Welcome Guest [Log In] [Register]

From Aug 2006 - Nov 2013 WeDig provided a live forum for diggers & fans of Vindolanda. It has now been mothballed and will be maintained as a live archive.

Here you will find preserved 7 years of conversation, photos, & knowledge about a site many people love. Vindolanda gets under the skin. (Figuratively and literally as a volunteer excavator!) It's a place you remember, filled with people you remember!

Thanks for 7 great years!

Viewing Single Post From: Mike's Geoblog
Mike McGuire
Member
[ *  * ]
I’ve alluded previously to the fact, which nowadays I think most people know, that the continents move very slowly around the face of the globe. “About the rate your fingernails grow” is a common analogy. What is less well understood is how this is possible. Quite commonly it is suggested (including in a recent BBC TV science series which should have known better) that there is a layer of molten rock below the crust. NO – THERE IS’NT. So how can the continents move? Here’s the simplest explanation I can give of what really happens.

The Earth has four layers. At the centre is the solid inner core which is mostly iron. Outside that, reaching out to just over half the Earth’s diameter of just under 14,000 km, is the liquid outer core, again mostly iron, which produces the Earth’s magnetic field. Nearly all the rest, making up about 80% of the Earth’s volume, is the mantle which is made of what we would recognise as “rock” – mostly silicate minerals. The crust is just a thin layer on the outside, typically 35-40km thick for the continents and only about 7 km thick for oceanic crust.

The layer we’re concerned with here is the mantle, which is mostly made of a rock type called peridotite. Much the most common mineral in this is olivine – the jewellers’ name for olivine is peridot. Olivine is green, so yes folks the vast majority of the Earth’s bulk really is green, not just the outer surface we live on (and, boy, is that green in the Vindolanda area at this time of year, especially now it’s back to the rainy season). Like all rocks which contain more than one mineral, peridotite melts over a range of temperature. At the surface, it starts to melt at about 1,100 degrees C, but this temperature increases as the pressure increases with depth into the Earth. By about 350 km deep the melting temperature is over 2,000 degrees C. The temperature increases as you go into the Earth and so there is a sort of competition between the increasing temperature trying to make the peridotite melt and the increasing pressure trying to stop it. In normal circumstances the pressure wins at all depths, though in a few special cases a bit of melting does occur – hence volcanoes and all igneous rocks including the Whin Sill (of which more later). However, there is a depth between about 100 and 200 km where the mantle is close enough to melting that it gets ever so slightly “squidgy” and can flow very slowly by a mechanism called solid state creep. Geologists call this layer the asthenosphere and the “non-squidgy” mantle above it together with the crust are called the lithosphere. In fact it’s sections of the lithosphere, referred to as plates, which move, carrying the continents with them, driven by complex combinations of forces which are not completely understood.

The existence of the asthenosphere was first discovered by seismologists studying how vibrations from earthquakes propagate through the Earth. They do so slightly more slowly through the asthenosphere than through the surrounding mantle. There are a few isolated instances where bits of peridotite have been “coughed up”, so to speak, at the Earth’s surface and laboratory experiments on these confirmed the seismologists’ discovery. This discovery had far-reaching implications. It provided a way in which the theory of continental drift, which had been pooh-poohed for many years as impossible, could be possible and thus could explain much else which had been a puzzle. And it also explained why vertical movements of the crust occur. Sections of the lithosphere can be thought of as “floating” in the asthenosphere and so Archimedes’ principle can be applied. Geologists call this phenomenon isostacy.

You may have wondered how it was possible for all the Yoredale Cycles to keep piling up on top of each other and yet still stay at about sea level. Isostacy provides the answer. Partly it was the weight of the accumulated sediments pressing the whole pile down into the asthenosphere. But this would not have been sufficient on its own. The lateral movements of the plates can cause the crust to become compressed in some places, as they did about 100 million years earlier when England and Scotland were welded together, and to become stretched in other places. It seems that, in the mid-Carboniferous when the Yoredale Cycles were deposited, this area of crust was being stretched and the whole Northumberland basin sagged down. The combination of these two effects caused the crust to subside at just the rate which was needed to create the repeated deposition cycles we now observe.

On a smaller scale, isostatic movements have occurred in Britain since the end of the last glaciation. Since the weight of all that ice was removed, the northern part of our island has been rising up – hence phenomena such as raised beaches in Scotland. In compensation, the southern part of Britain has been subsiding. The Romans knew what we now call the Scilly Isles as a single quite substantial land mass.

The stretching in Northumberland had one other important effect. Because the crust became thinner, the load on the top of the mantle was reduced to such an extent that a very small amount of melting was eventually able to occur. When such partial melting happens, some minerals in the peridotite melt sooner than others and the resultant liquid has a different composition called basalt. Liquid basalt is less dense than the rocks of the lower crust so it rose up through fissures until it reached a level where its density matched that of the rocks. Then it spread out sideways over a vast area to form what we now see as the Whin Sill. Outcrops of the Sill are found all the way from the Farne Islands in the north east to the Pennine ridge overlooking Penrith in the south west as well as down Teesdale at places such as High and Low Force.

Such “decompression” melting is one, but not the only one, of the main ways in which partial melting of the mantle occurs to produce the igneous rocks which eventually become the Earth’s crust.

Sorry if some of the terminology is a bit confusing. In common with most scientists, geologists tend to use Greek and Latin words to generate names for the new concepts they discover. This contrasts with my own former full-time profession computing (yes, I confess it, I am an ancient Geek) where new ideas are named by taking the American word for something completely different.

Next week, something a bit less abstruse – limestone.
Offline Profile Quote Post
Mike's Geoblog · Excavation & General Archaeology Discussions - Open to All!