows of -1.6 to -7.3). Similarly, O isotopes (�������O) show a slight increase in base level through time from lows of 6.5��� to 7.0��� (highs of 6.75��� ��� 7.5���). Dy/Yb and Sm/Yb ratios also increased systematically from highs of 2.11 to 3.45, and 2.76 to 6.67, respectively. Despite the temporal isotopic variation, there is little isotopic variation with indices of fractionation, suggesting this signal is the consequence of deep magmatic processing, here attributed to an expanding zone of melting, assimilation, storage, and homogenization (MASH) of mantle-derived magma in the deep crust. Upward expansion brought the MASH zone into contact with rocks that were increasingly evolved with respect to Sr and Nd isotopes, explaining the isotopic shifts. Downward expansion of the MASH zone enhanced garnet stability during basalt fractionation, explaining the increased Dy/Yb and Sm/Yb ratios. Mass balance calculations involving Sr, Nd, and O isotope modeling are consistent with a crustal component making up 10 - 30% of AVC lavas, implying that although the history of central Andean magmatism is replete with large scale crustal recycling, the current phase is largely a crust formation event. / Graduation date: 2012