Gold and Lava

This ask-a-Geologist query began as a science fiction writer trying to make his novel more realistic. His original question was framed around a scene where gold is mixed in a lava flow. Barring the obvious difficulties of how you would (a) concentrate and refine the gold and (b) why would you want to play with gold in a lava flow in the first place...

           Q: Does the gold just melt completely away? Would It sink? Would it meld into the Lava and you couldn't tell the different between the two? Would it rest on top and be visibly different? I'm writing a story and I want to be as scientifically accurate as possible. And considering I don't know what happens when Molten Lava touches a refined Precious metal. I thought i would ask.

– Jeremy A

A: To start with, your hypothesis is a long way removed from any possible reality. This is because gold is rarely found in nature in a form larger than tiny flakes. The rare nugget found in Alaska is just that: extremely rare, and secondary at that (i.e., the nugget is not the original form).

            If you want to modify your story to deal with a refined gold artifact, the following may be helpful:

            The melting temperature of pure, refined gold is 1064 °C. By comparison, the melting point of magma is in the range 700 °C to 1300 °C - depending on its silica content. Gold is also extremely conductive, both electrically and thermally. Gold will thus tend to conduct heat through it very efficiently. 

            The density of pure refined gold is quite high: 19.3 g/cc. A typical magma might range in density from 2.4 g/cc to 3.35 g/cc - again depending on silica content. 

            From these, I can suggest two possible scenarios:

1. If a refined gold object is set on an active flow down-range from Kilauea/Pu'u O'o (on the Big Island, Hawai'i), it will initially start to sink into the magma. Experience has shown (including my personal experience) that magma exposed to air quickly forms a crust and hardens inward - rapidly - from that crust. I've personally walked over active toes of magma flows there, because it was already crusting over. And yes, it was still rough on my boot-soles, but mainly because the crust is really mostly glass. Under these very specific circumstances, the gold object would end up being locked, partially sunk, in the cooling magma crust. 

2. In another possible scenario, the refined gold object might be thrown into an active lava tube (look at the photo in the upper right of this link: https://profile.usgs.gov/jwynn for an example of a skylight broken into a lava tube). In this case, the magma temperature is in the 1200 - 1300 °C range, and the gold object would first sink, then slowly turn liquid and begin to disperse in the magma as it continues down the lava tube. 

Hope this helps your book be a bit more realistic.

~~~~~

 

Age of the Earth in a Nutshell

Q: In geology class my professor told me that the earth's age is based of off meters how does this work

Thanks for your help

- Theo

A: 

The age of the Earth was initially estimated by scientists by mapping stacks of sedimentary rocks in the UK, then measuring sedimentation rates in similar environments (lakes, rivers, seashore, etc.). In the 19th Century this initially gave startling - even shocking at the time - estimates in the hundreds of millions of years range. In the early 20th Century radioisotopes became available, and these were used to extend the age of the Earth into the billions (billion = thousand million) of years age range*. This physically meant measuring back to the point in time when the mineral hosting the radioisotopes and their daughter-products was last melted. THEN it just became a game of searching all over the Earth for the oldest date-able minerals with uranium and lead in them (for example, a zircon crystal). The oldest rocks found so far are in Greenland and western Australia, and based on these the Earth's age is estimated to be at least 4.55 thousand million years old. This means it is at LEAST that old. 

* Note that in some countries like the US, the word "billion" means a thousand million, while in other countries (e.g., the UK), the word "billion" means a million million.

Paleomagnetism

Paleomagnetism = old + magnetism
It's the study of magnetic signatures in ancient rocks, and what they can tell us about the Earth long ago. This requires drilling out little core samples from rock outcrops, carefully marking them for orientation, then back in a laboratory measuring the orientation of the remnant magnetic field that comes from just the rock itself.

Q: Could you tell me what factors they looked at to determine that the north pole moved.

My book says it left markers but i couldn't find what markers it was referring to.

- Theo

A: There are magnetic minerals in most non-sedimentary rocks including, most commonly, magnetite. Each individual iron-containing mineral crystal grain has a magnetic moment - sort of like a tiny bar magnet - frozen in the orientation direction of the Earth's magnetic field at the time that the mineral solidified out of its original melt. This is called remnant magnetization. In an unweathered igneous rock, these tiny magnetic domains (if there are magnetic minerals in the rock) will all align in the same direction as the Earth's field. A fluxgate magnetometer can be used in the field to orient the sensor until it aligns with the rock's internal magnetic field. Geologists can use these to tell the difference between apparently similar volcanic flows in the field, just by the different remnant magnetic fields in the different flows.

In a sedimentary rock these magnetic domains will usually be randomly oriented, because the mineral grains have been jumble up as they were wind blown or washed down a stream. 

The geology sub-field of paleomagnetism is the study of how these orientations can depict the Earth's magnetic field at times in the past - but of course this means that you must also be able to date the rocks. After more than a half century of gathering data, paleomagnetic specialists can say definitively that there have been magnetic epochs when the Earth's magnetic poles were oriented just like it is now - and epochs when the magnetic field orientation has been reversed. Of intense interest in the geoscience world right now is refining the dating part, in order to see how *fast* the Earth's magnetic field orientation changes or flips over when it does. Does the orientation of the magnetic field flip overnight, or does it take 100,000 years? The difference is important.

During that switch-over time, our planet doesn't have the magnetic protection from energetic charged particles from the Sun like it normally does (these charged particles being deflected by the Earth's magnetic field are what creates the auroras). The implication here is that there will be a lot more radiation damage to creatures living on the Earth's surface during a magnetic pole switch-over transition time. The next implication: perhaps this equates with (a) a jump in biological diversity, and/or (b) a dying off of some species. 

~~~~~

 

Porosity in Sandstone and Carbonate Rocks

Q: Dear Sir, I would like to ask what are the classification of porosity in carbonate rocks?
- Tarek M

A: Carbonate rock porosity is a very big issue in the oil and gas world – perhaps THE issue. Water, gas, and oil must be able to move through the rocks or there is no oil and gas business. The nature of porosity in a source or reservoir rock can make all the difference in whether the hydrocarbons can be formed in the first place, and later economically extracted or not. Basically, can the oil mature and be concentrated in the first place – and if so, can it later then be extracted?

Here are some links that might help answer this for you:
http://pubs.usgs.gov/wri/1994/4013/report.pdf
http://www.slb.com/services/technical_challenges/carbonates/near_wellbore/rock_properties.aspx
http://en.wikipedia.org/wiki/Porosity
http://www.searchanddiscovery.com/documents/2004/mazzullo/images/mazzullo

        By comparison, sandstone porosity is pretty simple: it consists primarily of inter-grain spaces.

For carbonates (e.g., limestones and dolomites) however, porosity evolves with time and can be quite complex. Porosity in carbonate rocks is thus usually classified on the basis of the timing of the porosity: how the porosity changes and develops over time. Perhaps surprisingly, porosity often evolves over time. First, there is primary or depositional porosity, in which the pores are inherent in the newly deposited sediments and the particles that make them up. These pore types include inter-particle pores in carbonate sands (muddy or otherwise), and intra-particle pores (i.e., small passageways within tiny fossils such as foraminifera). Another pore type is fenestral: these are pores formed by gas bubbles and sediment shrinkage in tidal-flat carbonates, and also the growth-framework or structural pores common in reef buildups.

        In carbonate rocks (as opposed to sandstone) you can also have secondary pores – in other words pores that form as a result of later, generally post-depositional dissolution of certain constituent minerals. You can also have vugs: large pores that cut across the rock fabric. This means that the dissolution has not been controlled or driven by the fabric of the original deposition. Incidentally, you can also have pores that are basically dissolution-enlarged fractures. The natural tendency in most carbonate sediments is to for groundwater to cement in and overlying rock pressure to compact the pores during post-depositional burial. Most geologists believe that the bulk of the porosity in limestone and dolomite reservoirs is secondary in origin: post depositional, in other words.

Keep in mind in this discussion that carbonates and sandstones are discrete members of what is usually a continuum of rocks types in a stack of sediments. One can encounter sandstones, limestones, anhydrites, shales, and dolomite in relatively close proximity in a sedimentary package.

Hey! A Sedimentary Rock Looks Like an Igneous Rock!

Q: I encountered a sedimentary rock in lab called Greywacke and it reminded me of an igneous rock, Gabbro. How does one differentiate these 2 apart? Or basically igneous from sedimentary and vice versa.

- Feiruz R

A: Igneous rocks were formed when the material was a melt, so the individual mineral grains are tightly fused and intergrown, and the porosity in the rock is very low. A greywacke was formed by accumulation of cold, weathered detrital material. It might superficially LOOK like an igneous rock, but a porosity test would give it away immediately. A closer examination with a hand-lens will show angular grains in a greywacke that do not interlock seamlessly; the word "greywacke" means that this rock also includes very fine silt, so this tends to fill those inter-grain boundaries between the larger crystals - but with a hand-lens you can see this. A petrographic microscope makes it even more obvious that a greywacke is really a garbage can term, representing an accumulation of weathered material of all different grain sizes. 

Oxygen and Mcrobes and Algae and the Early Earth

Q: Hi!

Oxygen in our atmosphere was created by small creatures who had just invented a new process:  photosynthesis.

The waste product of photosynthesis is oxygen.

After an unfathomable number of  years, too much O2 built up in the atmosphere, changing the greenhouse gas, methane, into carbon dioxide, which isn’t such a strong greenhouse gas.  This caused the earth to cool off to the point where the first known ice age began, the huronic, I think.  It lasted for millions of years and enveloped the entire earth with ice.

Question 1: how did these photosynthetic creatures survive in an iced-over earth?

Question 2: what caused the end of this very long ice age?

 Just curious - Thank you!

 - Susan K

A: Your question suggests you are well along in studying this topic. I've stored a number of closely related questions and answers on this blog, and by way of a long answer, some of these may help:

http://askageologist.blogspot.com/2013/07/climate-change-is-it-real.html

http://askageologist.blogspot.com/2013/04/castastrophes-and-mass-extinctions.html

http://askageologist.blogspot.com/2012/06/snowball-earth-faint-young-sun-paradox.html

The medium-length answer: Our atmosphere passed through the oxygenation transition around 2.5 billion years ago, and it certainly involved photosynthesis - stromatolites (fossil algal clumps) have been found dating as far back as 3.3 billion years. However, there are also suggestions that mantle outgassing, tectonics, and oceanic current-shifts may have contributed. There really was a Snowball Earth episode, and there have been a series of cold-warm cycles since then. Scientists have been exploring - with limited data - what could have caused these events, and suggestions range from the fairly mundane to the exotic: asteroid impact, tectonic change that interrupted oceanic current flows, etc. The Chicxulub asteroid event 65 million years ago certainly knocked the oxygen levels in the atmosphere down dramatically, requiring millions of years to recover. This is almost certainly why bar-headed geese can easily fly over Mount Everest: birds have evolved a truly advanced respiratory system since that event. Evolution is well-documented to speed up under environmental stress.

Some short answers:

1. If there is one thing certain about microbial life, it is that it can survive almost anything. Microbes have been found kilometers deep in the Earth, and temperatures steadily increase with depth due to radio-isotope decay in the Mantle and Core of the Earth (the temperature rises to typically 60 C at 4,000 meters depth).

2. There are a lot of variables that may have been involved in the recurring cool-warming cycles, including the fact that the Sun has steadily grown in luminosity during its lifetime, as well as tectonics, and methane-emitting life forms. Likely a combination of these - and probably other factors - led to out-of-control feedback loops that dead-ended in climate extremes before the atmosphere eventually  recovered. 

Damage Earthquakes Cause

Q: What kind of damage can a(n) earthquake do?

- Carrera K

A: Earthquake damage can be very wide-ranging. In increasing order of destructiveness (roughly following magnitudes from M ~4 to M ~ 8) :

1. Small cracks form in drywall, stucco, and concrete walls of buildings.

2. Fragments of building fall into the surrounding streets - glass, or bricks from the corners of windows, etc.

3. Topographic settling, leading to serious internal structural damage in buildings, making them uninhabitable (San Francisco, Loma Prieta earthquake, 1979).

4. Large fractures form in soils and rocks. Water towers fail. Electric power is cut and gas lines rupture (White Wolf Fault, Bakersfield, CA, 1952).

5. Major fires start that are difficult or impossible to control (San Francisco, 1906 AD).

6. Collapse of concrete floors in buildings, crushing most of what lies within (Izmir, Turkey, 1999 and 2013).

7. Major infrastructure collapse, leading to water-borne diseases, and starvation happens on a wide scale (Haiti, 2010).

8. Tsunamis wreak broad damage to coastlines, killing most people within reach of the water (Aceh, Indonesia, 2004).

9. Allocthons and major landslides cover or sweep inhabited regions. Segments of coastline drop below sea level (Puget Sound, 1700 AD), and rivers temporarily reverse direction (Mississippi River, 1811).

10. Entire civilizations collapse and do not recover (the Minoan civilization, 1,500 BC, though how much was explosive volcanism and how much was caused by earthquake damage is unclear).