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Spontaneous reversal of magnetization in rocks and its apparent dependence on the oxidation state of the Fe-Ti oxide minerals is highly significant in view of the overwhelming evidence that exists in support of field reversal. An attempt has been made in this study to classify these "states" of oxidation in terms of characteristic mineral assemblages. The series which has evolved is a texturally progressive series of high temperature origin. The series is supported by known thermal equilibria, by whole rock chemical analysis, and by experimental oxidation of titanomagnetite, ilmenite and olivine. The magnetic and associated mineralogy is discussed in detail and a correlation study with some magnetic properties has been made in controlled sampling of traverses across fourteen single lavas from Iceland. These results show that the degree of oxidation is highly variable throughout a lava and that the maximum degree of oxidation is towards the central part of a lava. Several factors suggest that oxidation takes place deuterically at the time of cooling. The magnetic and petrological implications of this zonation is discussed. In spite of the fact that mixed magnetic polarities and a self reversal mechanism are absent, within single units, a strong positive correlation is nevertheless demonstrated between samples which are highly oxidized and samples which are reversely magnetized. The correlation is well defined but is not fully understood.

Geomagnetic excursions, brief deviations in geomagnetic field behaviour from that expected during 'normal' secular variation, remain some of the most enigmatic features of geomagnetic field behaviour. This thesis presents high-resolution records of geomagnetic excursions recorded at the Blake-Bahama Outer Ridge in the Western North Atlantic. The highest resolution record yet of the Blake geomagnetic excursion (~125 ka) is measured in three cores from Ocean Drilling Program Site 1062 (ODP Leg 172). These cores have sufficiently high sedimentation rates (>10 cm ka-1) to allow detailed reconstruction of the field behaviour at these sites during the excursions. Previous reconstructions of geomagnetic field behaviour during excursions from marine cores have been limited by low-resolution age models. This thesis discusses a new approach, whereby measurements of excess 230Th (230Thxs) are used to constrain relative variations in sedimentation rate. Modifications are suggested to the methods previously used to calculate the concentration of 230Thxs and a new MATLAB® program is developed and described that allows rapid and flexible calculation of 230Thxs. Using this new approach, the duration (6.5±1.3 kyr) and age (129-122 ka) of the Blake excursion are accurately constrained. A palaeomagnetic study is also conducted on two ODP Sites, 1061 and 1062 on the Blake-Bahama Outer Ridge to obtain a high-resolution record of the Laschamp geomagnetic excursion (~41 ka). The Blake excursion is found to be of 'long' duration (6.5±1.3 kyr) whilst the Laschamp excursion is relatively short (<400 years) showing that excursions do not have a characteristic duration, linked to the conductivity of the inner core, but instead occupy a continuous range of durations. The records of both the Blake excursion and the Laschamp excursion from the Blake-Bahama Ridge sites also show rapid transitions to excursional geomagnetic pole positions (less than 500 years), much faster than often quoted for full geomagnetic reversals. Based on current estimates for reversal durations, this would imply that excursions and reversals are controlled by different processes.

In this thesis, I use multiple techniques to investigate the active faulting and deformation of the Altay Mountains, Western Mongolia. The Altay are an intracontinental transpressional mountain range, which are deforming in the far-field of the India-Asia collision. An anastomosing network of dextral faults strikes NNW-SSE, and accommodates NE-SW oriented shortening by rotating anticlockwise about vertical axes. I begin by characterising the Altay faults, and add to what is already known about their surface expression with new observations of active faulting and three previously undescribed ancient earthquake ruptures. I use 10Be cosmogenic dating and uranium-series dating on pedogenic carbonate to estimate the average Quaternary rate of slip for two of the major fault zones in the Altay. The slip rate on the Ölgiy fault is constrained to 0.3-2.1 mm/yr-1. Results from the Hovd fault are ambiguous, demonstrating the complications encountered with application of Quaternary dating techniques. I measure palaeomagnetic directions from Cretaceous to Pliocene-aged sediments in the eastern Altay to constrain the degree of anticlockwise rotation. Results from thermal demagnetisation of specimens indicate that the eastern Altay has not undergone significant rotation, in contrast with previous studies from the Siberian Altay that reveal almost 40 degrees of anticlockwise rotation. This suggests that the eastern-most Altay fault is too young to have experienced significant rotation, or is kinematically different from the Siberian Altay. I apply apatite fission track (AFT) dating and track length modeling to the central Altay. Results from AFT dating show rapid cooling in the late Cretaceous due to the distal assembly of Central Asia, suggesting that there was pre-existing topography at the start of the Late Cenozoic phase of deformation, the timing of which is constrained to have initiated at least 20 Myr ago. My work demonstrates that combining results from techniques that cover a variety of time scales quantifies the evolution of active faulting and deformation in the region.

Magnetic minerals are abundant within our Earth's crust and can retain, through one of a number of processes, a remanent magnetisation induced by the Earth's magnetic field. Analyses of palaeomagnetic samples have been used for the past fifty years to improve our understanding of many of the Earth's major processes. Recent studies utilising newly developed imaging techniques, namely holographic transmission electron microscopy, have for the first time allowed direct observations of the magnetic structure in palaeomagnetic samples on a nanoscale. It is commonly observed that igneous rocks contain closely packed magnetic lamellae with a non-magnetic matrix, a result of the chemical process of exsolution. However, the results of current micromagnetic models, generated to predict the magnetic structure within such samples, are not in agreement with these direct observations. The results do, however, show strong similarities to the direct observations. The discrepancies between the direct observations and micromagnetic models indicate a lack of understanding of the magnetic interactions within such samples. To examine this two distinct hypotheses have been tested. Firstly, the geometry of the system has been altered to examine the effect of this on the magnetic structure of the grains. Secondly, a multiphase model has been produced. This multiphase model allows the simulation of more complicated systems that include more than one magnetic material in direct contact. This multiphase model has allowed us to examine the effect of varying the exchange in these multiphase structures and its effect on the modelled magnetic structure. Further, this multiphase model has allowed us to examine theoretical systems involving combinations of magnetic materials commonly found in palaeomagnetic samples.

If a rock is to retain a geologically meaningful magnetic record of its history, it is essential that it contains magnetic minerals which are capable of carrying stable magnetic remanence. Of the natural occurring magnetic minerals, magnetite is the most important because of its abundance and strong magnetic signature. The stability, i.e., the resistance to demagnetisation or reorientation, of magnetic remanence is related to grain size; in smaller grains the magnetic moments align to have single domain (SD) structures, in larger grains complex magnetic patterns are formed (multidomain (MD)). “Classical” domain theory predicts that SD remanence is stable, whilst MD remanence is not. However experimental evidence has shown that both SD and MD grains can have stable remanences. In this thesis the origin of stable MD remanence is examined. There are two opposing theories; one suggests that the stability is due to independent SD-like structures, the other postulates that the stability is due to metastable MD structure. A series of experiments were designed to examine the stability using a selection of characterised synthetic and natural samples. Low-stress hydrothermal recrystallised samples where grown for this study. For the first time, the stability of thermoremanence induced in hydrothermal crystals to cooling was examined. The results agree with previous observations for crushed and natural magnetites, and support kinematic models. The behaviour of SIRM and thermoremanences in MD magnetite to low-temperature cooling to below the crystallographic Verwey transition at 120-124 K (Tv) and the cubic magnetocrystalline anisotropy isotropic point (Tk) at 130 K was investigated. On cooling through Tv, SIRM was observed to decrease and demagnetise, however thermoremanence was found to display a large increase in the magnetisation at Tv, which was partially re- versible on warming. The size of the anomaly is shown to be dependent on the temperature at which the thermoremanence is acquired, internal stress and grain size. The anomaly is attributed to the large increase in the magnetocrystalline anisotropy which occurs on cooling through Tv . It is postulated that low-temperature cycling demagnetisation is due to kinematic processes which occur on cooling between room temperature and Tk. Characterisation of low-temperature treated remanence and partially alternating field demagnetised remanence, suggest that the stable remanence is multidomain. Low-temperature cooling of remanence in single sub-micron crystals was simulated using micromagnetic models. The models predict the observed anomaly for thermoremanence on cooling through Tv, and also the relative behaviour of SIRM and thermoremanence. The single domain threshold was calculated for the low-temperature phase of magnetite, and was found to be 0.14 microns, compared to 0.07 microns at room temperature.

Sheet intrusions provide key pathways for magma transport and storage as it flows through the Earth's crust. Many studies focus on the relationship between the intruding magma and the host rock through which it propagates, however the flow and emplacement processes occurring within the magma are often poorly constrained. Understanding this is vital for understanding the behaviour of active volcanic systems and the development of bodies of economic interest. To address this, I study how magnetic fabrics are preserved in a natural sill and dyke in combination with laboratory experiments using analogue materials to investigate the evolution of intrusion propagation and solidification. The geochemistry and petrology of sills of the Little Minch Sill Complex, Isle of Skye, are well documented, however the physical processes of emplacement, i.e. magma flow and solidification, are relatively understudied. The studied sill was 6 m thick, of crinanite composition and a sheet with evidence of magma fingers. Anisotropy of magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM) techniques were combined with petrology, to understand how magnetic fabrics vary across the sill length and thickness. AMS fabrics identified two groupings: Group A at the sill margins showed an initial magma flow aligned with the long-axis of the magma fingers, whereas Group B in the sill interior showed flow reorientation which was inferred to have occurred due to magma finger coalescence. AARM fabrics identified a post-emplacement flow regime, associated with migration of interstitial melt through the solidifying crystal mush. The contrasting fabrics demonstrate that multiple flow regimes were operational within the sill, and that these can be identified using multiple rock magnetism techniques. Within a basaltic dyke from the Skye Dyke Swarm, multiple magnetic fabrics are recorded and originate from contrasting processes. Within the dyke core, magnetic fabrics originate from titanomagnetite, however in margin regions pyrrhotite becomes the dominant source of the fabrics. I identified that within the dyke core a record of lateral magma flow was preserved, however closer to the margins the fabrics were more indicative of magnetite breakdown and growth of pyrrhotite from a sulphide rich hydrothermal fluid which had overprinted any primary flow indicator. There was also variation in fabrics along strike of the dyke, with alteration of the dyke core also evident nearer to branching in dyke strike. This study shows the complexity and variation in magnetic fabrics, how this can vary over short (~13 m) distances and can be affected by post-emplacement alteration. To understand how magnetic fabrics, develop during the propagation of sheet intrusions, a series of laboratory experiments were conducted. In these experiments plaster of Paris (magma analogue) seeded with magnetite particles, a pseudoplastic fluid, was injected into a box filled with flour (host rock analogue), a cohesive granular material. These materials were used to identify the evolution of AMS fabrics across multiple slices of model intrusions. The experimental intrusions formed a range of magma bodies including plutons, dykes, laccoliths and fingers, which indicate a range of processes occurring during initial emplacement and subsequent intrusion growth. AMS analyses of three parallel slices cut perpendicular to the flow axis, identified compressional and shear fabrics closer to the intrusion margins and towards the intrusion leading tips. Magnetic fabrics indicative of flow in the propagation direction were formed closer to the intrusion core and are like those observed in nature, thus demonstrating the potential of laboratory modelling for studying emplacement processes. In combination, these studies of natural intrusions and laboratory experiments investigate the development and evolution of magnetic fabrics across the length and thickness of intrusions. This is important as understanding the evolution of flow during dyke and sill emplacement is vital for determining and improving current emplacement models.

Explosive super-eruptions (≥450 km³) are amongst the most catastrophic events at the Earth's surface, with immediate and devastating regional environmental consequences. Recent catastrophic super-eruptions at Yellowstone are well-known, but the previous (Miocene) history of large explosive eruptions from the Yellowstone hotspot is less-well understood, even though some in the central Snake River Plain (cSRP) may have been similar in size, or larger. To test this, local successions of rhyolitic welded ignimbrites in the southern cSRP have been studied to distinguish and characterise individual eruption-units using a combination of fieldwork, whole-rock and mineral chemistry, rock magnetism and geochronology data to correlate them regionally. In the Rogerson Graben, the revised Rogerson Formation comprises five eruption-units, each designated as a member. In the Cassia Hills ~20 km further east, the revised Cassia Formation comprises thirteen eruptionunits, and a new deep drill-hole near Kimberly, reveals three rhyolitic eruption-units. Robust correlations between these sites and to the north of the Snake River Plain have revealed the presence of three new regionally widespread ignimbrite sheets: (1) the Brown’s View Ignimbrite (10.3±0.2 Ma; 2700 km²; 40 km³ DRE); (2) the McMullen Creek Ignimbrite (9.0 ±0.1 Ma; 12,000 km²; 589 km³ DRE); and (3) the Grey’s Landing Ignimbrite (~9.0 Ma; 18,000 km²; 708 km³ DRE).Using standard techniques to calculate eruption magnitudes, two of these record super-eruptions: the McMullen Creek eruption (magnitude 8.4, 1179 km3 DRE) and the Grey’s Landing eruption (magnitude 8.5, 1416 km³ DRE). These represent the largest super-eruptions within the cSRP and are also among the largest eruptions of the entire Snake River-Yellowstone volcanic province. The addition of the three correlations presented here has further reduced the total number of previously inferred eruption-units from 42 to 29. Therefore, mid-Miocene rhyolitic explosive eruptions in the central Snake River Plain were less numerous but significantly larger than previously thought.

This thesis studied 9 sediment cores from the Bohai Sea (JX91-2A and JX91-3B), the Huanghai Sea (JX91-7m and 7G) and the Arabian Sea (cores 1733, 1734a, 1735, 1736 and 1739) through a multi-disciplinary approach including geochemistry, sedimentology, rock magnetism and radiochemistry. The purpose of this study is to retrieve the sedimentary records, characterise and compare the virtually different sedimentation processes in the Chinese and Arabian regions, and to reconstruct regional and trans-continental palaeoenvironmental changes since the Late Pleistocene. The main conclusions drawn from this study concerning A) the China Seas, B) the Arabian Sea and C) palaeoenvironment in the two regions include: A1. The Huanghe River plays a key role in the sedimentation in the Bohai and Huanghai Seas, and its well documented channel switching can be directly correlated with the lithological changes in the sediments; A2. Estuarine turbidity current can be formed in the Bohai Sea in a different way from that in deepwater, owing to the extremely turbid riverwater and highly diluted seawater; but it cannot in the Huanghai Sea; A3. Huanghe estuarine sediment in the Bohai Sea can be linked to the recent soil erosion on the Loess Plateau through geochemical and palaeomagnetic indicators, viz. P, Ti, REE and magnetic susceptibility; B1. Sediments in the Gulf of Oman in the Arabian Sea are generally of aeolian origin, though coarser sands can be found either as saltation population in nearshore cores or as cyclone deposits on the Murray Ridge; B2. Monsoonal variations since the Late Pleistocene in terms of both wind strength and wind direction are responsible for the lithological changes in the Arabian sediments directly, and biogeochemical changes indirectly, through the wind-driven coastal upwelling system in the Arabian Sea.

This study involves the production of arrays of parallelepiped magnetite particles in the size range 0.1 - 1 μm. This was achieved by utilising electron beam lithography techniques which are often employed in the design of semi-conductor devices. These particles are required in order to understand the mechanism of reversal of magnetisation for pseudo-single-domain (PSD) particles important to paleomagnetic studies. The samples used by previous researchers are magnetite powders. In such samples, despite attempts to disperse the particles onto non-magnetic matrix, there is no way of eliminating particle clumping, and magnetostatic interaction. Low temperature magnetic measurements obtained using samples of cubic arrays of magnetite particles shows that the effect of particle clumping is to increase the amount of saturation isothermal remanence (SIRM) lost at the Verwey transition. The dependence of SIRM lost at the Verwey transition on particle size in the PSD size range, is consistent with the vortex domain structure predicted for unconstrained 3-D micromagnetic studies. The results shows that 'true' magnetic memory is a stress related phenomenon. Magnetic properties of cubic arrays of magnetite particles measured at room temperature are not consistent with the mechanism of magnetic reversal of coherent rotation of atomic magnetic moments, but are generally in good agreement with that of vortex nucleation and propagation in general. The method of domain classification using the coercivity ratio often used in rock magnetism, is shown to be not useful in classifying PSD in the submicron size range. Magnetic properties of rectangular parallelepiped magnetite particles are more complex than expected from the simple demagnetisation shape anisotropy contribution.