Glaciers and volcanic activity EarthCache
Glaciers and volcanic activity
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Intrusion or not an intrusion?
Attention: Do not step into the penguin colony! Seals and sea elephants may be present. You need a camera to log this cache!
Peterman Island
Petermann Island is located at the west side of the Antarctic Peninsula and is named after German geographer August Petermann. Main parts of Peterman Island consist of granodiorite, a material similar to granit. In contrast to granit it contains more of the plagioclase-type feldspar than orthoclase-type and pyroxene. Granodiorite is a so called intrusive plutonic rock. It is formed by an intrusion of liquid magma which cools in stocks below the surface. Slow cooling caused quite big crystals of quartz, feldspar and mica. Usually granodiorite is exposed only after erosion of the covering material or by geologic/tectonic uplift.
Source: Wikipedia
Photos
Photo 1 : Basaltic intrusion
Photo 2 : Epidot
Photo 3 : Accumulation
Photo 4 : Whaleback rock
Photo 5 : Cracks
Intrusion or not an intrusion?
At the location many dark areas in the light granodiorite are notable. Some are intrusions, some not.
- Intrusions in granodiorite can be formed by two different processes. One is the enclosing of parts of the covering material that does not melt completely and resist inside the granodiorite. The second process is caused by volcanic/tectonic forces that cause cracks into the granodiorite sized in mm up to m that are filled with fresh basaltic magma or by crystallization from hydrothermal aqueus solutions. Here are basaltic crack fillings present. See photo 1. Such inclusions always show sharp borders and edges.
Caused by the heat of the basaltic magma and hydrothermal effects (= chemical transport by supercritical water) iron ions from the basalt changed the dark pyroxene crystals in the granodiorite into light-green epidot crystals by replacing magnesium. See photo 2.
- Many of the dark areas in the granodiorite are not an inclusions but random accumulation of the darker minerals biotit and pyroxene. These areas are irregular/diffuse shaped or show (randomly!) sharp borders/edges but are not an inclusion. With a magnifying glass the dark areas look exactly as the lighter parts oft the granodiorite, just the distance between dark and light crystals is shorter – or, the darker minerals are accumulated. See photo 3. This may happen if real inclusions melt up completely in a very viscose liquid. The different material causes concentration gradients. High viscosity perevent diffuson processes which usually equalize concentration gradients in liquids.
The surface of Peterman Island is covered with many ‘whaleback rocks‘, see photo 4. They were formed by glacier forces which removed softer parts of the rock in a kind of sanding process.
Strong ice pressure during the movement of the glaciers caused small (sub-mm to mm-sized) cracks in the whaleback rocks. The orientation of these cracks correlate with the direction of the glacier movement, see question 1. The forces of frost and ice during winter and liquid water during summer times enlarged most of the cracks to the cm and dm range. Please see photos 4 and 5.
The last photo shows some quite old cracks that where filled with minerals and the 'geologic new' gaping crack caused by the glaciers.
To log this cache please mail us the answers for the following questions:
- 1) What do you thing: are the cracks orientated parallel or transverse to the glaciers moving direction. Describe in your own words.
- 2) Find some ‘Random accumulations’ . Is the mayority of these accumulations ‘regular’ shaped or diffuse?
- 3) Try to suggest (with your own words) what forces may cause these ‘Random accumulations’.
- 4) Optional task: If you find a basalt/granodiorite contact zone with epidot different from my photo, please add a photo showing the epidot crystals to your log.
- Add a foto from nearby that proofes your visit. Please do not spoil geological features!
Please log directly, we will contact you if anything is wrong.
Rheinflut1995
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