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The compaction of firn (the transition of snow to ice) is a process in glaciology that is difficult to constrain without the use of intrusive geophysical techniques. Full Waveform Inversion (FWI) is a seismic imaging technique used to obtain high-resolution images of the subsurface. This research introduces FWI to glaciological modelling and shows that FWI can improve upon the Herglotz-Wiechert travel time tomography method that's use is currently widespread. Using synthetic firn velocity models, acoustic FWI was shown to recover the compressional (P) wave velocity gradient of the firn column and could correct a velocity model that was a poor representation of the subsurface structure (up to a 5% velocity error). Additionally, FWI was able to detect ice layers of a minimum thickness of 10 m (half the minimum wavelength) within the firn column and provide constraints on their depth providing the ice slab did not occur in the shallow surface (20 m or less), where the steepest velocity gradient occurs. FWI is applied to two datasets. Firstly, legacy data acquired by the British Antarctic Survey on Pine Island Glacier were problematic, owing to an inconsistent, high-frequency source waveform that made FWI vulnerable to cycle skipping. Consequently, no improvement was observed to the seismic structure. Secondly, a dataset acquired specifically for this research was collected on Norway's Hardangerjøkulen Ice Cap. With this dataset a combination of Travel Time and Least-Squares FWI can be successful due to the use of a repeatable seismic source with lower frequency content (10 Hz), A VSP survey used to provide near-surface velocity constraints and near offset small receiver spacing (1 m). FWI provided a 6% change to the starting velocity model and improved the data match by 47%. The results showed that acoustic FWI must be considered from the seismic acquisition stage for the technique to be successful in characterising firn velocity structure. In this case, FWI can improve the model of the subsurface velocity structure from the HW technique and increase the match of the modelled seismic data.

Debris-covered glaciers comprise around a third of all glaciers in High Mountain Asia, and thus represent a critical source of meltwater for those living in the foothills of the region. Many of these debris-covered glaciers are situated at elevations exceeding 4,500 m above sea level and display a number of unique supraglacial features and mass balance characteristics arising from the spatially variable thickness of their supraglacial debris layers. Given their remoteness, these glaciers are difficult to access; field-based data are severely lacking, particularly for the subsurface environment, meaning both the parameterisation and validation of numerical modelling experiments are frequently based on assumptions and calculations. To address this gap, this research investigates the subsurface environment of a high-elevation debris-covered glacier, Khumbu Glacier, Nepal Himalaya, through a series of field-based experiments. In particular, the thermal regime, subsurface hydrology, and englacial debris content of Khumbu Glacier are targeted. The results reveal that Khumbu Glacier has a polythermal regime, with a notable proportion of temperate, and potentially warming, ice towards the terminus, which may support the presence of a persistent subsurface hydrology. The glacier's drainage system transports meltwater inefficiently during the early melt season, but has the capacity for more efficient transport with greater inputs. Meltwater transit through a large chain of linked supraglacial ponds is slow, which may promote enhanced surface ablation and, together with a warming thermal regime, may increase the glacier's sensitivity to future climatic warming through time. However, in contrast, the continued melt-out of a relatively high englacial debris content may act to suppress melt rates within only a few decades. Combined, the data presented in this thesis provide a suite of empirical observations that can reduce the current uncertainties inherent in contemporary numerical predictions of debris-covered glacier evolution and water resource availability within the region.

Like in other mountain ranges around the world (the Southern Alps, the Himalayas and the Rockies), glaciers in the European Alps are a major source of fresh water for human usage and actively contribute to sea-level rise. Among European Alpine glaciers, this study shows that 12% are debris-covered, representing 54% in ice volume. Debris-covered glaciers are those presenting a layer of rock debris on their surface. This debris layer has an insulation effect on the glacier, which influences the response to climate change of this type of glacier. However, little is known about their long-term historical evolution and their future behaviour in the face of climate change. By directly comparing debris-covered glaciers and clean-ice glaciers at several different spatial and temporal scales, this study shows the large impact that debriscovered glaciers have on the Alpine cryosphere. On a glacier-wide scale, this study shows that, over 200 years, a debris-covered glacier (Glacier Noir in the French Alps) has retreated 21% less than an adjacent clean-ice glacier (Glacier Blanc) under the same climatic conditions (a temperature rise of minimum 2°C since the 1960s). On the scale of the European Alps, this study shows that debris-covered glaciers will extend the water supply of 145 million inhabitants. By the middle of the 21st century, debris-covered glaciers will contribute more than 22% of the total glacier runoff from the Alps, with a slower release than clean-ice glaciers. The importance of debris-covered glaciers in terms of behaviour and quantity could change the way they are represented in numerical models and could also change forecasts of the fate of glaciers in the European Alps under future climate scenarios.

This study examines how longitudinal foliation develops in glaciers and ice sheets in a wide range of topographic, climatic, and dynamic settings, at a variety of spatial scales. Study locations include four valley glaciers in Svalbard (Austre Brøggerbreen, Midtre Lovénbreen, Austre Lovénbreen, and Pedersenbreen), a valley glacier in Canada (Sermilik Glacier), and seven outlet glaciers in Antarctica (Hatherton Glacier, Taylor Glacier, Ferrar Glacier, Lambert Glacier, Recovery Glacier, Byrd Glacier, and Pine Island Glacier). Detailed structural mapping of the valley glaciers from satellite imagery and field-based measurements were used to document the formation of longitudinal foliation in small-scale ice masses. These findings were ‘up-scaled’ and applied to much larger glaciers and ice streams. Longitudinal foliation develops in concentrated bands at flow unit boundaries as a result of enhanced simple shear. However, longitudinal foliation is not directly observable from satellite imagery at the surface of larger-scale valley glaciers. The longitudinal structures visible at the surface of larger-scale glaciers form at flow unit boundaries and are composed of bands of steeply dipping longitudinal foliation; however, they appear as individual linear features on satellite imagery as a result of the comparatively low spatial resolution of the imagery. The persistence of flowlines in the Antarctic Ice Sheet through areas of crevassing and net ablation (blue-ice areas) suggests that they are the surface representation of a threedimensional structure. Flowlines are therefore inferred to be the surface expression of flow unit boundaries composed of bands of steeply dipping longitudinal foliation. The survival and deformation of flowlines in areas of ice flow stagnation indicates that flowlines form in their initiation zones and not along their entire length. Furthermore, these ice stagnation areas indicate that flowlines record past ice dynamics and switches in ice flow.

The mid-latitudes of Mars host numerous ice-related landforms that bear many similarities to terrestrial ice masses. One particular landform has a strong resemblance to valley or debris-covered glaciers found on Earth and have subsequently become known as ‘glacier-like forms’ (GLFs). GLFs have detailed surface morphologies consistent with recent deposition and viscous deformation of ice, but there is still uncertainty regarding their formation, current and former volume, and dynamic evolution. Specifically, this thesis presents new observations and results that assess the current and former volume and dynamics of Mars’ mid-latitude GLFs. The findings presented in this thesis suggest that at the broadest level Mars’ midlatitudes appear to preserve a complex and spatially heterogeneous record of glaciation and that GLFs appear to be the manifestation of a more localised period of alpine glaciation. GLFs represent an active component of the near-surface water budget of Mars, locking away an estimated global equivalent water layer of between 3 ± 1 and 10 ± 3 mm. Evidence of recession and mass loss was identified in approximately one-third (n = 436) of the GLF population, suggesting that these landforms once contributed a larger volume of water to the near-surface water budget of Mars. Assessment of environmental and topographical controls over current ice volume and ice-mass loss revealed that their distribution and size is unlikely to be purely controlled by insolation forcing. Instead it is suggested that regional to local meteorological and topographical conditions also play an important role in GLF ice accumulation and/or preservation, with variation in physical environments providing microclimates favourable for accumulation and/or preservation of ice. The emerging picture shows that Mars’ GLFs appear to have been dynamically active, and that they have played an important role in altering the surface landscape of Mars through erosion, transport and deposition of material.

Accurate forecasting of the Greenland Ice Sheet's contribution to global sea level change requires detailed knowledge of how ice ow responds to surface water inputs. Both ice velocities and surface melt have increased significantly over the last decade but recent research suggests that ice ow acceleration over the summer is regulated by the seasonal evolution of the subglacial drainage system. To investigate these and associated processes, a network of continuously-operating, dual-frequency global positioning system (GPS) receivers was deployed on a 140-km-long land-terminating transect in West Greenland, providing centimetre-precise, high-frequency records of ice motion. These data reveal that the enhanced summer ow regime is comprised of discrete, transient accelerations driven by the diurnal melt cycle, rapid in situ supraglacial lake drainage and rainfall/melt events. In 2010, a comprehensive array of instruments captured the rapid ( ~ 2 hour) drainage of a large supraglacial lake via a 3-km-long fracture, hydraulically-driven through km-thick ice. A further pronounced, widespread and sustained acceleration driven by rainfall and melt, observed in late August 2011, suggests that the predicted increase in cyclonic activity over Greenland may drive widespread off-season melt, rainfall and ow acceleration across the ice sheet. Together these events provide new insights into the basal hydrodynamic controls on ice sheet motion. Furthermore, observations of a persistent year-on-year acceleration in ice ow between 2009 and 2012 at a high elevation site located ~ 50 km inland of the equilibrium line support the hypothesis that the observed inland expansion of supraglacial lakes is driving faster ice ow at high elevations. These observations contrast with the prevailing self-regulation model and reveal that despite surface melt increasing water inputs to the bed are still insuffcient to develop effective subglacial drainage in the ice sheet's interior.

Mars‟ mid-latitudes host a range of apparently flow-related landforms that strongly resemble terrestrial glaciers. Such „glacier-like forms‟ (GLFs) are strikingly similar to alpine valley glaciers found on Earth. These martian GLFs are believed to be composed of massive water ice but little is known about how they formed, how they evolve, and how they interact with the martian surface. This thesis presents various studies of Mars‟ mid-latitude GLFs that were designed to address some of these issues. New observations and new results were obtained using survey and mapping techniques. The findings presented in this thesis suggest that GLFs, observed ubiquitously in Mars‟ mid-latitudes, are all composed of a similar material and were formed in a similar way, most likely under Earth-like „mass-balance‟ conditions whereby ice accumulated at altitude and subsequently flowed, under its own weight, downhill and into an ablation zone where GLFs experienced net mass loss. It is likely these conditions existed on Mars during a past martian ice age. From their geographical distribution (relative to latitude, elevation and relief) it would appear that those GLFs that remain on Mars today are relict deposits that have survived where local conditions impair ablation, and that these residual GLFs appear currently to flow under the influence of local relief and gravity. From the examination of crevasse patterns it appears that this flow occurs under an englacial strain regime similar to that which commonly defines spatial flow patterns in terrestrial glaciers. This thesis also observes and identifies various small-scale features and textures that suggest that the evolution and subsequent flow of GLFs has been (or may currently be) sensitive to many environmental factors common on Earth, such as the agency of liquid water. This suggests that GLFs (and possibly other associated ice masses) may have been significant movers and shapers of Mars‟ surface sediments and structures, contributing widely to the evolution of Mars‟ present-day surface.

This study investigates sediment transport processes and sediment-landform development at two small valley glaciers of contrasting basal thermal-regime. A range of innovative techniques are applied to characterize and quantify glacier structure, glacially transported sediment facies and sediment-landform assemblages at Glacier de Tsanfleuron, Switzerland, and Midre Lov ́enbreen, Svalbard. Sediment transport at Glacier de Tsanfleuron is dominated by a rockfall-derived supraglacial debris stripe and abrasion-derived subglacial debris planes. Modeled sediment transport rates are equivalent to a glacier-wide erosion rate of 0.5 mm a−1 . Subglacially transported sediment ablates to form a complex network of debris stripes, the texture and morphology of which are consistent with sediment supply from palaeokarst sinkholes. These sinkholes dominate the limestone plateau on which the glacier is situated and also restrict glaciofluvial sediment transport by rerouting much of the glacier’s meltwater though the karst system. This results in a landsystem that is atypical of valley glaciers in the Alps. Sediment transport at Midre Lov ́enbreen is strongly influenced by glacier structure, identified at the glacier surface and in digital optical televiewing borehole logs. Glacier structure is successfully reconstructed in three-dimensions, revealing several visually distinctive englacial ice properties and deformation structures. Basally derived englacial sediment layers are found to be intercalated with primary stratification, elevated into near-vertical planes around a gently upglacier dipping central fold axis by large-scale lateral folding. Sediment analysis also allows supraglacial longitudinal debris ridges to be sub-classified into two types, with a new (Type II) form experiencing secondary deformation by small-scale flow parallel recumbent folding in association with vertical displacements across arcuate shear planes near the glacier terminus. Non-structurally controlled sediment transport at Midre Lov ́enbreen is dominated by low-intensity deformation of subglacial till, and is also characterized by the formation of supraglacial debris cones and proglacial flutes. Flutes at Midre Lov ́enbreen are interpreted to form beneath warm-based ice by the squeezing of partially fluidized till into cavities where freeze-on occurs due to a ’heat-pump’ effect across subglacial boulders. The results of this investigation demonstrate that, while basal-thermal regime may exert a strong influence on glacial sediment-transport processes that operate at the small-scale, bedrock physiography ultimately defines glacier morphometry and resulting large-scale flow structures. Additionally, although some of the sediment-landform types identified in this study are exclusive to a particular set of basal-thermal conditions, the composition and morphology of these sediment-landform assemblages are such that they are unlikely to be preserved in the geological record.

In order to increase coverage, mass balance changes of the world’s ice sheets are increasingly derived from surface elevation changes measured via satellite. Across the percolation zone of the Greenland Ice Sheet, meltwater, percolation and refreezing cause a re-distribution of mass through densification which may result in elevation change with no associated mass loss. Therefore, densification processes need to be quantified, spatially and temporally, and accounted for in mass balance measurements. This thesis investigates the relationships between patterns of elevation change and temporally and spatially variable accumulation and densification processes. In doing so, it provides an important contribution to the validation of the European Space Agency’s CryoSat-2 mission by placing error bars on the accuracy to which changes in satellite-measured ice-mass surface elevation represent real changes in ice mass. Temporal variability in near-surface (<10 m) snowpack and firn density and structure was measured in snowpits, shallow cores and using a neutron probe in the spring and autumn of 2004 at ~1945 m elevation (T05, 69o 51N, 47o 15W) in the percolation zone of the Greenland Ice Sheet. Results show that average snowpack density increased by 26% from spring to autumn, with a 5% (7.6 cm) increase in elevation, and a corresponding 32% increase in mass. Spatial variability was investigated at 11 sites along two transects at spatial scales of 1 m – 10 km. Whilst there was little variability in small scale (1 - 100 m) density changes, ‘seasonal densification’ increased at lower elevations, rising to 47% 10 km closer to the ice sheet margin at 1860 m a.s.l. The spatial variability in seasonal densification was further investigated in spring 2006 at seven sites located at ~10 km intervals along a 57 km transect spanning a 350 m elevation range. Snowpits and shallow cores reveal no significant variation in spring (prior to melt) snowpack density but following summer melt and refreezing cycles, seasonal densification decreased with increasing elevation at 32 kg m-3 per 100 m. Measurements at three sites ranging in elevation from 1860 – 2015 m and spanning three melt-seasons show inter-annual variation in the seasonal densification gradient. In order to obtain a longer time series of mass balance, a 17 m core retrieved in spring 2004 was analysed for stratigraphy, density and ionic and isotopic concentrations to identify annual layers. Unfortunately, the seasonal melt cycle (whereby on average 10% of the snowpack undergoes melt), results in a complex stratigraphy and density and ionic concentrations that cannot be resolved into a seasonal signature. However, the δ18O and δ D isotopes show clear sinusoidal fluctuations, which have been used to derive annual mass balance from 1986 to 2003. These show a mean annual accumulation of 53.7 cm w.e. (s.d. 12.9 cm w.e.) although the accuracy of these measurements is compromised by the percolation of meltwater through more than more year’s snowpack. These findings confirm that estimates of mass balance cannot be calculated solely from observed changes in surface elevation. However, predicting spatial and temporal variations in densification is not straightforward because of the complex inter-annual variations in the processes of accumulation, melt, percolation and refreezing.