Brimketill Lava Pool

The coordinates will lead you to Brimketill that is a lava rock formation in the shape of a pool on the edge of the ocean. It is also named Oddnýjarlaug, Oddný the troll's bath.

The local geology of Reykjanes/Brimketill.

The basic way to explain the geology of Iceland is that the island is placed over a hot spot.

Iceland has formed by the coincidence of the spreading boundary of the North American and Eurasian plates and a hotspot or mantle plume – an upsurge of abnormally hot rock in the Earth´s mantle. As the plates moved apart, excessive eruptions of lava constructed volcanoes and filled rift valleys. And that is how this part of Iceland has formed. With the volcanic act this beautiful landscape has been built up.

Brimketill is made out of lava rock/volcanic basalt, created by the volcanism of the Reykjanes Peninsula. This peninsula is renowned for its many cone-shaped peaks, its endless expanses of black lava, and its constant geothermal activity. The rock of lava that forms Brimketill is from the volcano Hengill that is still an active volcano, although there has not been an eruption here for about 2,000 years. In spite of that, there is still lava close to the surface, evidenced by the steaming fumaroles and many hot springs dotted across it. Hengill is an enormous mountain in south-west Iceland, covering an area of about 100 square kilometres.

The basin like shape of the lava rock pool was formed by coastal erosion. The lava around Brimketill is extremely rough, cracked and with high, sharp edges and surfaces. The field was most probably formed in the Reykjanes fires in 1210-1240. The waves pound the southern coastline of the peninsula with great force, having great time to build when the wind comes from the south, as there are no significant landmasses south of Iceland before the eastern tip of Africa. Such forces have shaped these jagged shores.

There are two types of volcanic basalt, described under in words to easier understand and remember the two different types:

Two Hawaiian words are used to describe the two types of volcanic basalt: 'A'a and pahoehoe. 'A'a basalts have rough surfaces (that make barefoot people cry, "Ah! Ah!" as they walk across it). This type of lava surface is rough with pointing and sharp edges. 'A'a is cooler, slower-moving basaltic lava. Pahoehoe basalts have a smooth glassy surface that looks like many ropes. The "ropes" form when the surface cools, becoming solid rock while lava flows beneath it. Pahoehoe is a hotter, faster-moving basaltic lava.

Brimketill formation:

Brimketill (Surf cauldron) is an extraordinary pool on the seaside on the westernmost part of Stadarberg. You could say and think that on a sunny day the rock formation looks mostly like a hot tube. Brimketill and the pools in its neighbourhood formed due to the constant beating of the waves against the lava rock coast.

The endless pounding of the surface has over time created a number of pools and dips in the basalt lava. The ocean has had an immense impact on the landscape of the Reykjanes peninsula. One can see the vast force of the North Atlantic Ocean at work by watching the waves crashing onto the rocks near Brimketill. The waves erode the rock by increasing the air pressure in fissures and cracks. On receding, the sea takes the air with it, creating under-pressure. This process erodes the rock over time. In addition, erosion occurs when the waves crash rocks and gravel onto the shoreline, as do winter frosts when water freezes in cracks and expands. Erosion also occurs on occasion by sandblasting when the weather conditions allow.

This formation goes under the same theme as wave-cut platform, shore platform, coastal bench, or wave-cut cliff that is the narrow flat area often found at the base of a sea cliff or along the shoreline of a lake, bay, or sea that was created by erosion . Wave-cut platforms are often most obvious at low tide when they become visible as huge areas of flat rock. Sometimes the landward side of the platform is covered by sand, forming the beach, and then the platform can only be identified at low tides or when storms move the sand. Wave-cut platforms form when destructive waves hit against the cliff face, causing undercutting between the high and low water marks, mainly as a result of abrasion, corrosion and hydraulic action, creating a wave-cut notch.

Hydraulic action

Hydraulic action is the erosion that occurs when the motion of water against a rock surface produces mechanical weathering. Most generally, it is the ability of moving water (flowing or waves) to dislodge and transport rock particles. Within this rubric are a number of specific erosional processes, including abrasion, attrition, corrasion, saltation, and scouring (all this as downcutting). Hydraulic action is distinguished from other types of water facilitated erosion, such as static erosion where water leaches salts and floats off organic material from unconsolidated sediments, and from chemical erosion more often called chemical weathering. It is a mechanical process, in which the moving water current flows against the banks and bed of a river, thereby removing rock particles.

A primary example of hydraulic action is a wave striking a cliff face which compresses the air in cracks of the rocks. This exerts pressure on the surrounding rock which can progressively crack, break, splinter and detach rock particles. This is followed by the decompression of the air as the wave retreats which can occur suddenly with explosive force which additionally weakens the rock. Cracks are gradually widened so each wave compresses more air, increasing the explosive force of its release. Thus, the effect intensifies in a 'positive feedback' system. Over time, as the cracks may grow they sometimes form a sea cave. The broken pieces that fall off produce two additional types of erosion, abrasion (sandpapering) and attrition. In corrasion, the newly formed chunks are thrown against the rock face. Attrition is a similar effect caused by eroded particles after they fall to the sea bed where they are subjected to further wave action. In coastal areas wave hydraulic action is often the most important form of erosion.

Similarly, where hydraulic action is strong enough to loosen sediment along a stream bed and its banks, this will take rocks and particles from the banks and bed of the stream and add this to the stream's load. This process is the result of friction between the moving water and the static stream bed and banks. This friction increases with the speed of the water and the roughness of the bed. Once loosened the smaller particles are actually held in suspension by the force of the flowing water, these suspended particles can scour the sides and bottom of the stream. The scouring action produces distinctive markings on streams beds such as ripple marks, fluting, and crescent marks. The larger particles and even large rocks are dragged along the bottom in a process known as traction which causes attrition, and are often "bounced" along in a process known as saltation where the force of the water temporarily lifts the rock particle which then crashes back into the bed dislodging other particles.

Hydraulic action also occurs as a stream tumbles over a waterfall to crash onto the rocks below. It usually leads to the formation of a plunge pool below the waterfall due in part to corrosion from the stream's load, but more to a scouring action as vortices form in the water as it escapes downstream. Hydraulic action can also cause the breakdown of river banks since there are water bubbles which enter the banks and collapse them when they expand.

Notice, it is highly recommended to NOT try swim and walk down to the pool area, stay on the ramp and pathway for your safety. Look also out for waves and the sea that might be rough at times and can hit the platform.

To log this cache.

To get to log this cache you will have to visit and answer the questions which are related to the coordinates given the earthcache.

When answers are collected, send them to CO for verification.

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Logs without answers to CO or with pending questions from CO will be deleted without any further notice.

Please do not include pictures in your log that may answer the questions.

Questions

1. Answer the questions under by visiting the coordinates, and study the formation at Reykjanes/Brimketill.

A. Describe the volcanic basalt that you are surrounded by on your way out to gz and at gz, Color, texture, formation and shape! Is it Aa or pahoehoe lava?

B.  At the end of the ramp and pathway you will to your right hand side see the lava pool that is on the header photo of the cache page, and another pool to your left. Watch the water/waves work on the lava field, shore line and in the pool. Describe the difference between the two pools, and how the erosion works/has worked differently with them both?

C. How big in size/diameter would you estimate the two pools to be? And for how long has the ocean been working and shaping them?

2. (It’s voluntary to post a photo in your online log of your visit)