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Welcome to Edmonton's first EarthCache! Fine-grained flood plain and overbank deposits of the North Saskatchewan found along Edmonton’s river valley record one of the most spectacular and catastrophic volcanic eruptions in North America since the end of the last Ice Age.
Mount Mazama of southern Oregon (about 1450 km southwest of Edmonton) is part of a chain of volcanoes that formed around 400 000 years ago. For most of its history, Mazama’s eruptions were not particularly violent. About 6800 years ago, Mount Mazama erupted explosively, with a force 100 times greater than the Mount Saint Helen’s eruption in 1980! The eruption drained the magma chamber which fed the volcano and the mountain collapsed inwards, leaving a deep crater called a caldera. The caldera, 9 kilometres wide and 1219 metres deep, is now occupied by Crater Lake, the second deepest lake in North America. By the end of the eruption, almost 70 cubic kilometres of rock had been displaced.
Ash from this eruption was sent high into the stratosphere, undoubtedly producing spectacular sunsets throughout the northern hemisphere. The ash plume was carried by winds northeastwards, blanketing almost 1.3 million square kilometres, including much of southern Alberta.
Mazama Ash is often found in exposures along river valleys in southern Alberta. Locally, the ash is preserved in North Saskatchewan flood plain deposits which consist of alternating layers of dark organic-rich beds and grey silt. The most prominent and accessible exposures of the Mazama Ash can be seen just upstream from the LRT Bridge on the south bank of the river. This is one of the most northerly occurences of the Mazama Ash in Alberta.
The ash horizon is a prominent centimetre-scale-thick silty layer with a light-pinkish hue and gritty texture due to high volcanic glass content. It occurs in a bed that is lighter in colour than the beds above and below, and it looks like someone took a knife to the exposure and cut a horizontal line through it. The layer thickens and thins suggesting that it was quickly reworked and redistributed by surface water after settling through the air. Here, it is about one centimetre thick, but in southern parts of the province it is thicker, simply because there areas were closer to the eruption. Its age was determined by radiocarbon dating of wood that was buried in the ash.
Like all volcanic ash deposits, the Mazama Ash has its own distinctive mineral composition, which makes it useful for correlating and dating deposits across large areas. It is a particularly important tool for archeologists studying Alberta’s past because they use it to date remains of older cultures. Any artifact found below the ash is considered quite rare.
To log this EarthCache, take a picture of someone in your party with their finger directly on the Mazama Ash. If you are a party of one, a picture of your finger on the ash layer will suffice. Don't forget to include your GPS in the picture. Please look at the pictures that I have posted before seeking for the ash. I have placed arrows on them that clearly show the horizon to give you a better idea of what you are looking for. Also, to proove that you have been to the site and learned something, you must make some observations. Please email me the scale of thickness of the exposed beds above and below the ash horizon (no need to bring measruing tape, just tell me are the beds mm-scale, cm-scale, m-scale, km-scale). Furthermore, describe the colour and grain size of the flood-plain deposits above and below the ash horizon (examples of grain size are mud, silt, sand, granule, cobble, pebble and boulder. If there is more than one size, state a range).
Mussieux, R. and Nelson, M., 1998. A Traveller's Guide to Geological Wonders in Alberta. The Provincial Museum of Alberta. pp. 254
Shaw, J. and Godfrey, J. D., 1993. River Valley Trail. In: Edmonton Beneath Our Feet. Edmonton Geological Society. p. 109-134
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- another view of the ashArrows point to the correct horizon, as indicated by the 'Geological Wonders of Alberta'. Picture by OBA.
- closer view of ash
- geographic extent of ash deposit
- side view of exposurearrows point to ash
- the ash is findablecongratulations to capnmoroni who was FTF (AND correctly identified the ash) as a waymark.
- the exposurethe ash sandwiched between flood-plain deposits. Look closely and you'll see arrows pointing to the ash.
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Coordinates are in the WGS84 datum