The Geology of Dunbar
If he had only known it as a child, John Muir spent his first years in a geologist’s paradise. It was only when he visited Dunbar much later in life that this became apparent to him. By then, of course, he was an experienced glaciologist and geologist. Dunbar and its vicinity are still a draw for rock enthusiasts. The opening of the John Muir Way allows easy access to the history written along John Muir’s native coast. Dunbar is built on a coastal headland or promontory with a complicated geology. Small volcanic necks, igneous dykes and beds of volcanic tuff (now cut into cliffs by wave action) cut through sedimentary deposits that become more obvious to the east and west of the town. A lot of the stone was exploited in Muir’s time. The Castle Rock was quarried for walls and buildings as a new harbour was created. The marls and mudstones to the west were burnt for cement and deposits of clay at nearby Belhaven were being worked for brick and tile manufacture. To the east many tons of fossiliferous limestones and shales were burnt every year for lime (used as mortar and field dressing). The overlying landscape has been heavily glaciated leaving a deep and fertile red clay-rich soil and with characteristic sculpting of the landscape. The intrusion of igneous features has led to some interesting features. The coastal exposures allow easy exploration of the succession of sediments. Evidence of the landscape and environment at the time the sediments were deposited can often be teased from the nature of the existing rock. For example,
at Catcraig, a couple of miles to the east of Dunbar, the shales and limestones are interspersed with a few thin seams of coal and on the foreshore the surface is marked in a dimpled pattern both indicating vanished forests. The variety and richness shown along the coastline helped stimulate advances in geological thinking taking place in Edinburgh during the Enlightenment of the 18th century. James Hutton, John
Playfair and those that followed them used newly developed techniques of scientific observation and reasoning to develop their theories. John Muir was later taught these same techniques at the University of Wisconsin. He used them to develop his theory of the glacial formation of Yosemite and other Sierra Nevada valleys. Others may have been less welcoming of methods that led to answers that contradicted Biblical teaching. When John’s father arrived in Dunbar, the local publisher had recently reinterpreted Hutton and other researchers for a lay audience. It can be assumed that in the light of John’s later struggles with his father that George Miller’s ‘Book of Nature’ would not have been welcome to the sight of Daniel Muir.
Columnar basalt
Basalt
Basalt is a very common dark-colored volcanic rock composed of calcic plagioclase (usually labradorite), clinopyroxene (augite) and iron ore (titaniferous magnetite). Basalt may also contain olivine, quartz, hornblende, nepheline, orthopyroxene, etc. Basalt is a volcanic equivalent of gabbro. Basalt is usually black or dark gray and relatively featureless. It is composed of mineral grains which are mostly indistinguishable to the naked eye. Basalt may also contain volcanic glass. Basalt may contain phenocrysts (larger crystals within fine-grained groundmass) and vesicules (holes that were filled by volcanic gases). Black color is given to basalt by pyroxene and magnetite. Both of them contain iron and this is the reason why they are black. So this is iron again which is responsible for the coloration of basalt. Plagioclase, volumetrically usually the most important constituent, is mostly pale gray in color. Basalt is a major rock type that occurs in virtually every tectonic setting. Basalt is clearly the most common volcanic rock on Earth and basaltic rocks (including gabbro, diabase and their metamorphosed equivalents) are the most common rocks in the crust2. Basalt is also common on the Moon and other rocky planets of the Solar System. What makes basalt so common? Basalt is the original constituent of the crust from which almost all other rock types have evolved. Basalt forms when mantle rocks (peridotite) start to melt. Rocks melt incongruently. It basically means that melt that forms has a different composition from the source rocks. Of course, it can only happen if rocks melt only partially but this is exactly what happens in the upper mantle. It melts partially to yield basaltic magma which is less dense and rises upward to form new oceanic crust in mid-ocean ridges or volcanoes and intrusives (dikes, sills) in many other tectonic regimes. Basalt is the source rock of other more evolved volcanic rocks like dacite, rhyolite, etc.
Tuff
Tuff is an igneous rock that forms from the products of an explosive volcanic eruption. In these eruptions the volcano blasts rock, ash, magma and other materials from its vent. This ejecta travels through the air and falls back to Earth in the area surrounding the volcano. If the ejected material is compacted and cemented into a rock that rock will be called "tuff". Tuff is usually thickest near the volcanic vent and decreases in thickness with distance from the volcano. Instead of being a "layer" a tuff is usually a "lens-shaped" deposit. Tuff can also be thickest on the downwind side of the vent or on the side of the vent where the blast was directed. Some tuff deposits are hundreds of meters thick and have a total eruptive volume of many cubic miles. That enormous thickness can be from a single eruptive blast or more commonly from successive surges of a single eruption - or eruptions that were separated by long periods of time.
Dike
A dike or dyke in geological usage is a sheet of rock that formed in a fracture in a pre-existing rock body. However, when the new rock forms within and parallel to the bedding of a layers rock, it is called a sill. 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 can be either intrusive or sedimentary in origin. For example, when molten rock intrudes into a crack then crystallizes, it is an igneous dike. When sediment fills a pre-existing crack, it is a clastic dike.
An intrusive dike is an igneous body with a very high aspect ratio, which means that its thickness is usually much smaller than the other two dimensions. Thickness can vary from sub-centimeter scale to many meters, and the lateral dimensions can extend over many kilometres. A dike is an intrusion into an opening cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies that a dike is always younger than the rocks that contain it. Dikes are usually high-angle to near-vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata through which the dike propagates so that the dike becomes horizontal. Near-horizontal, or conformable intrusions, along bedding planes between strata are called intrusive sills. Sometimes dikes appear in swarms, consisting of several to hundreds of dikes emplaced more or less contemporaneously during a single intrusive event. The world's largest dike swarm is the Mackenzie dike swarm in the Northwest Territories, Canada. Dikes often form as either radial or concentric swarms around plutonic intrusives, volcanic necks or feeder vents in volcanic cones. The latter are known as ring dikes. Dikes can vary in texture and their composition can range from diabase or basaltic to granitic or rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast volumes of mantle-derived magmas through fractured lithosphere throughout Earth history. Pegmatite dikes comprise extremely coarse crystalline granitic rocks - often associated with late-stage granite intrusions or metamorphic segregations. Aplite dikes are fine-grained or sugary-textured intrusives of granitic composition.
Volcanic plug
A volcanic plug, also called a volcanic neck or lava neck, is a volcanic object created when magma hardens within a vent on an active volcano. When present, a plug can cause an extreme build-up of pressure if rising volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption. Glacial erosion can lead to exposure of the plug on one side, while a long slope of material remains on the opposite side. Such landforms are called crag and tail. If a plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive upstanding landform.
An example of two volcanic plugs can be found at the Pitons, in Saint Lucia, as they rise abruptly out of the eastern Caribbean Sea.
The EarthCache
To log this earthcache as a found you have to visit the above coordinates. You also have to send your answer to the questions to me. You do not need to wait for my reply. Do not write any answers in your log. Logs that do not fulfill these requirements will be deleted. Te questions:
1. At the published coordinates: What type of rock do you think that you found here? How has the rock been formed. Describe the shape of the rock, why is it shaped like that?
2. At stage 2: If you look outside of the ruins, you will see a different kind of rock used in the wall of the ruins. How does that rock differ from the rock at the published coordinates? Do you think that that rock was formed in a different way? If yes, how?
3. How has glacial activity affected the area? Explain.
4. Optional: If you want you can upload a picture of you and/or you GPS at the coordinates to your log. This is completely optional and is not a logging requirement.
