Plucking and its mechanisms

Plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the erosion and transportation of individual pieces of bedrock, especially large "joint blocks".

This occurs in a type of glacier called a "valley glacier". As a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints (cracks) in the bedrock. The freezing and thawing action of the ice enlarges, widens, or causes further cracks in the bedrock as it changes volume across the ice/water phase transition (a form of hydraulic wedging), gradually loosening the rock between the joints. This produces large pieces of rock called joint blocks. Eventually these joint blocks come loose and become trapped in the glacier.

In this way, plucking has been linked to regelation (Regelation is the phenomenon of melting under pressure and freezing again when the pressure is reduced.). Rocks of all sizes can become trapped in the bottom of the glacier. Joint blocks up to three meters have been "plucked" and transported. These entrained rock fragments can also cause abrasion along the subsequent bedrock and walls. Plucking also leads to chatter marks, wedge shaped indentations left on the bedrock or other rock surfaces. Glacial plucking both exploits pre-existing fractures in the bedrock and requires continued fracturing to maintain the cycle of erosion. Glacial plucking is most significant where the rock surface is well jointed or fractured or where it contains exposed bed planes, as this allows meltwater and clasts to penetrate more easily.

Plucking mechanisms.

Glacial plucking is largely dependent on the amount of stress exerted on a clast overlain by glacial ice. This relationship is a balance between the sheer stress exerted on the clast and the normal pressure on the clast by a body of ice. Plucking is increased where there are preexisting fractures in a rock bed. As the glacier slides down a mountain, energy from friction, pressure or geothermal heat causes glacial meltwater to infiltrate the spaces between rocks. This process, known as frost wedging, puts stress on the rock structure as water expands when it freezes. Impacts from large clasts carried in the bedload can cause additional stress to the bedrock.

Mechanical erosion

Glacial plucking is the main mechanism of other small scale mechanical glacial erosion such as striation, abrasion and glacial polishing. The heavier the sediment load, the more extreme the erosion of the downhill landscape. Erosion is largely dependent on the amount of water flow and its velocity, the clast size and hardness with relation to the stability of the slope.

Glacial striation

A rock that has been subject to glacial erosion will often show a striation pattern in which the rock appears scratched. Long parallel lines will cover the rock and show the appearance of something having been dragged along the top of it.

Polishing

Glacial polishing is the result of clasts embedded in glacial ice passing over bedrock and grinding down the top of the rock into a smoother surface. The small rocks entrained by plucking act like sandpaper to the downhill slope. This creates an almost mirror like surface in the rock. Polish indicates a more recent process as it is often lost to weathering of the rock surface.

Glacial till

The joint blocks and rock fragments that are entrained and carried down the slope can be deposited as till. This leads to a whole set of depositional glacial landforms such as moraines, roche moutonnée, glacial erratics and drumlin fields.

43. Types of river regimes in permafrost areas

There are three basic types of regimes:

· simple regime - one maximum and one minimum per year

· mixed regime - two maximums and two minimums per year

· complex mode - several extrema

Simple regimes: Simple regimes can be nival, pluvial or glacial, depending on the origin of the water.

Glacial regime:

· Very high discharge in summer after the ice melt

· Very low discharge from the end of autumn to early spring

· Amplitude of monthly variation of discharge greater than 25

· Very high daily variability in discharge during the year

· High flow (several hundred l/s/km2). It is found at high altitudes, above 2,500 metres

Nival:

The nival regime is similar to the glacial, but attenuated and the maximum takes place earlier, in June. It can be mountain or plain nival. The characteristics of the plain nival are:

· Short and violent flood in April–May following massive spring thawing ofwinter snows

· Great daily variability

· Very great variability over the course of the year

· Great inter-annual variability

· Significant flow

Pluvial:

· high water in winter and spring

· low discharge in summer

· great inter-annual variability

· flow is generally rather weak. It is typical of rivers at low to moderate altitude (500 to 1,000).

Mixed regimes:

Nivo-glacial:Only one true maximum, which occurs in the late spring or the early summer (from May to July in the case of the Northern hemisphere), relatively high diurnal variations during the hot season, significant yearly variation, but less than in the snow regime, significant flow

Nivo-pluvial:Two maximums, the first occurring in the spring and the other in autumn, a main low-water in October and a secondary low-water in January, significant inter-annual variations

Pluvio-nival:A period of rainfall in late autumn due to abundant rainfall, followed by a light increase due to snow melt in early spring, the single minimum occurs in autumn, low amplitude

Complex regimes: The complex regime is characteristic of large rivers, the flow of which is diverly influenced by numerous tributaries from different altitudes, climates etc. The influences diminish extreme discharges and increase the regularity of the mean monthly discharge from upstream to downstream.

Stream Flow in Arctic:

Hydrologists recognize several different hydrologic regimes:

• Arctic nival - The nival regime represents stream flow derived from melting snow, thawing of the active layer, and direct precipitation.

• Subarctic nival - The freshet occurs several weeks earlier in the Subarctic environment compared to the Arctic environment.

• proglacial - The proglacial regime represents stream flow derived from the melting of snow and glacier ice, and direct precipitation.

• wetland/muskeg - The wetland regime represents stream flow through wetlands.

(In the present climate, most streams and rivers originating within the Arctic have a nival regime in which snowmelt produces high flows and negligible flow occurs in winter.)