I thought that I'd start this section on glaciation with a look at how glaciers form in case, like me, the students haven't studied glaciers before. An icy worldThe Earth goes through cycles of ice ages, cold periods which last for millions of years. During these ice ages, there are cooler glacial periods and warmer interglacial periods. The last glacial period began about 100,000 years ago and ended about 10,000 years ago. Around 20,000 years ago over 30% of the land on Earth was covered by ice, including almost all of the UK. Ice sheets in the UK were up to 3 miles thick and one reached as far south as London. Today, we're in an interglacial period and only 10% of the Earth's land surface is covered by ice. How do we know about these past changes in temperature? There are three types of evidence for the changes: chemical evidence from the changing chemical composition of ice and marine sediments, from fossil remains showing the adaptations of the organisms to the past climate and, importantly for geographers, from the landforms that were created by glaciers. How are glaciers formed?![]() Glaciers are often described as rivers of ice (see photo right). They move downhill and are so powerful that they carve valleys out of mountains. Glaciers form when multiple snowfalls in mountainous or polar regions fill valleys and sheltered hollows. Summer temperatures are important, as this snow must remain on the ground year round in order for glaciers to form. As these successive layers are compacted by further snowfall on top, they turn into ice. This process is demonstrated in this BBC video How glaciers form. This section of the glacier where the snow accumulates is called the zone of accumulation. As the snow and ice continue to pile up, the lower layers get pushed downwards under the force of gravity to the section of the glacier where less snow falls and where the ice melts to form meltwater. This part of the glacier is known as the zone of ablation. As it travels, the glacier transports this ice along, and in doing so erodes the landscape and picks up, transports and deposits material, creating the landforms that I'll look at in detail in the next few posts. The glacier is constantly transporting the ice from the upper part of glacier where it fell and moving it forward along the glacier, so even when the glacier as a whole is getting smaller and retreating, the ice is still being transported along. Whether a glacier advances or retreats depends on the difference between inputs and outputs of the glaciers. The difference between the accumulation (snow and ice) and ablation (ice melting) for one year is known as the glacial budget. If more snow forms and ice forms than the amount of ice that melts, there is a positive glacial budget and the glacier advances. However, if more ice melts than is formed in the zone of accumulation, there is a negative glacial budget and the glacier gets smaller and retreats up the valley. There is evidence that the world's remaining glaciers, in places such as Greenland, Antarctica and Europe, are shrinking and some are in danger of disappearing. The main cause of this is global warming as average global temperatures have increased by about 0.9°C in the last 150 years. Sources: BBC (2015) 'Glaciers' BBC Science. [Accessed on 19/08/15] Available from: http://www.bbc.co.uk/science/earth/water_and_ice/glacier British Geological Society (2015) 'Ice and our landscape' Geology of Britain. [Accessed on 19/08/15] Available from: http://www.bgs.ac.uk/discoveringGeology/geologyOfBritain/iceAge/home.html?src=topNav Coordination Group Publications Ltd. (2010) GCSE Geography: The Revision Guide. CGP. Royal Geographical Society (no date) 'Glaciation and geological timescales - Not an ice cube: how glaciers work' Teaching resources: Key stage 3 resources. [Accessed 19/008/15] Available from: http://www.rgs.org/OurWork/Schools/Teaching+resources/Key+Stage+3+resources/Glaciation+and+geological+timescales/Not+an+ice+cube+-+how+glaciers+work.htm
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Glaciers transport and deposit materialAs I looked at in the previous post, glaciers erode rock, creating distinctive landforms. This eroded material is then transported and deposited by the glacier, forming different landforms. A glacier works like a conveyor belt, constantly transporting material downhill. The ice continues to move downhill, even when the zone of ablution at the bottom of the glacier is receding. The material can be frozen inside the glacier, carried on its surface or bulldozed, which means material is pushed in front of the glacier. Material can include rock, gravel, sand and clay, but even ancient human remains have been found transported in glaciers. Deposition occurs when the ice melts at the snout (or end) of the glacier, which is why this part of the glacier, the zone of ablution, often looks grey and dirty, or if the ice is overloaded with material. Glacial deposits are deposited when the ice melts, and unlike river deposits which are sorted by weight, glacial deposits can be rocks of all shapes and sizes, from fine clay to rocks the size of a house. Fluvo-glacial deposits are washed out of the ice by meltwater, and these are sorted by weight, with heavier materials deposited first. Landforms produced by transportation and depositionMoraines These are piles formed from material deposited by the glacier as it melts. There are four different types depending on their position.
Drumlins Elongated hills of glacial deposits. They can be 1km long and 500m wide, and often occur in groups. They are formed from the debris carried along and then accumulated under a glacier, which was deposited when the ice became overloaded with sediment. One end is round, blunt and steep and the other tapered, pointed and gently sloping, showing the direction the glacier moved in. There is still disagreement among glaciologists as to how exactly drumlins were formed. Erratics As I said, glacial deposits come in a mix all shapes and sizes. Extremely big deposits are called erratics. They are large rocks or boulders and are often found on their own. They are often unusual shapes and a rock type which is uncommon in the area they were deposited in. This video from the BBC gives more information about and examples of erratics: Erratics in Switzerland Till Till is a poorly sorted mix of mud, sand and gravel-sized material deposited directly by glacier ice. Outwash plain A flat spread of sediment deposited by meltwater streams from a glacier. Photo examples of these landforms are shown in the slideshow below. Sources:
Coordination Group Publications LTd. (CGP) (2010) GCSE Geography: The Revision Guide. CGP. Royal Geographical Society (no date) 'How glaciers shape the land and what they leave behind' Teaching resources: KS3 teaching resources. [Accessed on 24/08/15] Available from: http://www.rgs.org/OurWork/Schools/Teaching+resources/Key+Stage+3+resources/Glaciation+and+geological+timescales/How+glaciers+shape+the+land+and+what+they+leave+behind.htm BBC (2014) 'Landscapes of glacial deposition' Geography: Glaciation. [Accessed on 24/08/15] Available from: http://www.bbc.co.uk/bitesize/standard/geography/glaciation/glacial_deposition/revision/4/ Although the main focus of the exam specifications is the current glaciated landscapes of the UK, the specifications do refer to the processes of erosion, transport and deposition that once took place in the UK. GCSE students should understand different types of erosion from previous geography topics such as rivers and coasts, but may not have studied them in the context of glaciers. How glaciers erode the landscapeAlthough glaciers move extremely slowly, they are incredibly powerful and have a huge effect on the landscape. One of the processes that it caused by the advance of a glacier is erosion. Note that 'weathering refers to the weakening and breakdown of rock in its position (without being moved); whereas erosion refers to the picking up and removal of the weathered rock, thereby wearing away the land' (Royal Geographical Society, no date) The weight of the ice in the glacier causes it to move downhill, and as it does it erodes the landscape in two main ways:
Then as the glacier moves, it rips out the pieces of rock.
of the glacier and wear it away, like sandpaper. The rock above glaciers is also weathered by freeze-thaw weathering. This is when water gets into cracks in rocks and freezes, causing the rock to expand and putting pressure on it. When it then thaws, the pressure is released. Repeated freezing and thawing widens cracks and causes parts of rock to fall off. At the top of the glacier, in the zone of accumulation, the ice doesn't move in a straighten line. Instead it turns in a circular motion called rotational slip. This erodes hollows and deepens them into bowl shapes. Landforms produced by glacial erosion
the valleys and forming a sharp edge between them.
eroding the mountain.
Formed by a hollow containing a small glacier which is eroded by rotational slip. It often contains a tarn - a small circular lake formed when the glacier melts.
eroded by a river are cut off as the glacier moves past, leaving truncated spurs.
tributary glaciers (similar to a river's tributaries) flow into the main glacier, but the glacial trough is eroded much more deeply by the larger glacier so when the glaciers melt the hanging valleys are left "hanging" at a higher level.
resistant rock was eroded more than the surrounding hard rock.
moutonnee can result. It often has a steep, jagged face as a result of plucking on the far side, and a gradual incline on the other side which is smoothed and polished by abrasion. Striations on the rock indicate the direction the glacier moved in. Examples of all these glaciated landforms in the UK can be seen in the slideshow below. Sources:
BBC (2014) 'Landscapes of glacial erosion' BBC Bitesize Geography: Glaciation. [Accessed 20/08/15] Available from: http://www.bbc.co.uk/bitesize/standard/geography/glaciation/glacial_erosion/revision/1/ Coordination Group Publications Ltd. (CGP) (2010) GCSE Geography: The Revision Guide. CGP Glaciers online (2015) Photoglossary: Alphabetic list. [Accessed on 20/08/15] Available from: http://www.swisseduc.ch/glaciers/glossary/ Royal Geographical Society (no date) 'Glaciation and geological timescales - How glaciers shape the land and what they leave behind.' Teaching resources: KS3 teaching resources. [Accessed on 20/08/15] Available from: http://www.rgs.org/OurWork/Schools/Teaching+resources/Key+Stage+3+resources/Glaciation+and+geological+timescales/How+glaciers+shape+the+land+and+what+they+leave+behind.htm |
AuthorMy name is Heather and I am going to begin my SCITT course to train as a Geography teacher. ArchivesCategories
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