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.
CAVE
TYPES
There are several cave types but we will only focus on the most
common ones.
Sea Caves
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.
Glacier 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.
Ice Caves
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.
Coral Caves
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.
Volcanic Caves
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.
Tectonic 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.
Solution Caves
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.
e.g.
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
References:
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.