Introduction
Rock hard... solid as a rock... etched in stone... heart of stone - all common expressions which reflect our default perception of rocks which is that rocks are hard, because we are used to seeing them most at the Earth's surface where the temperature and pressure are in the range where rocks are solid.
The fact is though that the rocks we see at the Earth's surface weren't always hard - or even solid, regardless of whether those rocks are sedimentary, igneous or metamorphic in nature.
Geologists can tell quite a lot about the history of a rock, about the forces is has been subjected to and the changes it has undergone, and in what order those changes happened by looking at certain characteristic features.
We normally associate volcanic activity with igneous rocks, but this EarthCache takes you to a location where you can see examples of deformation in a type of metamorphic rock that started off around 450 million years ago as an accumulation of volcanic ash on the sea bed which then, through the processes of lithification and low-grade metamorphism became a very attractive green slate.
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Based on your observations at the given coordinates and the information on the cache page, please tell me:
- There are clearly visible parallel layers in the slate. Are those layers laminae, beds or a mixture of both?
- The nature of the volcanic ash has given rise to slate with a grainy appearance. Are all the grains the same size or do they vary by layer?
- Do any of the slate slabs feature obvious SSDS? If so, please describe them and tell me how you arrive at this conclusion.
- Take minute or two to take a good look at the whole building frontage. Some of the slate slabs show evidence of brittle deformation - do you see joints or faults or both?
- These slate slabs include a mixture of metapmorphosed volcanic ash, chlorite and calcite. Roughly what percentage of visible rock surface is calcite?
- Optional task: feel free to add any photographs of your visit that do not show the specific features from the logging tasks - no spoilers please. In the interests of allowing everyone to experience the EarthCache fully for themselves obvious spoiler photographs will be deleted.
Background
One of the forces which shapes rocks is stress. Generally speaking, rocks respond to stress in one of two ways: they either bend or they break, depending on how soft or hard they were when subjected to that stress. When rocks bend or flow, like clay, it is called ductile deformation. When a rock breaks, it is called brittle deformation. Any material that fractures and breaks into pieces exhibits brittle behavior.
Sedimentary rocks are formed from fragments of other rocks, fragments which are typically transported by some flowing medium - water, wind or ice, before being deposited in layers which are then compacted and cemented together by some form of mineral cement to form solid rock.
This slate underwent most of those processes - but not all. Layer upon layer of volcanic ash settled on the sea bed and as those layers built up the lower layers were subjected to increasing pressure from the increasing weight as more and more layers were piled on top. In this particular slate those layers can still be clearly seen today because the temperatures and pressures which converted the shale-like collection of sedimentary layers into slate were comparatively gentle. Geologists would describe this rock as having been subjected to low-grade metamorphism.
Over the period of time this slate was being formed, in addition to volcanic activity, the surrounding area was included in a mountain building episode known as the Caledonian Orogeny. Mountains rose up in response to the rocks being subjected to compressive stress, contributing to the shaping of the area we know as the Lake District today. The slate, by now solid rock rather than soft sediments, developed fractures which were in turn filled with the white mineral calcite left behind by mineral rich fluids.
Bend or Break?
Because rocks change state, between soft and hard, in response to temperature and pressure, and these changes in state impact the way the rocks behave when subjected to stress, scientists can deduce quite a lot about rocks from certain resultant features within them.
Soft rocks and sediments subjected to stress tend to bend and fold, to flow, resulting in softer, more curvaceous features being left behind.
Hard rocks tend to resist stress more - but only until the amount of stress has built up to the point where it overcomes the integral strength of the rock and the rock cracks, breaks or even shatters, typically resulting in sharper, more angular features. The rapid release of built-up energy that occurs when solid rocks finally succumb to accumulated stress can result in earthquakes.
Rocks which have changed shape in response to stress - whether that be bending or breaking or a combination of the two - are said to have been subjected to deformation.
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Brittle Deformation - visible effects of stress on solid rocks include joints and faults.
If the rock cracks but the rock on either side doesn't move relative to the other side the crack is known as a joint.
If, after the rock cracks, the rock on either side of the crack moves relative to the opposite side, that's known as a fault.
The image to the right shows a very clear fault where the rock either side of the crack has slid up/down, leaving the horizontal layers misaligned.
If you look carefully though, to the right of that fault is a faint crack cutting through the rock, roughly parallel to the fault, and ending in the bottom-right corner of the image. That crack - where the horizontal layers are still aligned - is a joint.
It's not unusual to see a mixture of joints and faults in a rock that's been subjected to stress.
Liquefaction
The sediments which make up the green slate are mostly volcanic ash which, apparently, is relatively rare as most slate is formed from shale. The ash settled loosely on the sea bed, making it perfect material for a phenomenon called liquefaction.
When the irregularly shaped ash fragments came to rest on the sea bed, and before any compaction had taken place, there were lots of empty spaces between the fragments. Geologists call these pore spaces. As the sediments were submerged the pore spaces were filled with water - so we say that the sediments were saturated.
Subject granular, loosely compacted, water saturated sediments to powerful vibration or sudden loading from above i.e. by dumping a large quantity of new sediments on top of the saturated sediments and the result is liquefaction.
The water pressure in the pore spaces increases to the point where the individual sediments lose contact with each other and the whole body of sediments starts to act more like a liquid than a solid. Unsurprisingly, any deformation structures left behind in the sedimentary layers while they are in this soft, liquified state are called Soft Sediment Deformation Structures or SSDS for short.
There is much debate within the scientific community about how to tell which specific trigger(s) resulted in which types of SSDS. Some of those triggers include sediment loading (more dense sediments deposited rapidly on top of less dense sediments), storm currents (including tsumanis), seismicity (earthquake activity) and even meteoric impacts. Given the nature of the locality these particular sediments were laid down in it's entirely possible that at least some of the deformations in them arose from earthquakes.
Load Structures / Load Casts
Hard rocks can resist fairly high levels of stress before they deform, and the deformation that occurs is brittle i.e. joints (cracks with no movement) and faults (cracks followed by movement on either or both sides).
Soft sediments are less resistant to stress, especially when they are unconsolidated (loosely packed) and even more so when liquified. Deformation in soft sediments results in structures with soft bends and folds and rounded shapes rather than sharp cracks and angular shapes. Geologists call these more rounded structures load structures or load casts.
The volcanic eruptions which produced the ash which became this green slate will have thrown large amounts of material into the air. The denser, heavier material will have fallen into the sea and down onto the sea bed first, followed by progressively less dense, lighter material. Thus the lowest density ash ended up as the uppermost layer.
The repeating of the same sequence at the next eruption though would have resulted in the most dense material from that eruption settling directly on top of the least dense material from the previous one - creating an arrangement of layers primed for Soft Sediment Deformation - denser material on top of less dense material.
The sudden influx of a new load of sediment from above triggers liquefaction of the less dense layer below, leaving that lower layer in a state where it can no longer support the weight of sediment pressing down on it from above. Blobs of the dense upper sediment layer start to droop down into the less dense lower layer under the force of gravity. This increases the pressure in the pore fluid in the lower layer, forcing the fluid to squirt upwards, taking some sediment along with it back into the upper layer.
While load casts come in a variety of shapes and sizes they all arise from the same set of processes.
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SSDS - Flame structures - are one type of load structure which forms readily in fluidised / liquified sediments and are so called because they form upward-pointing flame-like shapes - which also makes them good way up indicator i.e. a means to measure if the rock you're looking at is the right way up.
A flame forms when a lobe of denser sediment sinks down into or is pushed down into a layer of less dense material below. In turn, this downward pressure increases the pressure in the fluid trapped in the pore spaces and causes a jet of the less dense material to be forced upwards, making the flame-like structure.
Sometimes the flames are very obvious and look very much like tiny, flickering flames frozen in time. They are not always so clearly defined though so sometimes not quite so easy to spot.
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SSDS - Ball and pillow structures - another type of load structure which arises under the same sorts of conditions as flame structures - but with some key differences.
Flame structures might be considered one-sided structures i.e. the more dense material sinks downward on one side resulting in the less dense material being forced upward on the other - forming the characteristic flame shape.
In ball and pillow structures though an elongated section of denser sediment becomes detached and pushes down into and settles in the less dense sediment below. As that denser sediment moves down it displaces the less dense material and that tends to be forced upward past both ends of the sinking sediment - rounding the ends and helping to produce the characteristic ball or pillow shape - often semi-circular or 'kidney-shaped'.
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SSDS - Convolute bedding / lamination - is complex folding or crumpling of beds or laminae - usually those made up of very fine sediments.
Sediments tend to settle in distinct layers of the same type and grain size. Geologists call those layers beds if they are more than 10mm thick and laminae if they are less than 10mm thick.
What particularly stands out about convolute bedding / lamination is that it is typically surrounded - above and below - by undeformed layers.
Geologists don't always agree on the nature of the underlying cause of this type of SSDS. Some postulate that the sediments in the affected section liquify and are deformed, while those either side are more dense and don't liquify and so are not deformed. Others postulate that the convoluted layers are eroded flat before new, flat sedimentary layers continue to be deposited on top of the eroded surface.
If you've carefully read and digested the information from this cache page your tasks at the cache location should prove relatively straight forward, although you may wish to take a printed copy of the page with you so that you can check your answers while there 
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Please submit your logging task responses before posting your log.