Pretty Tuff At First EarthCache

For this EarthCache, geocachers are invited to examine a short cliff that rises above a beach. The learning point of this EarthCache is to get the geocacher to become familiar with tuff. 

Everything you need to answer the questions is available by visiting the location and by reading this lesson. I don’t anticipate you will have to research anything extra online, although you’re welcome to do so if you wish to.

This area can be dangerous, so please be careful when visiting. The area is only available at low tide; the rest of the time it is underwater. Please check the tide times to avoid disappointment, and leave plenty of time to make your visit before the tide comes in again.

Tide times are available online here

The weather can change fast here, so be prepared for cold and wet conditions. 

The beach can be slippery, and the rocks can be uneven. Please wear adequate footwear. Please do not climb on the rocks. Please also be aware of waste and debris on the beach.

View of the cliff at GZ

On your approach to the published coordinates, you will have noticed that the Scottish Seabird Centre is built on a rocky headland surrounded by sandy beaches. The Centre is built on lava flows, which were once nearly-horizontal sheets and have later been tilted. The very hot lava flows from a nearby crater have formed a hard, dark, igneous rock called basalt.

During the Dination epoch, about 343 million years ago, Scotland was located south of the Equator and had a sub-tropical climate. Most of the North Berwick area was a flat muddy coastal plain covered with lagoons. In the muddy areas, sedimentary rocks were forming. At the same time, the area was very tectonically active, and a lot of volcanism took place. This was creating igneous rocks. Much of the sedimentary rock is more easily eroded than the igneous rock. Today, many local features are formed of this igneous rock, such as the Bass Rock (a phonolite plug), North Berwick Law (a phonolitic trachyte plug) and the islands of Craigleith (a laccolith) and Fidra and Lamb (basalt sills). 

The volcanic activity in the North Berwick area lasted for several million years. There were many different craters in this small area. At the start of the volcanic activity, the area was probably wet, and so the first eruptions were highly explosive as red-hot, mobile, basaltic magma turned groundwater to steam. The steam violently burst its way to the surface.

Volcanic ash fell, which produced red and green-coloured tuffs. Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is compacted into a solid rock in a process called consolidation. After the ash fell in the North Berwick area, volcanic eruptions occurred, producing four separate lava flows. Three of these flows were thick, and one was thin, and each can be seen distinctively in the local rocks. Further ash fell, creating further thin beds. Millions of years later, the area was subject to large-scale folding.

There is a lot to explore at GZ, but this EarthCache focuses on the tuff formations here. 

Tuff is an igneous rock that forms from the products of violent volcanic eruptions. These eruptions cause the volcano to blast rock, ash, magma, gas and other materials high into the air. This material then falls back to Earth in the surrounding area. If the material is compacted and lithified into a rock, then this rock is called tuff. 

At the published coordinates, you will be at the bottom of a small cliff immediately below the Seabird Centre. At the bottom of this small cliff you will see a thin red bedding layer. This red rock is tuff, formed by volcanic ash falling into shallow water. The iron content of the ash was oxidised to ferric oxide during, or after deposition.

The composition of magmas can vary widely, and every volcano is supplied with magma of a different composition. Here at GZ, above the tuff layer at the bottom of the cliff, is a lighter band of rock. This is the lowest lava flow, which has formed a trachybasalt. Trachybasalt is a volcanic rock with a composition between trachyte and basalt.

You will see a lot of other red rocks between here and the swimming pool, all formed from this volcanic tuff. In some areas the tuff is coloured green, where the iron has been reduced to the ferrous state (possibly due to the action of microbes after deposition.)

These tuff rocks are softer than the basalt layers. Tuff is one of the most common rocks in this area, but because it is softer than basalt and other crystalline igneous rocks, it is not as visible. The red tuff lies underneath the lava, and so this tells us that the layers of ash were created before the lava. The layering of basalt and tuff you see in the cliff before you is evidence of how the volcano behaved: first it exploded violently, and later lava oozed out. The layers of ash formed when showers of ash and rock fragments were blasted into the sky. This probably happened because there was lots of water around: hot magma and cold water make an explosive mix.

Formations of tuff are usually thickest near the volcanic vent. Rather than thinking of them as a layer, tuff is better described as a lens-shaped deposit. It can also be thicker depending on which way the wind was blowing (thicker downwind of the vent) or according to the side of the vent where the blast was directed.

Tuff deposits can contain xenoliths (inclusions of foreign rock) that were not created by volcanic activity. Rather, when the volcanic explosion happened underground, the force broke some of the overlying bedrock and launched it into the air with the ash, magma and other material. 

Tuff also varies by particle size. Near the vent, a tuff might consist mainly of large blocks of material in a volcanic ash matrix. With distance from the vent, the clasts will be smaller in size. At the edges of the rock unit, the tuff might be mainly composed of very fine ash.

In the tuff layer on the cliff, or the tuff rock on the floor in the area toward the swimming pool, you might see some grey-green circles (around up to 10 cm across) in the red tuff. They are known as reduction spots. They indicate that parts of the rock have chemically changed since they were formed - in this case, the iron content has reduced. The spots are roughly spherical in 3 dimensions.
 

Tuff deposits can be hundreds of metres thick. Each deposit might have come from one volcanic eruption, or more commonly, from successive surges of a single eruption, or even separate eruptions from the volcano that occurred over long periods of time.
 

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 cliff in front of you. What is it’s approximate height?
2. Look at the red tuff deposit in the cliff. Please describe it (dimensions, texture, features, size of tuff particles etc.)
3. Given your answer to Q2, do you think the volcano that formed this tuff was nearby or distant? Why do you think so?
4. If you can see any xenoliths in the tuff, please describe them.
5. If you can see any reduction spots in the tuff, please describe them.
6. Now, please consider the cliff as a whole. Identify the different layers of basalt and tuff rock. Which is thicker? Measure this thickest layer and try to explain why this layer is the thickest.
7. Optional, take a photo of yourself and/or your GPS in the general area of this EarthCache.  

Good luck, and thanks for visiting this EarthCache!