Dike, Sill and veins – Amazing Geology on Alderney

Dikes

A geologic dike is a flat body of rock that cuts through another type of rock. Dikes cut across the other type of rock at a different angle than the rest of the structure. Dikes are usually visible because they are at a different angle, and usually have different color and texture than the rock surrounding them.

Dikes are made of igneous rock or sedimentary rock. Igneous rock is formed after magma, the hot, semi-liquid substance that spews from volcanoes, cools and eventually becomes solid. Magmatic dikes are formed from igneous rock.

Sedimentary rock is made of minerals and sediments that build up over time. Sedimentary dikes, also called clastic dikes, are formed from sedimentary rock. They can arise through a variety of mechanisms, with sediment entering cracks formed in the affected sediment or sedimentary rock from below, from the side, or from above.

Dikes frequently intrude on open spaces between rocks, called fissures. A dike will either flow or build up in a fissure, pushing the surrounding rock to the side. A dike is, therefore, younger than the rocks surrounding it. Dikes are often vertical, or straight up and down. But since the Earth is constantly moving and shifting, the dike can end up horizontal after enough time goes by.

They are are discordant or cross-cutting, wall-like, minor intrusive structures which vary in thickness. They are generally vertical or nearly so having been intruded from below along joint or fault planes to form dilation structures. They may occur singly, in sets or in swarms. A dike swarm is usually created by the same geologic event, such as a volcano.

Sills

Sills are tabular versions of the dike and have usually been intruded along bedding planes in sedimentary rocks and may transgress to other planes via joints. Sills, on the other hand form when magma exploits pre-existing weaknesses, such as strata, foliations, or fractures to emplace within rocks. Frequently cited examples of sills are nearly horizontal intrusions that are parallel to the horizontal beds of the surrounding rocks, but actually sill can born inclined, if they exploit inclined mechanical discontinuities

Sills and dikes have chilled margins, this is an often feature and especially in sheeted dike complexes, where multiple chilled margins are a distinctive feature

Chilled Margin

A chilled margin is a shallow intrusive or volcanic rock texture characterised by a glassy or fine-grained zone along the margin where the magma or lava has contacted air, water, or particularly much cooler rock. This is caused by rapid crystallization of the melt near the contact with the surrounding low temperature environment. In an intrusive case, the crystallized chilled margin may decrease in size or disappear by later remelting during magma flow, depending on magma heat flux.

Characteristics

These gangetic rocks occur either in the form of so-called dikes (extended plate-shaped rock bodies) or in tubes (like the kimberlites in South Africa). Because of the rapid cooling during ascent, dike rocks, in contrast to plutonic rocks, often exhibit a porphyry structure (large, well-formed crystals in a fine-grained matrix).

Gangetic rocks whose chemical and mineralogical composition still largely corresponds to that of the original magma (such as in the plutonic rock granite and its derivative, rhyolite) are referred to as aschist (from the Greek aschistos: "unsplit"). These dykes are designated as follows: plutonite plus name; i.e. granite porphyry or syenite porphyry, for example.

More common, however, are diaschistic (from diaschizo: "to split") gangue rocks, such as aplite, pegmatite, lamprophyre and lamproite, which have clearly differentiated from their parent magmas as they rose. Lamprophyres are dark, fine-grained, and basalt-like. These are e.g. rocks with the names Spessartit, Monichiquit, Kersantite. Aplites and pegmatites are light-colored vein rocks formed from granitic magmas; they consist only of feldspars and quartz. Pegmatites (often containing rare minerals and ores) are coarse to giant grained and Aplite fine to medium grained.

Both types of intrusions (dikes and sills) are always younger than the rocks they intrude.
 

Veins

Veins are very narrow, generally wall-like structures, vertical or inclined, in which minerals have crystallised, usually by growing out from the walls of the fracture in which they formed

There are two ways in which veins can be formed, by open-space filling (crystals grow at 90° to the walls) or crack-seal growth (crystals quickly form variable or composite mosaics.)

Sources

wikipedia.de
www.jerseygeologytrail.net
eu.sctimes.com
www.mineralienatlas.de
blogs.egu.eu/divisions/ts/2022/05/30/features-from-the-field-dikes-and-sills/
education.nationalgeographic.org/resource/dike
www.geologyin.com/2014/06/dykes.html

So lets take a closer look of the geologic formation at the coordinates.

To log this Eartcache, go to the given coordinates, look at the marked area on the wall and please answer the following questions. This via email or the message center to me

1. Which of the above named intrusions do you see? If one of them is a dike or a sill, please justify your answer. (Look at the listing. What is the main diffenrence)

2. Describe the texture, the average width and the color of these intrusion.

3. These two kind of intrucions have a point of intersection. Which one is the older one?

4. What kind of rock is the surrounding rock, which keeps the intrusions?

5. Can you see a chilled margin?
   
   Additionally

6. Take a photo of yourself and/or a personal item (e.g. your GPS) at the house with the flint wall and attach it to your log!