The Basalt Caves of southern Iceland are known around the world for their unique physical and geographical properties. This Earthcache will take you to two caves in particular. All answers can be found between this cache page and the site of the Earthcache.
Types of Caves
There are many types of caves and among them are: lava, glacier, solutional, coastal sea, rare emerged sea caves and sand or wind caves.
1. Lava caves are tunnels or tubes in lava formed when the outer surface of a lava flow cools and hardens while the molten lava within continues to flow and eventually drains out through the newly formed tube.
2. Glacier caves are formed by melt water which excavates drainage tunnels through the ice. Of entirely different origin and not to be included in the category of glacier caves are so-called "ice caves," which usually are either solution caves or lava caves within which ice forms and persists through all or most of the year.
3. Solutional caves are where water has run down a crack in bedrock and has slowly dissolved the rock creating a cavity. The most common type of solutional cave is limestone. Solutional caves are the most frequently occurring caves and such caves form in rock that is soluble, like limestone, but can also form in other rocks, including chalk, dolomite, marble, salt, and gypsum. Most of the caves of the world as well as the largest caves are solutional caves. A process called erosion causes these caves.
4. Coastal Sea Caves are also known as littoral caves are caused by the waves mechanically eating away at the rock. Many of these caves may be on dry land now, but they used to be at the ocean's edge. Such caves demonstrate the enormous pressures exerted by waves and to the corrosive power of wave-carried sand and gravel. The primary process involved is erosion.
5. Emerged sea caves are a very rare type of cave. These caves are still located under the surface of the ocean and have been formed by many actions such as underwater volcanic activity and erosion.
6. Sand or Wind Caves may be formed by the sandblasting effect of silt or fine sand being blown against a rock face. Some rock shelters, which are true caves, are formed in this manner as well as from the erosion via water.
Formation: Sea Caves
Some of the most miraculous and mysterious caves known are sea caves. These caves are formed using the power of waves pounding against the shoreline. This cave probably started as a small crack or weak spot on the cliff side. The wind and waves (which can carry sand) then beat on this crack or weak spot until it falls off the cliff side. Caves that already exist and are widened by water erosion are called karst caves. You can find these caves near or opening on the shoreline. A sea cave can also be created by waves washing up the shore producing caverns, which are typically as deep as the waves can go. In this particular instance, you can see the effect that the tide has on the cave. During low tide, the water does not come anywhere near the cave. During high tide, the water comes up a lot higher on the beach, and the erosion of the rock is still taking place today.
While it doesn't matter what kind of rock is along the shoreline, it helps the formation of the cave if the rock is softer. In this instance, the rock that is being eroded away is Basalt. Basalt has a hardness between six and seven on Moh’s Scale of hardness. Looking at the cave today, this makes sense. This is logical because the rock is soft enough to be eroded by the wind and the water, however it is hard enough to not have completely been eroded away by now. The finished product of all of this erosion is this cave!
Formation: Columnar Joining
A definition of Columnar Joining: A structure that forms in rocks (most commonly in basalt) that consists of columns (mostly commonly hexagonal in shape) that are separated by joints or fractures in the rock that formed when the rock contracted, most often during cooling.
Basalt is an igneous, volcanic rock. For those of you who need a little Geology 101 refresher, “igneous” means that the rock formed from a melt and “volcanic” means that the melt erupted at the Earth’s surface as lava before it cooled to form the rock. After lava is erupted onto Earth’s surface, it cools. However, lava may take awhile to cool, and as it cools there may be a temperature gradient. Most commonly, the top of the lava flow will be cooler than the bottom of the lava flow.
When the lava cools, it contracts. This is because hot things generally take up more space than cool things. Think about hot steam, for instance. When you open the lid of a simmering pot or a tea kettle, that hot steam wants to escape and expand into the air. Or think about those balloons from your last birthday celebration. Have you ever notice how balloons tend to droop overnight? Partly, that may be because the helium in the balloons is escaping, but it’s also often because the gas inside the balloons cools down and contracts with the cooler nighttime temperatures. Sometimes, if you prop those drooping birthday balloons in the sun the next morning, they’ll pop back up again as the gas inside them warms up and expands.
When objects contract, they often crack or fracture. When contraction occurs at centers which are equally spaced (see the above diagram), then a hexagonal fracture pattern will develop. If the contraction is not evenly spaced, then other geometries of fractures, such as 5-sided or 7-sided fractures, may occur. Contraction may not be equally spaced if, for example, the thickness or composition of the lava flow varies. The fracture pattern that forms at the cooling surface will tend to be propagated down the lava as it cools, forming long, geometric columns. Thus, as lava cools to form basalt, it may crack in a hexagonal (or other) shape and form columns. These columns form in a variety of sizes– some are fairly small, and some are wider and much taller than people!
Qualifications to Log this Earthcache
Before logging this Earthcache, please send me an email with the following information:
1. Above, I described the process of Columnar Joining, but I did not specify exactly how this cave was formed. Based on the information above, and your observations at the site, how do you believe this cave was formed?
2. What is the term for a cave that already exists but is widened by water erosion?
3. Please estimate the height and width of the Basalt Cave.
4. To the right of the cave you will see the Basalt columns. Describe the average height, width, and texture.
5. Walk around the bend (watch out for the tide!) and you will find a second Basalt Cave (see additional waypoints). Please describe one difference between this cave and the first cave and provide a possible explanation for this difference.
6. OPTIONAL: Post a photo of you and/or your group with the Basalt Cave in the background.
Please also include the name of this Earthcache and the name of the cachers that your email is being sent for. You do not need to wait for permission to log this cache. If there are any problems with your answers I will let you know. Please do not post the answers to the questions in your log. Logs containing the answers or logs that do not email the answers to me will be subject to deletion.