A glacier is a large quantity of ice formed from snow that has accumulated and been compacted over a long period of time. About 70% of the Earth’s freshwater resource is stored in glacial ice.
How are glaciers formed?
Glaciers are formed from layers of snow converted to ice. When snowflakes fall, they are feathery and light. Due to partial melting and compaction, the accumulated snow becomes rounded ice crystals with interconnecting air spaces. This is called firn or névé. As firn is compacted further by the weight of the overlying snow, air spaces are reduced and dense glacial ice is produced. This can take many years. The density of the ice formed is about 0.9 mg m-3.
Gravity and the weight of accumulating snow and ice moves the glacier downhill. Rates at which glaciers move vary from 20 m per year to 10 km per year. At the bottom end of the glacier, either the ice melts or, if the glacier flows out over the sea or a lake, chunks of ice break off, producing icebergs.
This news story reports on data showing that glaciers in both New Zealand and worldwide are melting at the fastest rate since record-keeping began.
Erosion and deposition
Ice is very effective at eroding rock, or gradually wearing it away. Melting and refreezing occurs on the underside of the glacier, causing rocks to be frozen into the lower surface of the ice, producing an abrasive surface rather like sandpaper. Striations (parallel scratches) are left in the rocks as the glacier moves over them.
Sediment and rocks accumulated by the glacier are also carried in the glacier deposited a distance away from where they were collected. These deposits are called till. Ancient striations and till have been used as evidence of continental drift.
The past climate is recorded in the ice
As ice sheets can persist for many thousands of years, the air bubbles trapped in them can provide a record of past atmospheric composition. Dating of ice cores taken from glaciers is done in various ways, including measuring changes in electrical conductivity, recording seasonal changes in the ice, comparison of volcanic ash with known eruptions, comparison with other ice cores, a mathematical technique called isolation and modelling of accumulation and flow rate changes of the ice.
Glaciers on Antarctica
About 30 million km3 of freshwater is locked up in Antarctica’s ice. This is 90% of the Earth’s ice. Total Antarctic ice is estimated to cover 13.72 million km2 – 98% of the total land area. The ice sheet mostly covers land but on the western side of the continent it extends into the sea. It is so cold in central Antarctica (-30 to -80°C) that it takes sevearl thousand years for snow to form glacial ice. The oldest ice that has been found is from the EPICA Dome C core and is estimated to be about 950,000 years old. There are many valley glaciers as well as the ice sheet on Antarctica.
Glaciologist Trevor Chinn talks about the rapid shrinkage of the country's glaciers in this Radio NZ interview. Over four decades, Dr Trevor Chinn has photographed all of the South Islands glaciers as part of a world glacier inventory project.