Stacks and Stumps
A stack or sea stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea or near a coast. These landforms are formed by erosion, either by wind or wave action or a combination of both. Stacks are formed over large amounts of time when part of the headland is eroded by the force of the sea or water crashing against the rock, this is called hydraulic action. The erosion weakens cracks already present in the headland. Over time the erosion process chisels out a cave, then erroding the back of the cave to form an arch. The constant battering from the elements cause the arch to later collapse forming free-standing stacks. Without the constant presence of water, stacks also form when a natural arch collapses under gravity from wind errosion. Eventually, errosion will cause the stack to collapse, leaving as stump. This stump usually forms a small rock island, low enough for the high tide to submerge.
Stacks typically form in horizontally-bedded sedimentary or volcanic rocks. These rock types' medium hardness means medium resistance to abrasive (most effective and rapid form of erosion) and attritive erosion. A harder or more resistant layer may form a capstone. Weaker rock tend to slump and erode quickly, while harder rocks erode in different ways.
Abrasion: waves break on cliff faces and slowly erode it. As the sea pounds cliff faces it also uses the scree from other wave actions to batter and break off pieces of rock from higher up the cliff face which can be used for this same wave action and attrition.
Attrition: waves causes loose pieces of rock debris (scree) to collide with each other, grinding and chipping each other, progressively becoming smaller, smoother and rounder. Scree also collides with the base of the cliff face, chipping small pieces of rock from the cliff or have a corrasion (abrasion) effect, similar to sandpapering.
Logging Tasks:
Please be careful at GZ making your observations. You are on top of a cliff!
- From GZ how many stacks can you see? How many and where in relation to GZ (looking out to sea, left, right or below.)
- From GZ can you see any stumps? How many and where in relation to GZ (looking out to sea, left, right or below.)
- From GZ look Southward and you can see a stack. To the left is a fissure (you can see light through it). Consider it's proximity to the water and identify which type of erosion would be forming this stack?
- Now look at the top of the stack to the right of the fissure and describe it's shape and colour. How would you classify the hardness of the rock? Why did you classify it that way?
- Have fun and be safe!
Sources:
http://www.brighthub.com/environment/science-environmental/articles/119781.aspx
https://en.wikipedia.org/wiki/Stack_(geology)
https://en.wikipedia.org/wiki/Erosion