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Trace Fossils of Ancient Volcanoes

A cache by Geocaching@pt Send Message to Owner Message this owner
Hidden : Sunday, 08 October 2006
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Geocache Description:

- Trace Fossils of Ancient Volcanoes -
Avencas Beach (Parede) - 8 October 2006, 10H00 to 12H00

This cache is part of the 1st Annual International EarthCache Day

When we speak of fossils the images that immediately come to mind are the skeletal remains of the large quadrupeds that once roamed the earth back in the Triassic (245 Ma) – Cretaceous (65 Ma) periods.
No doubt, “trace fossils” will conjure up visions of the king of the Cretaceous period and Jurassic Park. However, trace fossils can be more subtle, smaller and you can be walking right over them without ever being aware of their existence.

Ancient and present day volcanoes are features that have left a definite fingerprint on the surface of the earth. The present day eruptions that we are presented with on the news bulletins of our global television coverage are perhaps the best known. The latest major eruption (at the time this cache went to press) was the August the 16th eruption of Tungarahua volcano in central Ecuador. However, the USGS Weekly Volcanic Activity Report keeps track of dozens of potentially hazardous and deadly volcanic activity throughout the world.
Less hazardous and clearly harmless clues of this activity have been left as fossils for geologists to study and for the public in general to marvel at. These fingerprints are dykes, sills, volcanic chimneys and basalt flows (columnar and pillow) left exposed as a result of natural erosion processes, excavations, archaeological digs and mining.

In Portugal we have several examples of these types of volcanic fingerprints. Here are but a few listed:

  • Dykes: Commonly found in several settings and common occurrences along the coastline where exposure is perhaps best. These are bodies of lava that cut across the bedding of the rock.
  • Sills: Commonly found in several settings and common occurrences along the coastline where exposure is perhaps best. These are bodies of lava that are parallel to the bedding of the rock.
  • Volcanic chimneys: Commonly found in the Lisbon area. These are remnants of the conduits of the lava to the surface. Clear and simple examples can be found at Cabeço de Montachique, where there are two caches nearby, 1- Passeio pelo Parque de Montachique and 2 – Sanatório Albergaria and at Les Aventures de Tintin to name but a few clear examples.

Basalt flows:

  • Columnar basalts: An example of this type of basalt can be seen at another earthcache created in Portugal by our friend Lynx pardinus. His cache at Penedo do Lexim is an excellent opportunuty to view examples of this type of basalt and well worth a visit. It forms during the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns. Because hexagons fit together efficiently with no vacant space (a tessellation), this is the most common pattern that develops. Pentagonal, heptagonal or octagonal joint patterns are also known, but are less common. Note that the size of the columns depends loosely on the rate of cooling; very rapid cooling may result in very small (<1 cm diameter) columns, and vice versa.


  • Pillow basalts: Spectacular examples of this type of flow are present in Caniçal (Madeira). When basalt erupts underwater or flows into the sea, the cold water quenches the surface and the lava forms a distinctive pillow shape, through which the hot lava breaks to form another pillow. This pillow texture is very common in underwater basaltic flows and is diagnostic of an underwater eruption environment when found in ancient rocks. Pillows typically consist of a fine-grained core with a glassy crust and have radial jointing. Size of individual pillows can vary from 10 cm up to several metres.

The cache:

If you're interested in learning a little more about the planet you live in, then let’s meet on the 8th of October for an on-site description and open air informal lecture on dykes and sills at Praia das Avencas (Parede) at the above coordinates. At 10H00, the prediction is for a low tide that will allow clear viewing of these features. On hand will also be a biologist that will try to answer any questions you may have regarding the diverse biota that live amongst these rocks.
The nearby beach will allow, the weather permitting in October, for a relaxed atmosphere and the more restless can brave the cooler water temperatures of the Atlantic ocean.
For more instructions of how to get to the location, please follow the instructions here. It will also give you a chance to brush up on the features you will observe

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