Rock formation
There are two different forces to form rocks: Weathering and Erosion.
Weathering is the breaking down of rocks by the action of the weather, plants or chemical action. In happens in situ - this means 'without movement'. This is the main difference between weathering and erosion. Erosion refers to rocks being broken down and moved away. Marine erosion is the wearing away and removal of rocks by the action of the sea.
Marine Erosion were defined in four main processes: They are corrasion (abrasion), attrition, solution and hydraulic action.
Corrasion (abrasion) this is where the bits of rock and sand that are carried in the waves are hurled at cliffs and grind them down. It is sometimes described as sandblasting.
Attrition is where the waves cause rocks and pebbles that they are carrying to smash into each other and break down. They become smaller and rounder.
Solution is where acids contained in sea water slowly dissolve certain types of rock. Sea water is very corrosive and slowly dissolves chalk and limestone. This increases the size of the cracks and joints in the rock so that the other forces of erosion can become more effective.
Hydraulic Action is the constant force of waves crashing on the shore. When waves crash against the cliffs they force air into cracks in the rock. The air is trapped, and pressure builds up. As the waves move back, pressure is released and the trapped air expands. Small explosions take place and weaken the rock, blasting fragments away.
The geology of Saxhólsbjarg.
It is a promontory reaching out into the ocean on the west coast of Iceland. It is the nearly westernmost part of Snaefellsnes.
The first step in the process of headland erosion is for the sea to attack the foot of the cliff. It will begin to erode natural weaknesses in the rock (eg. a crack or joint) causing them to be widened by hydraulic action, wave pounding and solution. Abrasion and hydraulic action then widen these cracks still further, creating small sea caves at the foot of the cliff. Over time, continued erosion of these caves leads to an arch being formed when the sea caves break through the headland. A combination of wave attack at the base of the arch and weathering of the roof of the arch (by frost, wind and rain) weaken the structure until eventually the roof of the arch collapses. This creates a stack which is a column of rock that stands separate to the main headland. Sea spray then weakens the stack, and eventually it is eroded away leaving a stump. This will be covered by the water at high tide.
At the coordinates you have a wonderful view to two different states of erosion of the headland.
To log this Earthcache:
1. How do you think is this promentory formed? Is it a natural cliff or formed by vulcanic eruption(s) and lava flow? Please explain your opinion based on accurate observations and descriptions of the given coordinates. Describe the rock formation with the Arch. What type of stone do you see (Tuff stone, basaltic tuff, basaltic columns, other), how is the structure, colour, size, shape more angular or rounded? Do you see different level in this?
2. Do you see any other signs of erosion in this formation in addition to the great arch as decribed above? In which phase of the explanation of the erosion-process dos it belong? What do you think will happen in the next thousands of years?
3. At the given coordinates you see in addition a single stack. As described above, stacks could be the result of the erosion itself or the collapsing of an arch under gravity. What is your opinion in this case and why?
You can imediately log this earthcache. I will contact you, if the answers will not fit.