Most Of The Sediment That Glaciers Carry Comes From ______.

Most of the Sediment That Glaciers Carry Comes From… the Landscape Itself!

Glaciers, colossal rivers of ice, are powerful agents of erosion and transportation. They carve out valleys, transport vast quantities of rock and sediment, and ultimately reshape the Earth's surface. But where does all that sediment come from? The simple answer is: the landscape itself. However, understanding the intricacies of glacial sediment transport requires a deeper dive into the processes involved. This article will explore the various sources of glacial sediment, the mechanisms of erosion and transportation, and the characteristics of the resulting deposits.

The Primary Source: Bedrock Erosion

The majority of the sediment carried by glaciers originates from the erosion of bedrock. This process is significantly different from fluvial (river) erosion. While rivers primarily erode through abrasion and hydraulic action, glaciers utilize a more powerful combination of processes:

1. Abrasion:

Glaciers act like giant sandpaper, grinding away at the underlying bedrock. The embedded rock fragments within the ice – ranging in size from fine sand to enormous boulders – act as abrasive tools, scratching and polishing the rock surface. This process is particularly effective in areas where the glacier base is frozen to the bedrock, creating a powerful scraping action. The intensity of abrasion is influenced by several factors including:

• Ice thickness: Thicker glaciers exert greater pressure, leading to increased abrasion.
• Basal sliding rate: Faster movement translates to more intense abrasion.
• Sediment concentration within the ice: Higher concentrations of rock fragments enhance the abrasive capacity.
• Bedrock hardness: Softer bedrock is more susceptible to abrasion than harder varieties.

2. Plucking:

This is a more forceful process than abrasion. As the glacier moves, meltwater penetrates cracks and joints in the bedrock. This water refreezes, expanding and exerting immense pressure on the rock. This pressure, combined with the glacier's sheer force, causes fragments of rock to be "plucked" from the bedrock and incorporated into the glacier's base. Plucking is particularly effective in areas with fractured or jointed bedrock. The size of the plucked fragments can range from small pebbles to massive blocks.

3. Freeze-thaw weathering:

While not strictly a process of glacial erosion, freeze-thaw weathering plays a crucial role in preparing the bedrock for glacial erosion. Water seeps into cracks and fissures in the rock, freezes, and expands. This repeated cycle of freezing and thawing weakens the rock, making it more susceptible to both abrasion and plucking by the advancing glacier. This pre-weakening significantly aids glacial erosion.

Secondary Sources: Mass Wasting and Supraglacial Debris

While bedrock erosion is the dominant source, glaciers also acquire sediment from other sources:

1. Mass Wasting:

Mass wasting encompasses a range of processes by which material moves downslope under the influence of gravity. These processes, such as rockfalls, landslides, and debris flows, can deliver large quantities of sediment onto the glacier's surface. This material then becomes incorporated into the glacier's ice and is transported along with the glacial flow. The steep slopes surrounding glaciers frequently contribute to increased mass wasting events.

2. Supraglacial Debris:

This refers to sediment that is transported on top of the glacier. This debris originates from various sources:

• Avalanches: Avalanches frequently deposit large volumes of snow and rock onto the glacier's surface.
• Rockfalls: Rocks falling from surrounding cliffs directly onto the glacier's surface.
• Wind-blown dust: Fine-grained dust and sand can be deposited on the glacier's surface by wind.
• Rainsplash: Rain can dislodge sediment from surrounding slopes, depositing it onto the glacier.

Supraglacial debris is significant because it can affect the glacier's albedo (reflectivity) and melt rate. Darker debris absorbs more solar radiation, leading to increased melting and potentially impacting the glacier's dynamics.

Sediment Transport within the Glacier

Once sediment is incorporated into the glacier, it undergoes transportation through various mechanisms:

1. Basal Transport:

This is the most significant mode of sediment transport, occurring at the glacier's base. The sediment is embedded within the ice and is transported along with the glacier's movement. Basal transport is characterized by significant abrasion and interaction with the underlying bedrock.

2. Englacial Transport:

This refers to the transport of sediment within the glacier's ice. Sediment is incorporated into the ice through various mechanisms, including plucking, freeze-thaw weathering, and the incorporation of supraglacial debris through melting and refreezing processes. Englacial sediment is transported along with the flow of the ice, moving slowly towards the glacier's terminus.

3. Supraglacial Transport:

As mentioned previously, supraglacial transport refers to the movement of sediment on the glacier's surface. This sediment is transported by various processes including wind, meltwater streams, and gravity-driven downslope movement.

Glacial Deposition and the Resulting Landforms

As glaciers melt, they deposit the sediment they have accumulated. This deposition creates a range of characteristic landforms:

1. Moraines:

These are accumulations of sediment deposited at the glacier's margins (lateral moraines), terminus (terminal moraines), or along the glacier's length (medial moraines). Moraines provide a wealth of information about the glacier's past extent and behavior. The sediment within moraines can be highly heterogeneous, containing a mix of various sizes and types of rock fragments.

2. Outwash Plains:

These are flat, extensive plains formed by meltwater streams flowing from the glacier's terminus. These streams transport and deposit sediment, creating well-sorted deposits stratified by size and grain shape. The coarser material is deposited closer to the glacier, while finer material is deposited farther away.

3. Eskers:

These are long, sinuous ridges of stratified sediment deposited by meltwater streams flowing within or beneath the glacier. Eskers provide valuable insights into the glacier's internal drainage systems.

4. Drumlins:

These are elongated hills of till (unsorted glacial sediment) that are streamlined in the direction of ice flow. Their formation is still debated but is likely related to interactions between the glacier's base and underlying sediment.

5. Kames:

These are conical hills of stratified sediment deposited by meltwater streams or by ice collapse. Kames often form in groups or fields, indicating areas of complex glacial deposition.

Conclusion

In conclusion, the vast majority of sediment transported by glaciers originates from the erosion of bedrock through powerful processes of abrasion and plucking. Secondary sources include mass wasting and supraglacial debris derived from various sources. The sediment is transported within the glacier through basal, englacial, and supraglacial processes, and finally deposited to create a variety of characteristic landforms. Understanding the sources and processes involved in glacial sediment transport is critical to reconstructing past glacial environments and predicting the impact of future climate change on these dynamic landscapes. The intricate relationship between glaciers and the surrounding landscape underscores the fundamental role of bedrock erosion as the primary provider of the vast quantities of sediment shaped and redistributed by these impressive forces of nature.