Stuff we linked to on Twitter last week

Posted on August 29, 2010 by Chris Rowan

A post by Chris RowanA post by Anne JeffersonAnother week, another link extravaganza for you to enjoy.

Blogs in motion

The National Ground Water Association joins the blogosphere with its creatively named NGWA Blog (http://info.ngwa.org/blog). Early posts suggest the blog will focus on the practical aspects of the hydrogeology profession.

Volcanoes, Earthquakes, Tectonics

Planets

Environmental

Water

(Paleo)climate

Fossils

General Geology

Interesting Miscellaney

Categories: links

Friday Focal Mechanism

Posted on August 27, 2010 by Chris Rowan

A post by Chris RowanThe earthquake that particularly caught my eye this week occurred on Tuesday off the coast of Mexico:

Tuesday 24 August: M 6.1, off W Coast of Mexico, Depth 11 km

This focal mechanism is pure strike-slip: that is, it is the result of two sides of a fault moving laterally past each other, with no real compression or extension involved. A tectonic map of the area shows that it occured on the Rivera Fracture zone, a transform segment of the northern East Pacific Rise, just before it terminates in the Gulf of California. The dextral, or right-lateral sense of motion is consistent with the fracture zone being a tectonic link between different spreading segments of the ridge system, a concept that I have discussed before, regarding a similar earthquake in Iceland.

Plate boundaries off the west coast of Mexico

The other interesting thing about this area is that the East Pacific Rise is very close to the subduction boundary on the Mexican coast, and continuing eastward subduction means that the distance between them is shrinking all the (geological) time. The ridge segment at the eastern end of the Rivera Fracture zone is extremely close to being subducted, pretty much cutting off the section of oceanic crust to the northwest from the Cocos Plate to the southeast. In fact, deformation along the zones marked with the purple dotted lines mean that this area is effectively moving independently of the rest of the Cocos Plate, and can therefore be regarded as a separate plate: the Rivera Plate. This is what happens when a ridge gets subducted: large transform offsets eventually become the boundaries of little plates strung along the subduction zone, as the ridge segments that they originally linked to get subducted.

Categories: earthquakes, focal mechanisms, tectonics

Yellowstone: what lies beneath

Posted on August 26, 2010 by Chris Rowan

A post by Chris RowanResearchBlogging.orgThe Yellowstone caldera is located over a ‘hotspot’, where volcanism – and in the case of Yellowstone, hydrothermal activity – is occurring far away from the plate boundaries where such things are normally found.

Plume above....plume below? Plume geyser in Yellowstone's Upper Geyser Basin. Photo: Chris Rowan 2010.

Hotspots are often linked to mantle plumes: if this is the case, the anomalous volcanism is the result of hot, buoyant rock from deep in the Earth rising up through the mantle to the base of the crust, and melting as it enters regions of (relatively) low pressure. As you may or may not be aware, there is a vigorous and long-running debate over whether this link is justified in all cases, with some arguing that mantle plumes may not exist at all (those interested in the gory details should check out mantleplumes.org). In the case of the Yellowstone hotspot, the linear hotspot track marked out by the chain of calderas along the Snake River Plain suggests that the hotspot remains stationary with respect to the North American plate moving over it, as you would expect if it was coming from deeper in the mantle. The oldest caldera on the track was active 16-17 million years ago, and is located in the same region as the Columbia River flood basalts, which also first started erupting about 17 million years ago; several other hotspots are also associated with flood basalts in this way, and they are interpreted as recording the moment when the voluminous head of an upwelling plume first reaches the base of the crust, triggering a large amount of melting and sending huge volumes of magma to the surface. It is possible that the Yellowstone hotspot is similarly connected to the Columbia River Basalts.

Geographical association between the earliest part of the Yellowstone hotspot path, and the Columbia River Basalts.

These features are suggestive of a possible plume origin for the Yellowstone hotspot but are far from definitive. In the latest attempt to settle the question, Obrebski et al. exploit the fact that there are more seismometers deployed across North America than ever before to produce the most detailed picture yet of the structure of the mantle beneath western North America. This picture is constructed using seismic tomography: an analysis of how the time that seismic waves from distant earthquakes arrive at a detector is changed by local variations in the temperature or composition of the material it is passing through. For example, if there is a region of hotter than normal material beneath a particular seismometer, the earthquake waves will travel more slowly and be detected later than expected; if it is colder than normal, the waves will move faster and arrive earlier than expected. By combining the signals from a large number of earthquakes, all of which take different routes through the mantle, and the records from a distributed grid of seismometers, you can produce a three dimensional picture of the structure of the mantle.

Obrebski et al.‘s latest model clearly images the ‘hot’ part of the hotspot, in the shape of a large, low velocity zone directly beneath the Yellowstone caldera. What is more, you can follow the low velocity zone downwards into the mantle, along a somewhat corkscrew path: it first descends to the northwest, before abruptly reversing its direction and descending to the southeast, and then changing direction again at about 800 km depth. This trail extends right to the base of the area modelled, 1200 km beneath the surface, almost half way to the core-mantle boundary, and much further than any potential plume has been traced beneath Yellowstone before.

Horizontal slice through the tomographic model at 200 km depth (left); the low velocity zone beneath the Yellowstone caldera follows a corkscrew path into the lower mantle (right). Source: Obrebski et al. (2010), Fig. 2.

But perhaps the most exciting thing about these new images is not what is there, but what isn’t. There has been a plate subducting eastwards beneath the Pacific Northwest since at least the Cretaceous period, and we would expect the tomographic model to pick up the cold subducted slab as a region of faster than average seismic velocities dipping to the east, and extending some way into the mantle. At shallow depths, the slab is easy to spot: in the horizontal slice through the model above, it is the the blue linear feature just inbound from the west coast. But as we look deeper in the mantle, we see something unexpected. A cross-section running E-W beneath Oregon (the top image in the figure below) reveals that there is no fast anomaly below about 300 km depth, even though there should be a slab there based on our understanding of the tectonic history of the subduction zone. A similarly oriented cross-section further to the south shows the slab extends much deeper beneath California and Nevada, to depths of at least 600 km. Curiously, however, further to the east there is another eastward dipping zone of fast (and presumably, cold) mantle, which looks a bit like a subducting slab but appears to be disconnected from any surface tectonics.

Vertical E-W cross sections showing the velocity structure of the mantle beneath (top) Oregon and (bottom) California and Nevada. Source: Obrebski et al. (2010)

Obrebski et al. argue that if the Yellowstone hotspot is indeed fed by a plume from the deep mantle, as these results seem to indicate, when it first ascended towards the surface it would have intersected with the subducting slab, and what we are seeing in the tomography is the aftermath of that interaction. Effectively, the upwelling plume burnt through the subducted plate beneath Oregon on its way to the surface, heating it up enough that it lost its rigidity and was assimilated into the asthenosphere, leaving isolated fragments further down dip that were no longer connected to the plate at the surface. Tectonic changes at the Cascadia subduction boundary around 19 million years ago, notably a large reduction in the rate of plate convergence, could be linked to this splintering of the subducted plate, and the reduction in slab pull that would have resulted. After overcoming this barrier to its upward passage, the plume head then reached the surface 2 million years later, causing the eruption of the Columbia River Basalts.

3-D plot showing the interaction between the subducting slab (blue) and the upwelling Yellowstone plume (yellow). Source: Obrebski et al. (2010)

This is all extremely cool in its own right, but I also find this quite interesting in the context of the whole mantle plume debate. This is hardly the first time that seismic tomography has been used to attempt to image mantle plumes beneath hotspots, and although they do occasionally seem to image conduits of hot material coming up from deep in the mantle, the dimensions of these features generally push at the limits of what is resolvable with tomography, raising the possibility that they could simply be artefacts of processing. But in this case, the tomography has also revealed that at some point in the past, material rising from the deep mantle completely disrupted the subduction system beneath the Pacific Northwest, and the debris of this past interaction is clearly associated with the current velocity anomaly beneath Yellowstone. This gives us more confidence that the ‘plume’ being imaged in this study is actually real.

Obrebski, M., Allen, R., Xue, M., & Hung, S. (2010). Slab-plume interaction beneath the Pacific Northwest Geophysical Research Letters, 37 (14) DOI: 10.1029/2010GL043489

Categories: geophysics, paper reviews, volcanoes

Castle geology

Posted on August 23, 2010 by Anne Jefferson

A post by Anne JeffersonBeing a giant geo-nerd, I tend to pepper my travels with a lot of geologically or hydrologically interesting places. A recent trip brought me to the UK and included a meetup with my coblogger in Edinburgh. Being an American tourist, I also felt compelled to visit at least one castle during my time in the UK, so I dragged Chris to Edinburgh Castle…where we naturally we ended up talking about the geology.

Edinburgh Castle from Princes Street

Edinburgh Castle from Princes Street. The low area with gardens in the foreground used to be a loch/lake. (Photo by A. Jefferson)

First off, Edinburgh Castle sits atop the core of a Carboniferous (340 million year old) volcano, now called Castle Rock. The rock is impressively elevated above the surrounding glacially sculpted cityscape. The volcanic root formed a bit of a speed bump for Pleistocene glaciers moving from west to east. Castle Rock was more erosion resistant than surrounding sedimentary rocks, so it was left standing higher than the surroundings. But it also protected the sedimentary rocks in its lee – forming a giant crag and tail structure. The Royal Mile, which stretches from the Castle to Holyrood Palace rides on the tail of this structure – which gradually decreases in height and width away from the castle. In the Google Earth image below, I’ve maximized the vertical exaggeration and you can get some sense of it, though I can tell you that a more visceral understanding is achieved by walking up or down the Royal Mile.

Crag and tail of Edinburgh Castle and the Royal Mile

Crag and tail of Edinburgh Castle and the Royal Mile (image from Google Earth)

Within the castle itself, the glacial legacy is on display in the building stones, which show a remarkable diversity of lithologies, both those found locally and those farther afield in Scotland. It is likely that castle builders would have made use of the rocks left on the landscape when the glaciers retreated, but the ice caps would have given them plenty of variety. In the middle of the photo below, I think we are looking at the Old Red Sandstone, which has an important place in the history of geology and of paleontology.

Stonework on St. Margaret's Chapel, the oldest extant building in Edinburgh Castle

Stonework on St. Margaret's Chapel, the oldest extant building in Edinburgh Castle (photo by A. Jefferson)

The stone construction seemed to vary between buildings and even parts of buildings within the castle. The photo above is from St. Margaret’s Chapel, built in the 12th century, and you can clearly see how the big rocks are matrix-supported by a sand and gravel cement. This wall is actually even different from other walls on the same building, where stones are much more rectangular, probably cut, and there is much less matrix, as shown in the photo below. The informative castle guidebook suggests that the wall pictured above may have been originally part of another structure, onto which St. Margaret’s chapel was built adjointly. I should also point out that the walls in St. Margaret’s chapel are incredibly thick (1 m or more), so there’s a lot of stone work we’re not even seeing from the outside.

St. Margaret's chapel showing variety of stone work

St. Margaret's chapel showing variety of stone work (photo by A. Jefferson)

Portcullis Gate and the Argyle Tower

Portcullis Gate and the Argyle Tower (photo by A. Jefferson)


In other parts of the castle, and indeed in the surrounding city, it was fun to watch for places where old doorways or windows had been filled in or where additions had been added to stone buildings. These were usually pretty easily spotted by a change in the stone construction style. For example, in the photo on the left, you can see the main gated entrance to the castle (the Portcullis Gate), which was constructed in the late 16th century. On top of it is the Argyle Tower, one of the youngest buildings in the castle, constructed in 1887. To me it also looks like there might be an intermediate strata between the cut and tightly placed blocks immediately around the gate and the much more modern top part.

Wall adjacent to Lang Stairs

Wall adjacent to Lang Stairs. The plaque on the wall commemorates the successful recapture of the castle from the English in 1314. (photo by A. Jefferson)

One of the most impressive features of the castle was the way the stone architecture worked with the irregular topography of the bedrock surface. Walls had uneven bottoms, and even, in some places, sides, like this wall near the Lang Stairs.

Finally, I would be remiss if I didn’t point out one important hydrogeological influence on Edinburgh Castle and its history. As I mentioned, the castle is much higher than the surrounding topography (peak is 134 m above sea level) and made of dense volcanic rock. This has advantages from the defensive point of view, but a significant disadvantage from the “having enough water to stay alive” point of view. During peace times, castle residents presumably brought water up the hill from the loch or from wells in town. But during sieges, they were reliant on the Fore Well. This well, constructed in the 14th century, is an impressive 34 m deep (through solid rock! in the 14th century!), but only the bottom 3 m of the well are below the water table. The well could provide 11,000 liters – but that’s not a lot to supply a bunch of soldiers and their animals. During at least one siege, most of the deaths seem to be attributable to lack of adequate water, rather than the warfare itself.

So that’s what happens when you take an American geo/hydro nerd to Scotland…she looks at castles and thinks about rocks and water.

Categories: by Anne, geology

Stuff we linked to on Twitter last week

Posted on August 22, 2010 by Chris Rowan

A post by Chris RowanA post by Anne JeffersonAs well as the two of us, there are many other earth, ocean and space scientists using Twitter these days: The AGU have more than 300 on their list. Listed below are the most interesting things we came across last week.

Blogs in motion

Volcanoes

Earthquakes & Tectonics

Fossils

(Paleo)climate

Water

Environmental

Planets

General Geology

Interesting Miscellaney

Categories: links