WHAT TO DO WITH A VOLCANIC PLUG

Painting of Edinburgh Castle ~1780 by Alexander Nasmyth. Public Domain.

The title says it all!  What does one person/state/country do with a volcanic plug? I asked this question to a group of beginning geology students many decades ago. Probably the most common answer was to quarry the basalt (or similar volcanic rock) and use the material for construction aggregate. Since volcanic plugs, or any volcanic material (except bentonite and wind blown ash), are unknown in the superficial sedimentary rocks of Kansas that was a reasonable guess. In further discussions I found that several of the students had seen cinder cone quarries during vacation trips/field trips through New Mexico and/or Arizona. There is a world of difference between cinder cones and volcanic plugs.

Sunset Crater is a large cinder cone located north of Flagstaff, Arizona, in the San Francisco volcanic field. The vent erupted ~1075. Photo Public Domain and courtesy of Mike Sanchez.
 

A cinder cone is a smaller conical hill composed of volcanic ejecta-- loose pyroclastic clinkers, volcanic ash, or scoria—that formed when a volcanic vent “blew” small fragments into the air and as they rained back down the fragments cooled and accumulated in a  cone-like structure.  These small cones are often quarried for construction aggregate, decorative yard rocks, and even for use in “backyard barbecue” grills as a cover for the burners. There is virtually no large-scale solidification of the clinkers, and they literally may be “scooped up” with a shovel or backhoe. Easy to quarry.

A volcanic plug is formed when molten magma hardens in the throat of an active volcano and blocks further eruptions from that particular vent. Sometimes the volcano becomes “unplugged” as massive pressure builds up under the plug and a very explosive eruption takes place—the volcano “blows its stack.”  At other times the pressure in the magma field simply opens a new vent. At other times, as the vents start to cool down, the plug in the throat also hardens into a very tough basaltic type of rock (I use this term to indicate any of the fine-grained, extrusive igneous rocks that form from the rapid cooling of lava rich in magnesium and iron. Some are actually diabase or dolerite or trachyte or a variety of other types). In fact, these throat rocks are often harder than the surrounding country rocks. As time moves on, erosion removes much/most of the surrounding rock and the volcanic plug becomes a positive topographic feature.  There are literally hundreds of these plugs scattered across the world but to me the most impressive are the volcanic plugs scattered across the red rocks of the Colorado Plateau. The most famous plug of the Plateau is probably Ship Rock near the Four Corners area of Colorado, Utah, New Mexico, and Arizona although nearby Agathla Peak, seven miles north of Kayenta, is much more intimate and easier to reach.

Agathla Peak, a volcanic plug, rises about 1500 feet above the desert floor in the Four Corners area.

So, to add a second choice to the question—preserve the plug as a scenic area or state park or wilderness or tourist attraction.

Edinburgh Castle. Public Domain photo courtesy of kids.kiddle.co.

Edinburgh Castle, with associated buildings (including the New Barracks 1799), situated on the summit of Castle Rock, a volcanic plug.

I find a third choice perhaps the most intriguing—build a fortified castle. I recently returned from a two-week train trip, with nightly stops at guest houses or pubs, visiting England and Scotland, from the southwestern beaches of England to North Sea beaches on the Firth of Forth in Scotland (I love the name of that locality). 

The UK has a magnificent high speed passenger rail system. We opted for a rail pass available to U.S. residents and spent a few dollars more for First Class. Wow. Luxury reserved seating with complimentary (somehow I paid for it) food and libations.

 

As most of us know, thanks to British BBC and/or U.S. PBS shows, the UK has numerous castles spread over the country. Some are in ruins while others are in quite good shape although the tremendous monetary expenditures needed for upkeep often necessitate opening the residences for tourist visitation. Most are built on flatlands complete with moats and swans; a few are built on higher points. The most impressive castle I observed on this trip was Edinburgh Castle (Scotland) situated on the high point of Old Town and constructed on Castle Rock. Impressive to a geologist since Castle Rock is a volcanic plug and was incorporated into the foundations of the buildings. In other words, the master stone masons did not “smooth off” the plug but cut dimension stone to fit the varied surface. I suspect that only an ole plugger like me would notice such a masterful piece of work; however, I was really impressed. The Castle was started sometime in the early 1200s; however,  Castle Peak was inhabited several centuries earlier. In the ensuing 1100 years Scotland and England were the sites of almost continuous warfare, and the Castle of sieges, at least 21 according to our tour guide.  Readers need to use a web browser and “look up” Edinburgh Castle.

Stonemasons quarried local limestone and sandstone as well as parts of the plug.

Notice how the building stones "fit" into then topography of the igneous rock foundation.

Soldiers of the Castle Garrison ca. 1845 (Top, Public Domain). A lone guard 2023 (Bottom).

The geological makeup of Scotland is extremely complex and many/most of the country seems composed of small orphan terranes stitched together during plate movements. The volcanoes that produced Castle Rock, and numerous other volcanic features of southern Scotland, are about 350 Ma, early Carboniferous (Mississippian in the U.S.) Period. At that time “Scotland” was part of the Supercontinent Pangaea and situated near the equator. There were periodic incursions of shallow marine waters and the southern Scottish volcanoes evidently intruded, at times, into these shallow seas. Many of the local quarried stones for the Castle buildings are of shallow marine origin of Devonian and Carboniferous age. Pleistocene glaciation helped create most of the modern landscape of Scotland and was critical in shaping the “modern” Castle Rock. As glaciers moved in from the north the volcanic rocks were more resistant to erosion than the surrounding sedimentary rocks and left the plug as a isolated highland. However, the harder plug (a crag) protected the sedimentary rocks on the leeward side and formed a somewhat tapered ramp (tail). The Scots like to call this a “crag and tail formation.”

Edinburgh Castle is built on a volcanic plug or crag (A),  the Pleistocene glaciation moved from left to right (C), B is the tail of rock on the leeward side protected from massive erosion by the crag. Public Domain figure.

I saw many impressive sites on the journey and we surprised our concerned children that an 80-year-old-guy who walks with a knee brace and cane, along with a very tough spouse, were able to do a fair bit of walking in the villages and completed the steep hike up Castle Rock. There were challenges as we picked small guest houses or lodges that normally do not have elevators. The toughest was a four-story lodge in Edinburgh with narrow steps that required schlepping two, forty-pound roller bags along with backpacks up 80 steps—one at a time. The scariest time was trying to board a Birmingham train station escalator (lift was out of order), losing my balance, and falling backwards. Strange feeling looking up while in a slanted position and people screaming “shut it down”. My heavy backpack protected my head and the people behind me dodged the tumbling roller bag and held me steady until the contraption shut down. EMTs were quick to clean me up and stop the bleeding from a series of “wolverine claw marks” running up my leg (from those bitty pointy teeth on the steps). But life goes on and old geologists are tough old birds and used to tumbles. We caught a later train and had a extra ale in a nifty inn in Shrewsbury.