Xenolith or Intrusion? EarthCache

This cache has been archived.

AnnieMaroo: More than half of the finders on this EC are sending wrong answers because they are not looking the correct "head"
Thank you to all who visited and learned about intrusions
Annie

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Hidden : 12/2/2014
Difficulty:
2 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:


To log this Earthcache please send your answers through the geocaching.com messaging system. Please copy and paste the questions (so I can see them along with your answers) Once you've sent your answers, go ahead and log your find with out waiting for a reply. I will contact you if there are any problems with your log entry. Please do not post your answers in your log or post phoptos that would give away all the answers. Please note that I will only answer emails sent through the geocaching messaging system. /span>

Questions and Tasks:

At Home: Read the brief definitions provided below so that you have an understanding of what a Xenolith and an intrusion are. Look at the photo called Polynesion Head. It is not a name of a type of rock. I just thought it looked like a Polynesion statue head.

At GZ: Look very closely at the rock. Feel it. If it is raining or really cloudy, you will need to bring a very bright flashlight. The head is located aproximatley half way between the telephone poles. The wires are not the man made item you will be looking for. 

1) Using the definitions below and in your own words, please tell me the evidence you found at the site to decide if the "head" in the rock is a Xenolith or an Intrusion.

2) List the differences differences between the "country rock" and the "head" rock. Examples of what I want you to look for are listed below
Texture: rough or smooth (or both) in the
"Head"
"Country Rock"
Colours contained in the:
"Head"
"Country Rock"
Colours along the edges where both igneous rocks meet and the thickness of this colour difference, if there is any.

3) Using meters estimate how high up from the sidewalk the "head" located. Approximately 2 meters above the "head" in the rock is a man-formed item, please tell me what that is.

4) True or False:
There are crystals the size of the end of a pin in both the country rock and the "head" rock.

5) True or False:
The rock that makes up the"head" is the oldest.
In your own words please explain what helped you make your choice.



A Xenolith (Ancient Greek: "foreign rock") is a rock fragment which becomes enveloped in a larger rock during the latter's development and hardening. In geology, the term xenolith is almost exclusively used to describe inclusions in igneous rock during magma emplacement and eruption.
Xenolith is a geologic term for foreign rock fragment within a host rock.
To be considered a true xenolith, the included rock must be identifiably different from the rock in which it is enveloped; an included rock of similar type is called an autolith or a cognate inclusion.

An Intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly cools into a solid, the different parts of the magma crystallize into minerals. Intrusions are one of the two ways igneous rock can form; the other is extrusive, that is, a volcanic eruption or similar event. Technically speaking, an intrusion is any formation of intrusive igneous rock; rock formed from magma that cools and solidifies within the crust of the planet.
An intrusion is magma (melted rock) which cools and becomes solid under the Earth's surface. It occurs when there are lines of weakness such as faults, joints, or bedding planes in the crust. Magma enters these lines of weakness.
By contrast, an extrusion is magma which cools into rock above the surface of the crust. Both intrusive and extrusive rocks are classed as igneous rocks.
The process of forming intrusive rocks may take millions of years. As the rock slowly cools into a solid, the different parts of the magma crystallize into minerals. The crystals are usually larger than in extrusive rocks.

Important Principles of Geology

There are a number of important principles in geology. Many of these involve the ability to provide the relative ages of strata or the manner in which they were formed.

The principle of intrusive relationships concerns crosscutting intrusions. In geology, when an igneous intrusion cuts across a formation of sedimentary rock, it can be determined that the igneous intrusion is younger than the sedimentary rock. There are a number of different types of intrusions, including stocks, laccoliths, batholiths, sills and dikes.

The principle of cross-cutting relationships pertains to the formation of faults and the age of the sequences through which they cut. Faults are younger than the rocks they cut; accordingly, if a fault is found that penetrates some formations but not those on top of it, then the formations that were cut are older than the fault, and the ones that are not cut must be younger than the fault. Finding the key bed in these situations may help determine whether the fault is a normal fault or a thrust fault.

The principle of inclusions and components states that, with sedimentary rocks, if inclusions (or clasts) are found in a formation, then the inclusions must be older than the formation that contains them. For example, in sedimentary rocks, it is common for gravel from an older formation to be ripped up and included in a newer layer. A similar situation with igneous rocks occurs when xenoliths are found. These foreign bodies are picked up as magma or lava flows, and are incorporated, later to cool in the matrix. As a result, xenoliths are older than the rock which contains them.

The principle of uniformitarianism states that the geologic processes observed in operation that modify the Earth's crust at present have worked in much the same way over geologic time. A fundamental principle of geology advanced by the 18th century Scottish physician and geologist James Hutton, is that "the present is the key to the past." In Hutton's words: "the past history of our globe must be explained by what can be seen to be happening now."

The principle of Original Horizontality states that the deposition of sediments occurs as essentially horizontal beds. Observation of modern marine and non-marine sediments in a wide variety of environments supports this generalization (although cross-bedding is inclined, the overall orientation of cross-bedded units is horizontal).

The principle of superposition states that a sedimentary rock layer in a tectonically undisturbed sequence is younger than the one beneath it and older than the one above it. Logically a younger layer cannot slip beneath a layer previously deposited. This principle allows sedimentary layers to be viewed as a form of vertical time line, a partial or complete record of the time elapsed from deposition of the lowest layer to deposition of the highest bed.

The principle of faunal succession is based on the appearance of fossils in sedimentary rocks. As organisms exist at the same time period throughout the world, their presence or (sometimes) absence may be used to provide a relative age of the formations in which they are found. Based on principles laid out by William Smith almost a hundred years before the publication of Charles Darwin's theory of evolution, the principles of succession were developed independently of evolutionary thought. The principle becomes quite complex, however, given the uncertainties of fossilization (Paleontology), the localization of fossil types due to lateral changes in habitat (facies change in sedimentary strata), and that not all fossils may be found globally at the same time.

Definitions:
Country rock: a geological term meaning the rock native to an area.
Igneous rocks: rocks formed from the solidification of molten magma.
The "head" is differnt coloured rock that is shaped like a head - please look at the photo called Head near the end of the cache page and just before the logs start to see it before you go to the cache site.

Resourses:
The Geology of Southern Vancouver Island by Chris Yorath
Wikipedia
Geology.about.com
E of Stellalabella

Additional Hints (Decrypt)

Nafjre dhrfgvbaf

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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