Fossils near Avaiki cave
Location: Among large limestone boulders alongside and beneath the Avaiki cave
Access: Can be reached by walking down the Avaiki trail.
Lesson: Related to rock cycle, fossilization, and geologic time.
Terrain rating due to >.5 mile walk to cache area, then rough boulders to climb around on in the vicinity of the cache. Watch your step.
HISTORY
Approximately 450 million years ago, the island of Niue (but also large areas of eastern Australia and New Zealand) lay beneath a deep ocean. Geologically speaking, this was the Silurian era.
About 440 million years ago tectonic activity caused the east coast of Australia move eastward. This caused that Niue became a warm, shallow tropical sea that was teaming with life.
During that time, there were periods of violent volcanic activity, interspersed with quieter periods when silt and sand were deposited to form sandstone layers. Coral reefs formed in the shallow waters. These became siltstone and sandstone beds, interspersed with some fossiliferous (containing many fossils).
The Volcanic activity resulted in the formation of different rvolcanic rock (for example such as dacite, rhyolite and tuff). Subsequent tectonic activity resulted in the beds being folded and faulted which is now observed as exposed dipping beds. These different formations can be observed in and around the Avaiki area.
The Limestone here at this location just outside the cave is particularly rich in fossils. The site is a perfect example for Late Silurian sedimentary sequences.
ROCK
Rock is a naturally occurring, solid mixture of minerals and organic matter. There are three types of rock. Sedimentary rock forms when the weight of sediments press down on layers of sediment, cementing them together. So when rocks of any kind are weathered -- or broken down into little pieces to become small bits of sediment -- they have the potential of being recycled and remade back into rock (sedimentary rock) again!
The other two types of rock are igneous rock-- which is formed out of the cooling of lava -- and metamorphic rock. Metamorphic rock forms when heat and temperature change the texture and mineral content of other types of rock. These rocks can be weathered and broken down too, and in time that sediment can be cemented into sedimentary rock.
FOSSILS
When organisms die, they usually decay quickly. If they are covered by sediment or soil right away, their bones or shells may be protected from decaying. Over a long time, that sediment gets pressed down and cementented together, making a sedimentary rock. The trapped bones or shells become part of the rock! This is called fossilization and it usually happens in sedimentary rock.
ROCK LAYERS AND GEOLOGIC TIME
Over tens of thousands and even millions of years, all those rocks end up in layers under our feet and of course under our oceans. If we could look at a cross section of the rocks underneath us, we'd see all those layers! The ones closer to us are younger. The ones way down deep are ancient! That is, if there hasn't been any faulting or folding -- shifting around -- of the original layers. This logical rule about the youngest rocks being above the older rocks in undistrubed rock layers is called the law of superposition. We use this law to figure out the order of events in Earth's history. Determining the age of objects and rocks this way is called relative dating.
THE FOSSIL RECORD
Relative dating can also be used with fossils we find, the fossil record. You can see thousands of fossils right here under your feet! Fossils of plants and animals show how they changed and evolved throughout Earth's history. An organism that changes a lot, that develops visible adaptations as it evolves, helps us in figuring out geologic time too. For example, a fish that lived 50 million years ago may look a lot different than its ancestor that lived 200 million years ago. When we find a fossil of the older fish, we know the rock around it is 200 million years old. If the fish didn't change at all over time, we wouldn't be able to tell the younger rocks with that fossil from the older rocks with the same type of animal.
How does the fossilization process work?
The process of fossilization takes place in successive phases, which are described in more detail below:
Death, decomposition (e.g. decay, putrefaction, mummification, charring), embedding, degassing, diagenesis and metamorphosis.
Depending on the surrounding circumstances, these phases can also take place repeatedly or the order can be reversed. It can be the case that an organism is embedded immediately after its death or that it dies in the first place when it is embedded (in media such as bitumen, quicksand or ice). An organism can also be exposed again long after it has been embedded, decayed, and then finally become embedded again.
To put it simply, you can say: This area was once flooded. In addition to ichthyosaurs, mussels, ammonites and corals also cavorted in the sea water. Their skeletons and shells were made of calcium and were deposited on the seabed after their death. Together with eroded rock debris, they formed a layer that grew thicker over millions of years. Heat and pressure pressed the thick limestone layers into solid sedimentary rock. As the sea receded, fossils appeared wherever their impressions were well preserved.
How many fossils are still preserved?
How many fossils According to scientifically based estimates, around a billion animal and plant species have emerged since the beginning of the Phanerozoic 541 million years ago and most of them have become extinct again. Some estimates even put it at 1.6 billion. Far less than one percent of this species richness has been preserved as fossils, not only because of the special circumstances required for fossilization, but also because many fossils have been destroyed over the course of millions of years as a result of weathering, erosion or rock metamorphism.
Sources:
https://de.unionpedia.org/Fossilisation
To log this EarthCache, please follow these instructions:
You can park your car at the beginning of the Avaiki cave trail.
Please answer the following questions by sending a message through the Geocaching message center or email, with answers (in english or german please) to my profile:
1. How many fossils can you discover and what size are they? (little help: It is shaped like the logo for Shell gas stations, and it was a scallop when it lived).
2. Is it an animal or plant fossil? Assign it to a group of animals or plants! What can you infer about the environment in which this fossil probably formed?
3. What do you call the process by which these corals remain imaged in this rock "for eternity"?
4. A picture of your team, your mascot or GPS is mandatory.
You are welcome to log your answers straight away but please message or Email me with your answers. I'll let you know if anything is unclear. Please don't forget: the answers must be received within 10-14 days or the log may be deleted.
Enjoy your stay