Dikes on Dalkey Island EarthCache

Dikes on Dalkey Island

Dikes

A dike is a sheet of rock that formed in a crack in a pre-existing rock body. However, when the crack is between the layers in a layered rock, it is called a sill, not a dike. It is a type of tabular or sheet intrusion, that either cuts across layers in a planar wall rock structures, or into a layer or unlayered mass of rock. Dikes, also named dykes, can be either magmatic or sedimentary in origin. A magmatic dike is formed when magma (lava) flows into a crack and solidifies as a sheet or tubular intrusion. It can cut through layers of rocks or the contagious rock mass. Clastic or sedimentary dikes form when sediments fill a pre-existing crack in the rock

A magmatic dike is a sheet of igneous rock that cuts across older rock beds. It is formed when magma fills a fracture in the older beds and then cools and solidifies. The dike rock is usually more resistant to weathering than the surrounding rock, so that erosion exposes the dike as a natural wall or ridge. It is from these natural walls that dikes get their name.

The thickness of a dike is much smaller than its other two dimensions, and the opposite walls are roughly parallel, so that a dike is more or less constant in thickness. The thickness of different dikes can range from a few millimeters to hundreds of meters, but is most typically from about a meter to a few ten of meters. The lateral extent can be tens of kilometers, and dikes with a thickness of a few tens of meters or more commonly extend for over hundred kilometers.

Mafic magma (fluid magma low in silica) usually reaches the surface through fissures, forming dikes. At the shallowest dephts, dikes form when magma rises into an existing fissure.

Types of Dikes

Magmatic dikes, formed by the intrusion and solidification of magma within cracks in pre-existing rock, exhibit a fascinating variety in terms of their characteristics and occurrence.

Dikes Types Based on Geometry

- Vertical dikes: The most common type, these dikes intrude perpendicularly to the host rock layers, often appearing as vertical walls when exposed.
- Sills: Intrusions that follow the bedding planes of the host rock, forming horizontal or gently inclined sheets.
- Cone sheets: Sets of concentric, cone-shaped dikes dipping away from a central magma chamber, mimicking the shape of a volcanic cone.
- Ring dikes: Circular or elliptical intrusions, often associated with caldera collapses and volcanic centers.

Dikes Types Based on Composition

- Simple dikes: Composed of magma of a single, uniform composition. These are the most common type and can range in composition from basaltic (fine-grained and dark) to granitic (coarse-grained and lighter).
- Basaltic dikes: These are the most common type of simple dikes, composed mainly of basalt or diabase, and are often associated with volcanic activity.
- Granitic dikes: Formed from felsic magma with high silica content.
- Pegmatite dikes: Characterized by very coarse-grained crystals and often containing rare minerals.
- Aplite dikes: Characerized by very fine-grained crystals and a typical light color.
- Composite dikes: Formed by multiple injections of magma with different compositions, resulting in a layered or mixed appearance.

Your task to log the Earthcache:

Answer the following questions via message in English or German via my geocaching profile:

1.) Describe the differences between the granite (the host rock at the site) and the rock forming the dike with your own words (color, structure, grain size, surface, form of weathering...)!
2.) Which rock is older - the granite or the rock of the dike? Why?
3.) How thick and long (estimated) is the visible part of the dike at the site?
4.) What type of dike (read the text) do you think this dike is?
5.) What rock is more resistant to weathering, and why? Describe the situation in your own words!
6.) Optional: Post a photo with your log, showing you and/or your GPS near the location!


After you've sent me the message with your answers, feel free to log! If there's something wrong, I'll contact you via message!

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