The term “impact crater” usually refers to the result of a hypervelocity collision of a smaller body on the surface of a larger one. On Earth most of the cratering record has been erased because of tectonic processes and weathering. The cratering record on the Moon shows a nearly complete record of impact events.
The crater that is the subject of this Earth cache, however, has been formed by a different means.
Since it was first surveyed in 1932 the area of Ardnamurchan has been of great interest to scientists from all over the world who come to study its remarkable cratering record.
The Earth’s tectonic plates which make up the outer layer or crust are constantly moving, colliding and dipping under and over each other. Some of these collisions have formed great mountain chains one of which, similar to today’s Himalayan Range, included all of the current Scottish Highland millions of years ago. Today only the roots of these ancient mountains remain. The Ardnamurchan Peninsula was far enough away from the main zone of deformation of these ancient mountains to have preserved a record of the land’s upheavals and burials.
About 65 million years ago a plume of molten material (known as a Hot Spot) that had been slowly rising from deep within the Earth reached the surface. When it broke through the crust a series of volcanic eruptions began which lasted for three million years. Although weakened, the Hot Spot has remained active long after North America and Britain have drifted apart - and to this day is still forming the volcanic landscape of Iceland. When large ice sheets began to cover the north, their advances and retreats caused much of Ardnamurchan’s ancient volcanic landscape to be ground away by ice leaving today’s raised beaches and rounded hills.
What has fascinated scientists about the geology of Ardnamurchan are its ancient calderas (although some geologists feel the term caldera may be inaccurate). Calderas are circular depressions formed above distinct magma chambers where the contents of the magma chamber are rapidly expelled due to the collapse of the roof of the magma chamber after an explosive style volcanic eruption. In the area around Achnaha these caldera are so pronounced that they can be seen in satellite imagery. Consisting of Eucrite, a variety of Gabbro. they have been called, “ …one of the most perfect annular intrusions in the British Tertiary Volcanic Province (BTVP).”
The Information board you see at the coordinates located in the car park at Sanna describes the Great Eucrite ring complex centred around Achnaha and how the rings may have formed with reference to cone sheets, ring dykes and lopoliths. The board’s text is supported by geological maps, photos and block diagrams. To get to Sanna, you drive through the heart of a massive ring dyke complex evidenced by the hills that surround you and in the flatness of the central plain. While you are driving on the challenging road to Sanna from Kilchoan as you approach the tiny village of Achnaha you will be crossing a circular plain surrounded (Centre 3) by a ring of sharply defined rocky hills, the very centre of the crater.
There is a question to answer here. Estimate the diameter of the crater of Centre 3.
Of the three ring complexes (described by JE Ritchie in 1920) which mark the position of a subterranean magma chamber and its feeder pipe, Centre 3 is the youngest. It reveals the remains of the ‘solidified contents’ of the magma chamber and represents the final stage in the intrusive evolution of the Ardnamurchan complex regarded by some to be “an almost perfect set of nested ring-dykes.” The area also shows evidence of the surges and flows down the slope of a volcano with air-fall or pyroclastic rocks; and by the occurrence of very large blocks and bombs in the area’s cliff deposits.
The region around Achnaha was originally described by Richey and Thomas (1930) a being formed of seventeen ring-dykes intrusions which generally become progressively younger towards the middle of Centre 3. Recently, however, Smith (1957), Bradshaw (1961), Wills (1970) and Walsh (1971, 1975 and in Gribble 1976) have doubted whether it is a ring-dyke complex at all saying it is most likely a saucer- or funnel-shaped intrusion whose contacts dip gently inwards instead of the previously postulated outward dip. Even more recent research supports this theory saying, “ … the Great Eucrite ring-dike is in fact a lopolithic intrusion with an overall funnel-shape geometry.” (O’Driscoll, Troll, Reavy, and Turner. 2005).
Ah, science …
To claim this Earth Cache answer the following four questions:
1. In Ardnamurchan most of the lava that poured out onto the surface has been eroded away. What is it you now see geologically between Sanna and Achnaha?
2. What are the three ways that rings such as those found around Achnaha may be formed?
3. When the American and Eurasian tectonic plates began to drift apart 55 - 60 million years ago, and the North Atlantic began to open, what three islands together with the peninsula of Ardnamurchan formed a chain of great volcanoes?
4. In your opinion which theory is more valid: Richey & Thomas’ “ring dyke” theory - or Smith & O’Driscoll et al’s “funnel-shape intrusion” theory?
5. At the given waypoint near Achnaha there is a layby. Here, you are at the very centre of the crater. From this viewpoint can you estimate the diameter of the crater which is Centre 3?
Please do not include your answer in your log, instead email me with your findings.
If I do not receive the answer, you may have your log deleted..... Educational guidelines for Earthcaches are set by Geocaching.com and GeoSociety.org (Earthcache) and have to be adhered to.