Month: May 2008

Volcanic Rocks

The book uses lots of names for volcanic rocks and other igneous rocks that can get confusing.

Original Source

Volcanic (extrusive) rocks are classified in almost exactly the same way as plutonic (deep intrusive) rocks. The principal difference is that volcanic rock names are substituted for their plutonic equivalents:

Plutonic Rock	Volcanic Equivalent
Granite	Rhyolite
Granodiorite	Dacite
Tonalite	Quartz Andesite
Syenite	Trachyte
Monzonite	Latite
Diorite	Andesite
Gabbro	Basalt
Monzogabbro (or diorite)	Latite-basalt (or andesite)

This diagram below show how the different kinds of igenous rock can be distinguished by their chemistry using a “simple” plot of Silicon against the total amount of Potassium and Sodium.

Chapter Three should need no added support, so I have passed over to chapter four.

This chapter of the book is probably the most difficult for all of you, even if you do have some background in science. To be very honest there is no science required that is not covered in the Virginia SOL but I know most of you probably did this, or the equivalent, a long time ago. The aim of this chapter is to introduce you to the physical science behind volcanology, and although there really is nothing beyond you, I do not expect you to “get” it all. Just try. I will point out some of the things you should try to take with you to the rest of the course.

Magma
Magma is complex stuff. It is a chemical mix of different oxides of the metals Fe, Mg, Ca, Na, K, Al, Si with minor Ti, Cr, Ni, Zr and a host of other trace elements present in quantities measured only in parts per million. It is, however, not just a simple mixing bowl. These metal oxides are not distributed at all evenly throughout the melt. A better way to think of magma is of a matrix, or a mesh. Think of a bowl of spaghetti, (but even more “joined up) made from Oxygen (O) Silicon (Si) and Aluminum (Al) with the other oxides such as Iron, Magnesium, Calcium, Sodium and Potassium occupying the holes. The more joined up the spaghetti (the greater the degree of polymerization) the more viscous (harder to stir) it becomes. In this bowl of magma, water acts like a pair of scissors, cutting up the matrix into smaller pieces that flow past each other more easily, and thus the magma becomes less viscous (easier to stir). An important by product of this is that that the other metal oxides are then able to diffuse (move) through the melt more easily. This allows crystals to grow faster and bubbles to form larger -and faster too.

Magma chambers are not usually simple caverns in the ground. In most cases they are very complex (well they just had to be, right?) and take many forms. There are cases, especially in high level magma chambers (i.e. not too deep in the crust), where such “caverns” of magma may form, even within the the actual physical edifice of a volcano, but more often chambers are much more complex; more like a sponge, where the surrounding rock (country rock) is the sponge, with magma as water in the sponge. In this case the magma often partially melts some of the country rock that then mixes with the magma as it cools and crystallizes. Even as this happy hadean fusion is consummated, more fresh magma will well up from below to replenish and reheat the magma that was emplaced earlier. This is a dynamic and interactive process that has the potential to produce a broad range of magma compositions and many different forms of volcanic eruption.

Bubbles
Until you have stared thoughtfully at the bubbles in as many pints of Guinness as I have, you could not have the slightest clue just how complex bubbles can be. This section tells a simple tale for you to remember. Bubbles grow in magma in the same way as they do in any fizzy drink. Gas is dissolved in the magma until the pressure drops enough for it to be released (open the can!). As magma rises to the surface, and the pressure drops, bubbles start to form. In low viscosity magma, such as basalt, the bubbles can form easily, grow large, and can escape from the magma relatively easily. In MORE viscous magma (complex spaghetti) like rhyolite, the bubbles find it hard to form, and are not able to grow so large. As a consequence pressure inside the bubbles builds up. Think of blowing some air into a large balloon, and then the same amount of air into a small balloon. The small balloon builds up pressure until the surrounding rubber breaks and the balloon bursts. Well, this also happens with bubbles in viscous magma. The pressure builds up to the point where the surrounding magma, stretched to breaking point in an incandescent froth, fractures under the pressure of all these bubbles to produce an almighty explosion, and a subsequent violent eruption, not of magma, but of a fragmented foam of magma and gas. This is a classic pyroclastic eruption.

Mount Etna (listed on your Google Earth file) is one of Europe’s great volcanoes and has been active for a long time

This (silent) BBC Video is worth a look.

Make sure you read or watch some of the background videos/sites on continental drift before reading this chapter.

• Active continental margin and island arcs; where one plate often slides under another
• Volcanoes in the middle of the ocean; often due to rising hotspots in the mantle (there are other possible causes)
• Continental rift valley; where plates are moving apart and tearing apart the crust of a continent
• Volcanoes in the middle of nowhere; but not in Va….yet…..

Please take time to look at at least a few of these volcanoes using Google Earth.

Convection in the mantle. We need to be very clear about a common misconception. Apart from very limited partial melting in the upper mantle, the mantle of the Earth is solid. Rock hard. Over time at the high temperatures and pressure of the mantle it can still flow as if it were liquid,, but all the time it remains essentially solid. The only part of the deep earth that is molten is the outer core.

In figure 2.13 you will see that the temperature of the earth rises much faster in the crust than in the mantle, this is because radiogenic isotopes that decay and release heat are concentrated in the crust rather that the mantle.

Page 35-35. MORB stands for Mid Ocean Ridge Basalt, the typical lava erupted at….mid ocean ridges where tectonic plates are moving apart! 🙂 Apparently you were just supposed to know this? Go figure? So much for careful editing!

Summary of main kinds of basalt, each chemically distinctive).

• MORB (mid ocean ridge basalt)
• Tholeiite
• Alkali
• IAB (island arc basalt)
• Ocean island basalt (OIB)

Page 38; Fractional crystallization.

These early-formed minerals are heavy and sink through the magma to accumulate on the floor and sides of the magma chamber. Hence “cumulates”. As they selectively remove certain elements from the magma, the magma that remains progressively changes in composition. Think pingpong balls. if I start with 50 green and 50 red pingpong balls in a bag (magma chamber) and selectively remove the green ones (as in the formation of cumulates) the bag is then left with an increasing proportion of red balls. In a magma chamber, this results in an increasing concentration of sodium Na, Potassium K, Silicon, Si, Aluminum Al, sometimes iron Fe, and always, water H₂0)

What do you think of his definition of a volcano? Does it fit what you observe on Earth? With this definition, some phenomenon we do not think of as volcanic are dragged into the volcanic world. Thermal springs, for example, are a consequence of internal thermal processes, but we do not think of them as actively volcanic. The problem here is that we need to extend our earth-bound definitions to encompass a plethora of extraterrestrial phenomena on frozen worlds where liquid water (or methane or ammonia) is as much lava to the substance of that planet/moon as our incandescent silicate lava is to the mantle of the Earth.

A note on units
You may or may not be familiar with the units used in this text. A simple Google search will get you to a satisfying definition in most cases, but you can always ask me if there is something that is unclear or confusing.

• Joules is a measure of energy like the more familiar (all too familiar!) calories
• 1 x 10⁶ is the same as 1,000,000, or 1 followed by 6 zeros, that makes it a million
• A Watt (or smaller milliwatts= thousands of a Watt) is a measure of power, or the “flow” of energy.
• Myr is millions of years
• PPM is a measure of concentration in parts per million

All the chemical elements have symbols

Web Elements

• U is uranium
• Th is Thorium
• K is Potassium

Isotopes
Each element has an atomic number based in the total number of both protons and neutrons in the nucleus. Each element can also exist in a number of forms based on a naturally variable number of neutrons in the atomic nucleus. These “forms” are called isotopes. U²³⁵ U ²³⁶ (read “uranium 235 and uranium 236”) are both naturally occurring isotopes of uranium, each with the same number of protons in the nucleus, but different numbers of neutrons. Many of these isotopes are unstable over time and break apart (decay) to produce different elements with the loss of energy. Thus, radioactive potassium (K) 40 decays over time to produce Argon gas and Uranium decays to produce different isotopes of Lead.

Ions are atoms either missing some of their electrons or carrying one or two too many! They have an electrical charge that can be positive (cation) or negative (anion).

Kinetic energy is the energy of movement. Hit your hand on the desk (ouch!) Hit is faster (bigger “ouch”). Welcome to the world of kinetic energy. This is a function of the mass of an object and its velocity. The faster the same mass is moving, the more kinetic energy it has, and as with all energy, this can be converted itno other forms of enrgy, most commonly thermal energy, on collision with another object.

Tidal energy is due to the effect of gravity of one object on another. In this case, gravitational energy is converted into internal deformation of a planet/moon and this leads to the generation of heat. Take a large paper clip. Twist it until it breaks. Quckly feel the broken ends. They will be hot due to the release of energy through deformation (in this case by your hands instead of gravity).

Remember the Earth is like an onion (or Ogers if you are Shrek fan). There are layers. The outer, very thin, layer is the crust. The thickest layer (100km -2,900Km deep) beneath is the rocky mantle (still solid for most part). Some 6,000Km deep lies the metallic core. There are two parts to the core, a liquid, molten outer core and a solid inner core.

Kimberlites…where we get diamonds from!

Do not try to remember the names of all the minerals or rock textures mentioned in this book. The list of minerals on page 12 is quite important, however, so have a go at remembering (or least recognizing) these ones.

On page 13 he writes about Sodium and Potassium as “Na₂O and K₂O”. These are not the elements (Na and K), but oxides of the elements; we geologists just get sloppy sometimes and that infuriated newcomers like yourselves.

This blog is a way for me to supplement and support the content of this course as we go through chapter by chapter. This is not an easy course, but it will be very rewarding if you put time into it. There are a number of hurdles that you will have seen already.The first hurdle is an apparently endemic fear of science and scientific notation that plagues the liberal arts field. Let’s deal with that one first. There is a good deal of scientific convention used in this book and, no doubt, some of this will be unfamiliar to you. Let me state quite clearly that there is nothing that you need to know, nothing that you need to understand in order to do well in this course, that is beyond you with just a little help from me and the other students.

A second hurdle is created by the vast amount of material available on the web and through the electronic library. You need to discipline yourself, limit the time you spend surfing around, and just focus on reliable and acknowledged sites such as the USGS or foreign equivalents and, of course learned societies and universities. By all means look more broadly, but do not forget to keep your focus. Please do not be tempted to use Wikipedia or other online encyclopedias as more that a starter option. We will go much deeper than that, and anyway, I know all of that so well that I am bound to spot it! 🙂

Welcome to this Volcanology blog.