A cave is a natural underground cavity large enough for a human to enter and large enough that some portion of it does not receive direct sunlight.
Caves are formed by various geologic processes that can involve a combination of chemical processes , erosion from water, tectonic forces, microorganisms, pressure, atmospheric influences and digging.
Like everything, there’re several types and categories of caves based on their origin. Primary caves are developed as the host rock is solidifying like lava tubes or coral caves while secondary caves are carved out of the host rock after it has been deposited or consolidated in with there is the most part of them.
There are several cave types but we will only focus on the most common ones.
If the cave is created along sea cliffs by erosion processes as they weather away weaker areas, they can be any size from crevices to large chambers and we term these Sea Caves
Eolian Caves or Wind Caves.
When cave formation is caused by wind erosion of sandstone cliffs, these type of caves are known as Eolian Caves or Wind Caves.
Long tunnels form near the snouts of glaciers between glacial ice and the underlying bedrock. Water from the surface drains down through crevasses in the glacier enlarging the crevasses and melting away the ice at the base of the glacier.
These caves can be carved out of glaciers or snowfields by water and/or wind or in a rock cavity containing ice formations. As moisture in a cave is frozen it clings to the walls and continues to build up. When slight melting occurs or water enters the cave, it runs along the walls creating formations similar to calcite speleothems.
When colonies of coral in shallow water expand and unite, they form lacy or bulbous walls around an open area. When the shoreline is pushed up or sea level falls, the cave is exposed. Waves and wind erode the coral, enlarging the cave, sometimes even destroying it.
These type of caves are all created from flowing lava and the effects of volcanic gases. Categories of volcanic caves include lava tubes, pressure-ridge caves, spatter cone chambers and blister caves.
A massive movement of bedrock separates rocks along joints or fractures. The cave created in this fashion is usually a small, high, narrow fissure consisting of a single passage. The ceiling is often a flat section of rock that did not move, or moved in a different direction. Massive, brittle rocks such as sandstone and granite are the best rocks for tectonic caves; however, they can also occur in basalt and limestone.
This is the category of caves that is classified as caverns. They are formed by the dissolution of soluble rocks such as limestone (calcium carbonate; CaCO3), dolomite (calcium magnesium carbonate; (Ca, Mg)CaCO3), gypsum (calcium sulfate dihydrate; CaSO4•2H2O) and salt (halite; NaCl).
Limestone is primarily solid CaCO3. All types of limestone begin with dissolved calcium carbonate, CaCO3, which is contained in most fresh water and in sea water.
Limestone Slowly Dissolves in Rain
Rain contains dissolved carbon dioxide, CO2 . As rainwater containing dissolved CO2 trickles
through the ground, it reacts with the calcium carbonate of limestone. The product of this reaction is calcium hydrogen carbonate, Ca(HCO3 )2 . Calcium hydrogen carbonate dissolves easily in the water and washes away.
H2O (l) + CO2 (g) ? H2CO3 (aq)
Carbonic acid then can ionize in water forming low concentrations of hydronium and carbonate ions:
2 H2O (l) + H2CO3 (aq) ?CO32- (aq) + 2 H3O+ (aq)
The outcropping rocks in the region of this EarthCache are composed of marly limestones of Lower Cretaceous age (Albian; 112 to 99 Ma.). These limestones are rich in fossils like Choffatella decipeins, Ostrea praelonga and other gastropods. The same limestone rocks are more prominent to the west of the area, namely between the Safarujo river mouth and Ericeira village where they are more massive. At the cache location there remain narrow bands of these limestones.
LAPA da SANTA is a limestone cave whose ceiling is constantly dripping water.
Caves were used by our primords to be safe from the weather and dangerous animals. And this one probably was used as a shelter for those purposes. Close by a what it seems to be a ancient mill (probably for olive oil or wine) can be found.
Nowadays the cave is filled of water and it’s used to feed sheep that sometimes graze nearby and for some wild life such as rabbits and foxes.
How can you log the cache?
To log this Earthcache as a find, you have to answer correctly this questions and send them to me via my GC.com mail before you log it:
• What, and why, is the type of this cave?
• What is written on the square at the door?
• You’ll also have to post a picture of you near the cave entrance.
Logs without the corresponding correct answers and uploaded photo will be deleted
Telo, J., Carreira de Deus, P., de Oliveira, J. Rodrigues, L. e Nery, F. 1954. Carta Geológica de Portugal 30-C (Torres Vedras), Escala 1:50000. Serviços Geológicos de Portugal.
Zbyszewski, G., Moitinho de Almeida, F. e Torre de Assunção, C., 1955.
Notícia Explicativa da Folha 30-C (Torres Vedras). Serviços Geológicos de Portugal.
NOTE: A PARKING WAYPOINT IS GIVEN. PLEASE DO NOT ENTER THE PROPERTY WITH YOUR VEHICLE!
The most exciting way to learn about the Earth and its processes is to get into the outdoors and experience it first-hand. Visiting an Earthcache is a great outdooctivity the whole family can enjoy. An Earthcache is a special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. Earthcaches include a set of educational notes and the details about where to find the location (latitude and longitude). Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth. To find out more click HERE.