Mass balance and the equilibrium line

A glacier is an open system. The input is accumulation (snowfall, avalanched snow, blown snow, hoar frost). The output is ablation (melting, sublimation, evaporation, calving of icebergs, wind scour). The store is the ice itself.

• Positive regime (net gain): accumulation exceeds ablation over the year, so the glacier thickens and the snout advances.
• Negative regime (net loss): ablation exceeds accumulation, the glacier thins, and the snout retreats — note that the ice itself still flows downhill; it is the position of the snout that moves back, because the ice is melting faster than it is delivered.
• Steady state / dynamic equilibrium: the two are equal over the long run, so the snout holds its position while ice moves continuously through the system.

The equilibrium line (ELA) — often marked in the field by the firn line — divides the glacier in two: the accumulation zone above it (net gain, over the year) and the ablation zone below it (net loss). A rising ELA in successive years is one of the clearest signals of a warming climate: the accumulation zone shrinks, the ablation zone grows, and the balance turns negative.

Advancing, retreating — or moving?

Your turn — net mass balance

A net balance of −0.7 m w.e. means this glacier thinned by the equivalent of 0.7 m of water, averaged across its whole surface, in one year. Repeat that for a decade and the snout will be somewhere very different.

Your turn — the whole-glacier budget

The net balance above is an average. To get the volume change you must weight each zone by its area.

Gain = 12 million m³; loss = 24 million m³; net = −12 million m³. The accumulation area ratio (here 8/20 = 0.4) is the number glaciologists actually watch: a healthy mid-latitude valley glacier typically needs a ratio of roughly 0.5–0.6 to stay in balance, so 0.4 tells you this glacier is shrinking before you calculate anything.

Warm-based and cold-based ice

By form and scale: cirque (corrie) glaciers occupy hollows high on a mountainside; valley glaciers flow down pre-existing valleys, often fed by several cirques; piedmont glaciers spread out where a valley glacier reaches a lowland; ice caps and ice sheets bury the underlying relief entirely and flow outwards under their own weight, independent of topography.

By thermal regime — and this is the distinction that explains everything else:

• Warm-based (temperate) ice is at or near pressure melting point throughout. Meltwater is present at the bed. The glacier can therefore slide, meltwater lubricates the contact, debris is picked up and dragged across the bedrock, and erosion is rapid. Typical of steep, mid-latitude, relatively warm mountain glaciers (the Alps).
• Cold-based (polar) ice is well below pressure melting point and is frozen to its bed. There is no basal water and essentially no basal sliding: the glacier moves only by slow internal deformation, and it erodes very little. Landscapes beneath cold-based ice can survive almost unmodified for hundreds of thousands of years.

Pressure melting point (PMP) is the temperature at which ice will melt at a given pressure. Increase the pressure and the melting point falls below 0 °C. This one fact underpins basal sliding, regelation, roches moutonnées and plucking — so learn it properly, and use the abbreviation in essays.

Why doesn't polar ice erode?

How ice flows

Ice is a solid, but under stress and over time it behaves as a very viscous fluid. Two mechanisms, and their balance depends on the thermal regime.

• Internal deformation (creep). Under the weight of the ice above, individual ice crystals slip past one another along their basal planes, and the whole mass slowly deforms downslope. It is the only mechanism available to cold-based ice, and it is slow.
• Basal sliding. Available only where the bed is at pressure melting point. A thin film of meltwater reduces friction and the glacier slides bodily over its bed. This is the fast mechanism, and it is what makes warm-based glaciers such powerful erosive agents.
• Regelation. The elegant sub-process. Where ice meets an obstacle on the bed, pressure on the upstream side rises, so the pressure melting point falls and the ice melts. The meltwater flows around the obstacle, where pressure is lower on the downstream side — and refreezes. This is how ice negotiates bumps, and it is precisely how debris is frozen into the base of the glacier ready to abrade.
• Deformation of a soft bed. Where a glacier rests on saturated, unconsolidated sediment, the sediment itself shears — an important mechanism under ice sheets and a cause of surges.

Flow patterns and their landform signature:

• Extending flow — where the gradient steepens, ice accelerates, thins and stretches. Tension opens transverse crevasses. Erosion is enhanced, so the bed is cut deeper: this is how rock basins and ribbon lakes begin.
• Compressing flow — where the gradient flattens, ice decelerates, thickens and piles up. Debris is thrust upward along shear planes. Erosion is enhanced by the increased thickness in some settings; deposition dominates near the snout.
• Surging — a glacier's velocity increases dramatically for months or years, usually because meltwater has built up at the bed or the subglacial sediment has begun to deform, decoupling the ice from its bed. Surges are episodic and largely independent of the climate signal — a fact worth deploying in any essay that claims glacier behaviour is a simple thermometer.
• Velocity varies within a glacier too: fastest at the surface centre-line (least friction), slowest at the bed and margins. Hence crevasse patterns curve down-glacier at the sides.

Regelation in action

Glacial erosion, glacial deposition, or fluvioglacial?

Tap a term, then tap the column it belongs to. The glacial/fluvioglacial distinction is one of AQA's favourite questions — get it automatic.

Plucking, abrasion, and how debris gets there

• Freeze-thaw (frost shattering) — water enters joints in the rock walls above the ice, freezes, expands by roughly 9%, and levers the rock apart. This supplies the angular debris that falls onto the glacier surface. It is a weathering process, not a glacial one — say so.
• Plucking (quarrying) — meltwater at the bed penetrates joints in the bedrock and refreezes, welding rock fragments to the base of the glacier; as the ice moves on, it rips them out. Requires warm-based ice and, crucially, well-jointed rock. It leaves a jagged, torn surface.
• Abrasion — the rock fragments now embedded in the ice act as tools, scouring the bed like sandpaper. It produces striations (scratches, which record the direction of ice flow), grooves, polished surfaces, and fine rock flour (which gives glacial meltwater its milky turquoise colour). Requires debris; a clean glacier cannot abrade.
• Entrainment and transport — debris travels supraglacially (on the surface, from rockfall), englacially (within the ice, having fallen into crevasses) and subglacially (at the bed, from plucking and abrasion). Because ice is a solid, it does not sort by size or weight — a boulder and a clay particle travel side by side at the same speed. Remember that; it is the whole basis of the next distinction.
• Deposition (till) — ice deposits its load simply by melting. Everything it was carrying is dumped together.

Erosion is a partnership. Plucking supplies the tools; abrasion uses them. A glacier that has not plucked cannot abrade. And both require pressure melting at the bed — which is why "how effective is glacial erosion?" is always answered with it depends on the thermal regime, the rock type and jointing, the ice thickness (pressure), the velocity, and the debris supply.

From the corrie to the trough

• Corrie (cirque, cwm). Snow collects in a shaded hollow, compacts to ice, and the ice rotates within the hollow (rotational slip). Plucking steepens the back wall; abrasion under the deepest, thickest ice over-deepens the basin; less pressure at the exit leaves a rock lip. When the ice goes, a lake — a tarn — is trapped behind that lip. A bergschrund (the crevasse between moving ice and the back wall) allows meltwater down to the wall, feeding freeze-thaw.
• Arête — a sharp knife-edge ridge where two corries erode back to back. Pyramidal peak (horn) — where three or more corries erode back into the same mountain.
• Glacial trough — a former river valley widened, deepened and straightened into a U-shaped cross-profile. The interlocking spurs of the old river valley are sliced off as truncated spurs. Hanging valleys are the tributaries: their smaller glaciers had less erosive power, so they could not cut down as fast as the trunk glacier, and they are left perched high on the trough wall, discharging as waterfalls.
• Ribbon lake — a long, narrow lake on the trough floor, occupying a rock basin over-deepened where the ice was thicker, moved faster (extending flow), or crossed a band of weaker rock.
• Roche moutonnée — a bedrock knob with a smooth, abraded, striated, gently sloping stoss (upstream) face, and a steep, jagged, plucked lee (downstream) face. It is a landform that literally displays the two processes on its two sides — pressure melting on the up-glacier face allows abrasion; refreezing on the lee face allows plucking. It also tells you the direction of ice flow.
• Striations — parallel scratches on bedrock, recording flow direction. Crag and tail — a resistant crag protects softer rock in its lee, leaving a tapering tail (Edinburgh Castle rock is the textbook example).

Name that landform

Tap a description on the left, then the landform it defines. The description gives the mechanism — which is the part that earns the marks.

Reading a roche moutonnée

Till, moraine, drumlin, erratic

Till is material deposited directly by ice. Because ice cannot sort, till is unsorted (every grain size jumbled together, from clay to boulders), unstratified (no layers) and angular to sub-angular (clasts have not been rounded by water). Learn those three adjectives — they are worth a mark each and they are the basis of every "distinguish between" question in this topic.

• Lateral moraine — a ridge of angular debris along the margin of a valley glacier, sourced from frost-shattered rockfall off the valley walls.
• Medial moraine — where two glaciers merge, their inner lateral moraines join to form a debris stripe running down the middle of the combined glacier.
• Terminal (end) moraine — a ridge across the valley marking the furthest extent the ice reached. Debris carried to the snout is dumped there as the ice melts; if the snout holds position, the ridge grows.
• Recessional moraine — a series of ridges up-valley of the terminal moraine, each marking a stillstand during an overall retreat. A sequence of them is a chronology of deglaciation.
• Push moraine — a moraine bulldozed and shoved into ridges by a temporary re-advance; the clasts are often tilted up-glacier, which is how you identify it.
• Drumlin — a streamlined mound of till, up to a kilometre or so long: blunt, steep stoss end facing up-glacier, tapering gently down-glacier. They occur in swarms ("basket of eggs" topography). The formation mechanism remains debated — deposition around an obstacle, remoulding of pre-existing till by overriding ice, and subglacial sediment deformation are all argued. Say so: examiners reward candidates who know the mechanism is contested.
• Erratic — a boulder of a rock type foreign to the area in which it now sits. Because you can identify the source outcrop, erratics are powerful evidence of the direction and extent of former ice movement (e.g. Norwegian rock found in eastern England; Lake District Shap granite scattered across northern England).

What the meltwater does — and how to tell

Meltwater flows on top of, inside, beneath and beyond the glacier, often under great pressure, and it behaves like any river: it sorts its load by size and it rounds its clasts by attrition.

• Outwash plain (sandur) — a broad, flat spread of sediment beyond the terminal moraine, laid down by braided meltwater streams. Sediment is graded down-valley: coarse gravels close to the ice, sand further out, and clay in distal lakes — because meltwater loses competence with distance. This down-valley grading is the visible proof of sorting.
• Esker — a long, sinuous ridge of sorted, stratified sand and gravel, deposited in a subglacial or englacial meltwater tunnel and left standing proud when the surrounding ice melts away. Its most striking property: an esker can run uphill — impossible for a normal river, but not for water flowing under hydrostatic pressure inside a glacier.
• Kame — a mound of sorted, stratified sediment deposited in a hollow or depression on or against the ice, and let down onto the ground surface as the ice melts. Kame terraces form along the valley side, where meltwater flowed between the ice margin and the valley wall.
• Kettle hole — a block of ice, buried in outwash, melts in place; the sediment above collapses into the void, leaving a depression that often fills with water. A landscape of kames and kettles is kame-and-kettle topography.
• Proglacial and subglacial meltwater channels — cut by huge volumes of meltwater; often dry today, and frequently out of all proportion to any stream that occupies them now.

How to answer "distinguish between glacial and fluvioglacial deposits" (a standard 4- or 6-marker): do not just list adjectives — give the reason. Ice is a solid: it carries a boulder and a clay particle at the same velocity, so on melting it dumps them together, unsorted, unstratified and still angular. Water is a fluid: its competence depends on velocity, so it drops the coarsest material first and the finest last, in layers that record changing discharge, and it rounds its clasts by attrition en route. The property follows from the physics.

Whose deposit is this?

Beyond the ice: the frozen ground

Periglacial environments are cold, frost-dominated regions at or near the margins of ice — and, importantly, much of lowland Britain was periglacial during the Quaternary, which is why periglacial landforms turn up in southern England.

• Permafrost — ground that remains at or below 0 °C for at least two consecutive years. Classify it as continuous (very cold, hundreds of metres deep), discontinuous (patchy) or sporadic (isolated islands of frozen ground).
• The active layer — the surface layer that thaws each summer and refreezes each winter. It is the key to almost every periglacial landform: because the permafrost beneath is impermeable, meltwater cannot drain away, so the active layer becomes saturated, weak and mobile.

Processes:

• Frost heave — freezing water expands and lifts the ground; because stones conduct heat better than the surrounding soil, ice forms beneath them preferentially and pushes them upward. Repeated cycles gradually work coarse clasts to the surface.
• Solifluction (gelifluction) — the saturated active layer flows slowly downslope over the impermeable permafrost, on gradients as low as 1–2°. It produces solifluction lobes and sheets, and it is why so many British hillslopes are draped in a structureless deposit of mixed material (head).
• Nivation — the combination of freeze-thaw, chemical weathering and meltwater removal beneath a snow patch, which excavates a hollow. Nivation hollows are how corries begin — a genuinely useful link between the two halves of this option.
• Frost shattering — as above, producing angular scree and, on flat plateau surfaces, blockfields (felsenmeer).

Landforms:

• Patterned ground — frost heave plus sorting: stone circles and polygons on flat ground, drawn out into stone stripes on slopes as the sorted stones creep downhill.
• Ice wedge — winter contraction cracks the frozen ground; summer meltwater runs into the crack and freezes; repeated over centuries, a downward-tapering wedge of ice grows. Filled with sediment after the ice goes, fossil ice wedges are diagnostic evidence of former permafrost.
• Pingo — an ice-cored mound. In the open-system (East Greenland) type, groundwater flows under pressure into a spot beneath thin permafrost and freezes, doming the surface. In the closed-system (Mackenzie) type, a lake drains or infills, permafrost advances into the saturated ground beneath, and the trapped water freezes and expands upward. Collapsed fossil pingos leave ramparted depressions — several are recorded in East Anglia and Wales.
• Thermokarst — the irregular, hummocky, waterlogged terrain produced when permafrost thaws and the ground subsides. Increasingly significant, because it is happening now, and because thawing permafrost releases CO₂ and methane — a positive feedback linking straight back to the carbon cycle.

Why does the active layer flow?

People in a glaciated landscape

AQA requires a case study of a glaciated landscape, its use by people, and the conflicts and management that follow. Whichever region your school uses — the Lake District, the Alps, the Himalaya — structure it identically:

• Why people value it. Water — glaciers and snowpack act as a natural reservoir, storing winter precipitation and releasing it as meltwater in summer, exactly when demand is highest; hundreds of millions of people in Asia depend on the seasonal contribution of Himalayan and Tibetan snow and ice to river flow, though the proportion varies greatly between basins and between seasons, and is often overstated. HEP — steep troughs, hanging valleys and reliable meltwater are ideal. Tourism — dramatic relief, skiing, hiking, ribbon lakes. Farming — thin, stony soils on till support hill sheep farming, little else. Quarrying — moraine, sandur and esker gravels are valuable aggregate.
• The conflicts. Tourism brings congestion, footpath erosion, second homes that price out local people, and pressure on fragile alpine vegetation. Ski development requires piste engineering, lifts and (increasingly) artificial snow, which consumes water and energy. Reservoirs drown valley floors and alter downstream flow. Quarrying scars the landscape but employs local people. Conservation designations (national park, SSSI, World Heritage) restrict development — and the people who bear that cost are rarely the people who enjoy the benefit.
• The hazards. Avalanches. Glacial lake outburst floods (GLOFs) — meltwater ponds behind an unstable moraine dam; the dam fails and releases a catastrophic flood downstream. As glaciers retreat, more and larger such lakes form, and GLOF risk is widely reported to be rising across the Himalaya and the Andes. Rock and ice avalanches. Slope instability as permafrost thaws in the high Alps, which destabilises the rock faces that mountain infrastructure is bolted to.
• Ice loss and sea level. Mountain glaciers worldwide are losing mass; the great ice sheets of Greenland and Antarctica hold, between them, enough water to raise global sea level by many tens of metres, and their current loss is a significant and growing contributor to sea-level rise. Do not invent tonnages or millimetre-per-year figures — the published estimates carry real uncertainty and are revised often. Describe the mechanism (surface melt, meltwater lubricating the bed, ice-shelf loss removing the buttress that restrains outlet glaciers) and the direction of travel, and hedge the magnitude.
• Management approaches. Zoning (honeypot sites concentrate visitors and protect the rest); footpath repair and pitching; visitor management and park-and-ride; planning controls on second homes and on ski expansion; GLOF risk reduction by controlled lowering of moraine-dammed lakes and by early-warning systems; sustainable tourism accreditation. Evaluate by asking who bears the cost and who takes the benefit — that is the question that separates a good answer from a list.

The best evaluative line in this whole topic: a glaciated landscape is a relict system. The Lake District's corries, troughs and ribbon lakes were made by ice that has been gone for around ten thousand years. So the landscape people are fighting over is not being maintained by the process that made it — which means damage is effectively irreversible on any human timescale, and the case for conservation is a case about preserving inheritance, not managing an ongoing process.

Writing the evaluative essay

Paper 1 essays are marked on AO1 (knowledge), AO2 (application and analysis) and — decisively — a conclusion that answers the question. Two question types dominate this option:

• "Assess the extent to which the effectiveness of glacial erosion depends on the thermal regime of the ice." Argue that thermal regime is necessary but not sufficient: without pressure melting at the bed, there is no basal sliding, so no plucking and no abrasion, and the landscape is preserved rather than destroyed. But even a warm-based glacier cannot abrade without a debris supply (which depends on jointing and on frost shattering), cannot pluck without well-jointed rock, and erodes faster where ice is thick (pressure), fast-moving and under extending flow. Judgement: thermal regime is the gateway condition — it determines whether erosion happens at all — while geology, debris supply and velocity determine how much.
• "To what extent are the landforms of a glaciated landscape the product of glacial rather than fluvioglacial or periglacial processes?" Use the sorting test as your evidence base. Concede that the large-scale relief (troughs, corries, arêtes) is unambiguously glacial. But the surface deposits over vast areas — outwash plains, eskers, kames — are fluvioglacial, and the slope forms and superficial deposits of formerly periglacial regions (head, solifluction lobes, blockfields, fossil ice wedges) are periglacial. Judgement: scale decides the answer — glacial processes made the shape of the land; fluvioglacial and periglacial processes made most of what is lying on top of it.

Habits that lift a script: (1) use pressure melting point explicitly — it is the pivot of half this option; (2) always distinguish the agent (ice, meltwater, frost) before naming the landform; (3) exploit the relict-landscape argument — most of what you are describing is not in equilibrium with today's climate; (4) hedge figures you cannot source, and never invent one.

Sharpening the judgement

The big ideas to know

That is the whole of AQA 3.1.4. Press Finish to see your score.