Keywords for this lesson:
fracture - any separation in a geologic formation
fissure - a fracture or crack in rock along which there is a distinct separation
tectonic event - a geologic occurrence produced by the movement of Earth's continental plates
hydrothermal solution - water that has been heated to some temperature above ambient surface temperature as a result of natural geologic processes. Often, hydrothermal solutions contain significant amounts of dissolved gases and solids in addition to water.
groundwater - water that exists underground in saturated zones beneath the land surface
sigmoidal - having the shape of the letter S
This EarthCache invites geocachers to look more closely at the veins in a rock in the centre of Rustington. The learning point of this EarthCache is to familiarise the geocacher with the processes that form veins and en échelon veins.
Everything you need to answer the questions is available by attending the co-ordinates and reading this lesson. There's no need to research anything extra online, although you’re welcome to do so if you want to.
At GZ you’ll find a ‘late Ice Age’ erratic boulder which has a number of thick white-coloured lines across it. These are quartz veins.
Quartz is often found in veins that cut through rocks and take the appearance of thin, intrusive quartz sheets blanketed between layers of host rock.
Veins
Veins are mineral deposits which form when a pre-existing crack (fracture or fissure) within a host rock is filled with new mineral material. Quartz is often found in veins that cut through rocks. The crack can form in a number of ways, such as during the folding of the rock in mountain-building processes, by shattering during tectonic events, by a decrease in pressure during the uplift of a rock, or because a rock cools down and shrinks.
Veins of quartz and other minerals are usually not thin tubes, but rather thin sheets. The veins can form under various conditions, and depending on these conditions, may or may not bear quartz crystals in them. White-coloured veins are sometimes made up of other minerals, such as calcite.
The simplest type of a quartz vein is the filling-in of crack that’s already present in a rock. Hydrothermal solutions rich in minerals, that originate at great depths inside the crust of the Earth, which are under great pressure move upward through the rock layers. The suspended minerals in this groundwater then precipitate as quartz as it reaches the cracks in the rock, which is at a lower temperature and pressure. They were then laid down to form the quartz crystals which now form the visible veins.
En échelon veins
As well as the thicker veins present on the rock at GZ, you will also notice some smaller, thinner veins in a different pattern to the larger veins across the surface of the rock. En échelon veins are an indication of movement of the rock mass at some point, and can tell us more about their geophysical formation. Also known as"en échelon gash fractures,” these are a related type of structure within rock caused by noncoaxial shear.
Shear causes pressure on both sides of the crack (fault.) It can be possible to see evidence of the shearing from the en échelon veins. With the larger vein already formed, smaller gashes, which are often orientated approximately 45 degrees from the fault trend, are formed and then filled with quartz or calcite. The en échelon veins change shape due to rotation and deformation in the shear zone, from the straighter orientation of the larger veins. Subsequent growth of the fracture, as it rotates in the shear zone, causes the en échelon vein to take on a sigmoidal shape.
En échelon veins can take various forms in a rock: parallel or subparallel, closely-spaced, overlapping or step-like.
To log this cache, please visit the published co-ordinates and answer the questions below. Once you have obtained the answers, please send them to me via email or through the Message Centre. You are free to log your find once you have contacted me. You don't have to wait for a reply. If there are any questions about your answers, I’ll contact you.
Logs without answers will be deleted. Please don’t include close up pictures in your logs that may answer the questions.
1. Look at the main quartz veins in the rock. Explain their distribution in the rock - are they all linear, are they formed in a pattern, or does it just look random?
2. Look at the angle of the main quartz veins in the rock. From the horizontal angle, which angle do they go in, and how thick is each of the main quartz veins?
3. Look for the smaller, en échelon veins. What shape are they?
4. In reference to the en échelon veins, please describe the pattern in which they appear in the rock.
5. Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache, but please do not give any answers away.
