Dike or Sill – Amazing Geology on Herm

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

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.

When the rock surrounding the dike or sill is less resistant to erosion, they are often left in nature as free-standing walls or ramparts

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
https://geologyistheway.com/igneous/sill/
www.sciencedirect.com
 

Attention: To answer the questions, you don't have to go to the beach, you can do it from above. Be aware of the cliff

To log this Eartcache, go to the given coordinates and answer the following questions. This via email or the message center to me:

1. What do you think, is it a dike or a sill? How would you justify your observation?

2. What do you think the dike or sill is made of?

3. Describe the texture, the average width and the color of the dike or sill.

4. What do you think is the reason, that the dike or sill is free standing?
   
   Additionally

5. Take a photo of yourself and/or a personal item (e.g. your GPS) and attach it to your log!