Walking on Glaciers

New Zealand was a joy to visit as it offers many scenic natural wonders!  In fact, much of its beauty seem to have something to do with glaciers.  For one, the scenic Milford Sound and its surrounding cousins in South Island were carved by glaciers long ago. As you travel northwards from the sounds, you will be met by two actual glaciers in Fox and Franz Josef.  We joined a walking tour on Franz Josef, the bigger of the two brothers.  The tour was an intriguing experience and I decided to do some research upon returning.

What Are Glaciers? 

Glaciers are perennial masses of ice that moves over land. They form when precipitation accumulates faster than it disappears on the surface of a terrain (a.k.a. ablation). As layers of ice and snow build upon each other, the granular ice at the bottom fuses to form firn. 

Glacial Types

• Alpine glaciers - form on mountain slopes.  Those that form on valleys are called valley glaciers. Franz Josef is an alpine glacier.
• Ice sheets - are ice that cover entire land masses, like the ones in Greenland and Antarctica. 
• Tidewater glaciers - are glaciers that terminate in the sea.  The ice chunks that break free into the sea are called icebergs.

Glacial Structure

• The origin of a glacier is called glacier head.
• The end (bottom) of a glacier is called terminus. 
• Accumulation zone: this region experiences a net gain in glacial mass.
• Ablation zone: this region experiences a net loss in glacial mass.
• Equilibrium line: this region separates the accumulation and ablation zones. The net gain in mass is equal to the net loss. 

Glacial Movement

Glaciers move downslope due to gravity.  Picture ice sliding over the terrain, lubricated by a layer of water.  This layer of water is primarily is formed from friction between ice and rock, but can also be attributed to geothermal heat contributing from the Earth's interior.  The top layers of the glacier move faster than the bottom layers, often resulting in crevasses and cracks on the surface. Glacier speeds vary depending on the glacier and slope of the mountain. They range from 10cm to 30m a day. Glaciers sometimes experience surges due to failures of the underlying rocks and will move a lot quicker than usual. 

Glaciers also undergo cycles of advances and retreats.  Franz Josef is currently in the advance phase, although it had retreated over several kilometres between 1940 - 80. 

Glacial Geology

As glaciers move across an area, it absorbs rocks and sediments either through plucking or abrasion. Plucking is the softening and uplifting of rocks due to sub-glacial water seeping into cracks and then expanding during freezing. Abrasion is caused when the ice and its rock fragments slide across the terrain, essentially sandpapering the surface below. This often produces rock flour, fine rock particles that can suspend in water. 

Glacial moraines are formed by the deposition of material usually  exposed after a glacial retreat.  They often appear as mounds of till, a pile of mixed rocks, gravel and boulders.  Terminal moraines are formed at the foot of a glacier. Lateral moraines are formed on the sides of a glacier. 

After glacial passage, V-shaped valleys usually become U-shaped.  Besides widening and deepening, glacial erosion also smooths the terrain, leaving a relatively flat bottom. 

-PTS