Icelanders say that Sönghellir on Snæfellsnes provided shelter for the settler Bárður Snæfellsás and his family, after relocating from Norway to Iceland at the end of the 9th century, while their farm was being built. Snaefellsás was apparently half troll (some say giant), half human and also a magician. He named both Snæfellsnes and Snæfellsjökull glacier and is now the glacier’s protective spirit.
To understand how the cave formed, however - and in order to claim this earthcache - you have to understand something of volcanoes and their interactions with the environment in which they occur.
Please read the requirements for claiming this cache carefully. It is set at a higher difficulty rating than many as you will need to do more than just read the description or look at infoboards to find the answers that are needed to claim the cache. You will need to spend some time at the site, looking around and thinking about the questions. All of the answers are at to be found on the ground, not on the internet! The questions have been placed at the beginning of the description to assist cachers with GPS devices which display limited amounts of text. Some supporting earth science follows the questions.
Finally, don't forget to visit the cave of Sönghellir itself and experience its wonderful acoustic and sounding properties. Who knows, you may even hear an echo of the voice of Bárður Snæfellsás himself percolating down from the depths of the glacier above!
Let us look first at the different ways that lava can flow away from an erupting vent:
Rift eruptions along fissure lines often give rise to low viscosity (i.e. runny, fluid) lava that pours from the volcanic vents in long, free-flowing rivers of molten rock that run rapidly downhill from the eruption site. This fast moving, highly fluid, lava is termed a Pahoehoe flow. (The word "pahoehoe", pronounced "pa-ho-ee-ho-ee", is Hawaiian in origin and means "smooth, unbroken lava" The word is sometimes rendered as Pa hoj hoj.)
Pahoehoe flows typically advance rapidly, as a river of molten rock, often running between cooling lava levées, or, at the front of the lava flow, as a series of small lobes or toes that continually break out from a cooled crust.
LEFT: Pahoehoe lava stream flowing between solidifiied lava levées;
BELOW: Pahoehoe lava 'toes'
Pahoehoe lava rapidly loses its gaseous content, so that it solidifies into a dense rock formation with a smooth surface, with few vesicles (bubbles) visible to the naked eye. The surface texture of pahoehoe flows varies widely, displaying all kinds of bizarre shapes, often referred to as lava sculpture. It typically forms fan-shaped ropey textures, such as this example from Þingvellir.
'Ropey texture' on the surface of a pahoehoe flow formation
When pahoehoe lava is erupted under water or ice, it forms distinct rounded pillow lava, like those exposed in this cliff face:
Exposed pillow lava bed
A'a lava
By contrast, dense, higher viscosity lava moves much less freely than its low viscosity counterpart. A'a lava, (pronounced "ah-ah", from the Hawaiian word meaning "stony rough lava") creeps forward rather than flowing, with the lava developing an outer skin of coarse, cinder-like material, which constantly breaks up and is re-entrained into the slowly moving mass, or else topples and slides down the advancing front of the flow as the build-up of molten material behind the cooling front pushes it ever onwards from the source of the eruption. This photograph is of one such cinder flow on the island of Heimaey in 1973.
Active A'a flow front, Heimaey, 1973
Some of the youngest lava deposits in the Snæfellsnes peninsula consist of A'a flows less than 1700 years old, which make for striking features as they cut across the landscape. A'a lava deposits are often topped by strange twisted and contorted rock structures, lying at odd angles, or towering up above the surface of the flow. These are formed from the cooled cinder crust of the flow breaking up and being rolled into new positions by the still moving lava beneath it. Apart from a coating of reindeer moss (which frequently hides deep crevices between the massive -- and unstable -- cinder block components, making them very hazardous to walk across) these flows look today very much as they did when they were deposited. This is a view of the Hellnahraun flow, between Hellnar and Arnastapi, which originated high up on the slopes of Snæfellsjökull a few thousand years ago.
Typical historical 'rubble-topped' A'a flow
Flows which start out as Pahoehoe may turn into A'a flows as they lose their gaseous content, or cool, thus becoming more viscous, so it is quite possible to find a mix of these types along the line of progression of extended lava fields.
A'a flows which erupted under ice, or which encounter rapid cooling through contact with sea-water, develop crumbling, heavily vesicled crustal edges, such as these seen in the cliffs at Djupalónssandur, as well as complex, curving pseudopillow fracture systems (visible in the lower part of the cliff).
Sea-water quenched lava at Djupalónssandur
Pyroclastic flows
A very different form of flow from the two just described can occur when, instead of a steady, fairly gentle ejection of molten lava, a volcano erupts explosively. This explosive activity may have a variety of different causes, such as the collapse of crater walls or, indeed, of the entire eruptive column back into the volcanic vent, or else through the sudden release of highly volatile eruptive matter which is relieved of its constraining overburden of rock and ice. This latter is prevalent in Icelandic volcanic systems which can lie dormant for many years before becoming active again, with the new eruption starting when subterranean lava pressures have built sufficiently to suddenly blow out the old, solidified vent plug, as well as any overlying material such as deep glacial ice lying in the old summit crater. These explosive eruptions, in addition to producing a vast cloud of debris which is carried high into the atmosphere, eventually falling as ash particles over a wide area, also generate high speed pyroclastic (literally, "fiery rock shard") events. These flows consist of a high-density fluidised mass of hot, dry solidified rock fragments, mixed with steam and intensely hot volcanic gases, collectively known as tephra. Speeds of over 450mph can be achieved by these flows; these massive currents of hot gas and rock can be highly destructive, eradicating everything in their path.
High-speed pyroclastic flow cloud
Because the resulting rocks have essentially been deposited gravitationally, in the wake of high speed currents, they often look sedimentary in nature, such as in these tufa beds at Svalþúfa, which are nearby the cache, on the coast of the peninsula.
Tufa cliffs
Pyroclastic eruptions encountering sudden quenching -- typically in Iceland as a result being erupted beneath glacial ice -- produce a highly dense pyroclastic deposit called hyaloclastite, which consists of sharp, angular glassy rock fragments of varying sizes, contained in a closely packed, fine-grained matrix dominated by glassy shards.
Hyaloclastite bed
Cave formation in volcanic deposits.
Each of these different types of volcanic deposition mechanisms has a distinct means by which voids can develop in them, which later appear as caves in the resulting rock formations. When pahoehoe flows become massive but slow moving, as can happen when the flow ponds behind natural barriers in the terrain, the flow may grow a rigid skin of hardened lava, beneath which the main flow remains molten and free-flowing. This results in a stream of molten rock flowing within a solid encasing tube. As the eruption dies down, and the flow of magma reduces or is cut off, the molten rock drains out of the enclosing tube, leaving a smooth-sided, linear cave in the rock. These cave structures can often be quite extensive.
Pahoehoe lava tube
Similar but less extensive lava caves develop in A'a flows in much the same way. As with A'a flows generally, these are much rougher and blockier structures than tubes within pahoehoe flows.
Lava cave in A'a flow
Until recently it was thought that pyroclastic flows were too fast moving and turbulent in nature ever to develop voids of any great size, and caves of volcanic origin within pyroclastic lava beds are indeed rare. Recently, however, studies have indicated that hyaloclastic flows of sub-glacial origin, or which encounter glacial ice masses in their flow paths, often entrain large ice blocks into their flow. When this happens, the encased ice mass may become a pocket of high-pressure steam trapped inside the cooling and hardening rock shard cloud. The formation of these hyaloclastic caves has been little studied, but they are increasingly being recognised as present within Icelandic volcanic deposits of all ages. They generally form isolated or self-contained, non-interconnecting structures which are only exposed by subsequent large-scale erosional events, such as glaciation, river valley formation, or coastal sea-wave action.
Typical hyaloclastic cave mouth
In addition to these intrinsic flow void structures, which form through the physical and mechanical properties of the lava itself, or its emplacement processes, volcanic deposits along sea coasts are frequently subjected to post-depositional wave erosion, which over time opens up lines of weakness in the deposits to make large, open-mouthed caves. Sea caves of this kind are common in all Iceland coastal cliffs, as here, at Arnarstapi.
Erosional coastal cave
The main difference, of course, between this latter type of cave and those described earlier, is that seacaves are formed by the erosional forces of the sea applied over long periods subsequent to the formation of the rock structures around them, whilst the others are created at the time of the rock emplacement, remaining unseen and undiscovered, until later erosional processes or rock collapse events bring these ancient caves to light.
The Quietly Crew have created this cache to celebrate our first trip to Iceland and as a "thank you" to the warm-hearted and generous people of this beautiful island who helped to make our stay so enjoyable and memorable. Wherever you are from, we hope you enjoy visiting this site and learning about the fascinating processes that have contributed to its formation.