Basalt Columns at Súgandisey EarthCache

Basalt Columns at Súgandisey

Súgandisey

Súgandisey is a beautiful island with an elegant shape, tall and sheer with its basalt columns. It protects Stykkishólmur harbour from the weather and waves, and is one of Stykkishólmur´s most distinctive landmarks. The lighthouse on Súgandisey used to be at Grótta in Seltjarjarnes but was moved here in 1948. By the stairs up to the island, you can still see the remains of a hoist that was used to drag gas kegs up to the lighthouse, which converted to electricity in 1995. The island was used as grazing land for sheep farmers in Stykkishólmur, as is still common today on islands in Breiðafjörður. Súgandisey was connected to the land in 1989. The embankment out to Súgandisey greatly improved the harbour facilities in Stykkishólmur, and it also functions as a road to the ferry and seawall for the harbour. Hiking on Súgandisey is popular among both locals and travellers as it has fantastic views in every direction.

Which makes this site geologically unique is not just the occurrence of basalt columns alone. When you walk to waypoint 2 you will have the chance to observe a very special basalt formation created by coastal erosion, too. This formation is a essential part of this earthcache.

Basalt

Basalt is a basic igneous rock of extrusive origin (cast rock or volcanic rock), with a characteristic pentagonal or hexagonal column shape. The colour of basalt is dark, as is characteristic of rocks with a basic composition. Often the rock appears blue, gray-blue, dark grey or black. As a result of weathering, the colour changes and becomes light grey. The reason for the colouring of basalts are the rock-forming aggregates (the minerals that make up a rock), which are at the same time to differentiate from other rocks or the basis for the definition of basalt.
Basalts as representatives of the effusion rocks are formed by the rapid cooling of low-silica magmas that come to the earth's surface. Therefore basalt is assigned to the rock group of volcanic rocks - igneous rocks that arise on the earth's surface.
Basaltic lava is characterized by a very high fluidity, which is why the lava can form extensive rock coverings that cover both the mainland and almost the entire seabed of the world's oceans.
Typical for basalt are geometrically perfect, pentagonal or hexagonal columns, often the rock has a completely prismatic separation. The cause of the formation of pentagonally or hexagonally aligned basalt columns are the thermodynamic processes during the cooling of the lava. The effect that leads to the columnar alignement of the basalt is called the Bérnard effect. Accordingly, on the one hand, the heat of the molten rock on the underside and, on the other hand, the atmospheric cooling due to the air on the top, create vertically structured convection cells that appear pentagonal or hexagonal when viewed from above.

The basalt columns at Súgandisey

Basalt columns can be found here on Súgandisey both on the south coast and on the entire east coast. The basalt columns stand out due to their regularity, their height (especially on the south wall) is impressive and highlights the geological importance of this place. What is also striking and underlines the geological peculiarity of this place: The basalt columns rise exactly at an angle of 90 degrees. This means that the position of the pillars has not changed since their formation - and this is, as you can see from the degree of weathering, an older lava flow. The situation has not changed due to tectonic movements or further lava flows - here you can see directly how the pillars were formed during the cooling process of the lava.

Marine erosion (abrasion)

Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landward retreat of the shoreline can be measured and described over a temporal scale of tides, seasons, and other short-term cyclic processes. Coastal erosion may be caused by hydraulic action, abrasion, impact and corrosion by wind and water, and other forces, natural or unnatural.
Abrasion is the term used to describe marine or limnic erosion with a grinding and leveling effect. The surf waves of the sea erode the rock as well as moving pebbles (surf pebbles) in the coastal region. This form of erosion attacks the mainland on a broad front and leads to surf caves as well as smaller and larger cavities in the rock, which collapse over time. Sometimes the erosion also leaves single columns near coast, called sea stacks. In the course of the coast there is a steep wall, the cliff, and at sea level an increasingly wider area, the abrasion plate.
A stack or sea stack is a geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by wave erosion. Stacks are formed over time by wind and water, processes of coastal geomorphology. They are formed when part of a headland is eroded by hydraulic action, which is the force of the sea or water crashing against the rock. The force of the water weakens cracks in the headland, causing them to later collapse, forming free-standing stacks and even a small island. Eventually, erosion will cause the stack to collapse, leaving a stump. Stacks can provide important nesting locations for seabirds, and many are popular for rock climbing. Isolated steep-sided, rocky oceanic islets typically of volcanic origin, are also loosely called stacks or volcanic stacks.
A special feature on the east coast of Súgandisey is a single sea stack, which is made up of vertical basalt columns. You can observe this sea stack very nicely from the given coordinates at stage 2 if you look to the south.
 

Your task to log the cache:

Answer the following questions via my geocaching profile:

1.) At the coordinates you are standing in front of a rock face made of basalt. Take a closer look at the columns and tell me the average diameter of the columns! What is the estimated hight of the rock face?
2.) Take a walk to stage 2 and take a look to the South. Here you can observe a single sea stack made of basalt. Give me a short description of the sea stack, estimate the hight and width of the stack, its distance to the rock face of the cliff and give me a short description in your own words, how this sea stack has been formed!
3.) Optional: Post a photo with your log, showing you and/or your GPS near the given coordinates in the header!


After you've sent me the message with your answers, feel free to log! Only if there's something wrong, I'll contact you via message!

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