Tuesday, January 26, 2010

Pale Blue Dot

No one could put it more heartfelt and passionate than Carl Sagan could. Listen to Sagan's narration from his book the Pale Blue Dot.

Wednesday, January 20, 2010

Ancient carbonate deposit and stromatolites

In 2008 a group of us visited Pering mine near Kuruman. Besides the very interesting minerals we collected the geology was fascinating.

The Pering deposit is a carbonate (limestone/dolomite) hosted zinc-lead mineralized breccias within the Campbell Rand member of the Ghaap Group, Transvaal Super Group (classified as the oldest Mississippi Valley Type (MVT) economic deposit in the world. It has been dated at approximately 2,550my old (Paleo-proterozoic in age), while all the other MVT deposits in the world are younger, ranging from middle Proterozoic (1,200my) to Jurassic (175 my) in age. For this reason Pering is unique and deserves more attention from a mineral collector’s point of view.

One could get a glimpse into the fascinating development of this unusual deposit as we drove down the ramp road into the large pit. The upper layers of dolomite still have well preserved stromatolite domes some as large as 5 metres across, with intercalated thin shale layers within the thick units of dolomite. These lower dolomites are characterised by smaller domed deformed stromatolitic lenses in the lower section of the exposures in the opencast pit. Stromatolites were simple algae-like plants requiring a warm shallow marine sea to thrive. These fossils represent the oldest living organisms on the planet, peaking about 1250 million years ago. These cyanobacteria are thought to be responsible for increasing the amount of oxygen in the primeval earth’s atmosphere through their photosynthetic action of taking in carbon dioxide and dispelling oxygen as waste (Allwood, et al, 2006).

Evidence of several fluctuations in sea-level resulted in the formation of limestone and dolomite deposits in deeper waters, with thin shale layers forming during shallow lagoonal/esturine conditions. Fossil ripple marks along bedding planes in the thin shale layers were evident at many of the collecting sites, indicating shallow gentle tidal flow when these sediments were deposited in the inter-tidal zone. These abrupt changes in sedimentary units were clearly visible on the mining benches as we explored the cliff-like rock walls of the open pit for pockets where we might be lucky to find mineral specimens.

Thursday, January 7, 2010

Magnificent Desolation

Buzz Aldrin uttered "Magnificent Desolation" on seeing the surface of the Moon for the first time. There are many parts of our planet that resonate with me and I can describe as "Magnificent Desolation". I have added a few pictures here on some of the places that I have seen that would earn the title of "Magnificent Desolation".
Pictures from the top down: Peruvian Andes, Peruvian Andes with limestone cliffs, Rosh Pinah region in southern Namibia. More pictures to come.

Ancient Earth - Evidence in the Greenstones

I have for a long time been fascinated and intruiged with the formation of old rocks(especially Archean rocks and continental crust). I have read everything one can possible read on the subject and for a non-geologist like me I have still have many questions. I live very near to a number of greenstone remnants here in Johannesburg and only a 5 hour drive from Barberton where major outcrops occur.

From what I can gather; The oldest piece of continental crust dated so far is the so-called Acanasta-Gneiss from the Slave Province in NW Canada (4.06 Byr. old).

Nevertheless, it is now assumed that fragments of older solid parts of the earth consisting of the crust and outer mantle and liquid water existed long before the Acanasta-Gneiss. This is evidenced in the oxygen isotope 0^18 record in zircons (small zirconium oxide crystals that resist weathering very well). According to recent research, Earth between 4.4 and 4.0 was not a magma-red glowing hostile planet but a place covered by tranquil oceans with small islands protruding from these waters. (I'll discuss the oxygen 18 isotope and zircons in a later blog).

Mean temperatures were – from a geologic point of view – cool, i.e. in average about 200oC or somewhat less. Beneath this value, a portion of the water, previously only existing as vapour, condensates under the high pressure conditions and formed oceans. The scarcity of previous continental fragments is probably the result of the so-called late heavy bombardment at about 3.9 Ga., one of the main meteoric bombardments of the young Earth.

Areas underlain by Archean rocks are typified by two main types of rock bodies: ‘greenstone belts’ and ‘granite-gneiss complexes’. Other cherts and iron-rich sediments, known as banded iron formations, are also found in the Archean sedimentary belts. For example, Zimbabwe consists of mostly of gneiss and various granitic rocks; the remaining rocks are largely greenstone belts.

The oldest large, well preserved greenstone belts are those of South Africa, which date from 3.6 billion yrs. An idealized greenstone belt consists of three major rock units: the lower and middle units are dominated by volcanic rocks and the upper unit is sedimentary. The volcanic rocks of greenstone belts are typically greenish (see above pictures of greenstone rocks from Johannesburg area) due to their low grade metamorphism (chlorite minerals). The occurrence of pillow basalts in the Barberton region indicates that much of the vulcanism responsible for the igneous rocks of the greenstone belts was subaqueous; shallow water and subaerial eruptions are indicated by pyroclastics.

Sedimentary rocks are a minor component in the lower parts of greenstone belts but become increasingly abundant towards the top. The most common ones are successions of graywacke (sandstone containing clay and rock fragments) and argillite (slightly metamorphosed mudrocks). Small-scale graded bedding and cross bedding indicate that the graywacke-argillite successions are deposits of ancient turbidity currents. Others were deposited in deltas, tidal-flats, barrier islands and shallow marine shelf environments.

In summary, detrital Archean rocks seem to indicate the presence of basins of moderate depth flanked by volcanoes that spewed out lava.

Tuesday, January 5, 2010

Rock of Ages

We were at dinner on the 31st December with a group of friends overlooking the horizon with the full moon rising. It is a "Blue Moon" my wife said. That got us talking about the moon and we got onto the subject of the lunar rocks brought back by the Apollo Missions and that lead the coversation to how to how old the moon was.

Well, the abundances of radioactive elements in rock samples can be used to tell the age of the rock in a process called Radioactive Dating. The lunar material was analysed and samples from Mare Imbrium and the Ocean of Storms brought back by Apollo 11 and Apollo 12 are about 3.5 billion years old, which is comparable to the oldest rocks found on the surface of the Earth.

What is interesting is that the ejecta blanket from the Imbrium Basin (which was formed by a gigantic meteor impact) was returned by Apollo 14 and found to be about 3.9 billion years old.

However, Lunar Highlands rocks returned by Apollo 16 are about 4 billion years old. The oldest lunar rock found was located by Apollo 17 and appears to be about 4.5 billion years old. So, the oldest material from the surface of the Moon is almost as old as we believe the Solar System to be. This is more than a billion years older than the oldest Earth rocks that have been found. Thus, the material brought back
from the Moon by the Apollo missions gives us a window on the very early history of our Solar System that would be difficult the find on the Earth, which is geologically active and has consequently, obliterated its early geological history.

The amount of cratering is usually an indication of the age of a geological feature. But more of this in a future blog.


Minerals, Rocks and Rationality

I am a collector of minerals and rocks. A recent acquisition of mine is a number of specimens of Albite with Quartz and Sphalerite from Rosh Pinah mine in the south of Namibia. On first seeing these specimens, I assumed that the white crystals were calcite, but looking a bit closer they seemed to be something different. I removed a small section of a crystal from the back of one of the specimens and had it identified by XRD for crystal structure and SEM for it's chemistry. Both techniques confirmed that the mineral is albite. This came as a surprise as the albite is typically found in volcanic and metamorphic rocks and not (or rarely) in sedimentary rocks as is the case at Rosh Pinah. I did some research on this and consulted several geologists and the consensus is that the albite crystals could have been formed in a late diagenetic stage at low hydrothermal temperature and be related to hydrothermal post-ore fluids. So, this was as far as I know the first find of well crystalised albite from this mine and also perhaps the first time it has been positively identified by proven scientific means from this locality.


In the Valley of Iridium

pics: The K-T site at Gubbio showing the Cretaceous (lower grey limestone), the thin 1 cm layer of iridium rich clay in the middle and the darker brown clay of the Tertiary period. The Valley of Iridium - the Bottaccione Gorge near Gubbio.

5 Jan 2010

It’s 2010 and another circuit around the Sun begins. It’s traditional to take this time to look back, and to look ahead. Looking back at 2009, one of the highlights for me was to visit the K-T boundary at Gubbio. Looking forward I'd like to learn as much as possible about this geological event.

I had the privilege last year to visit the world-famous K-T boundary site in Italy. It has been an ambition of mine to visit this site since reading about it in Time magazine in the early 1980’s. I was so intrigued by the site and the history of its discovery that I spent a part of my Italian holiday preparing a write-up on it. Back home in my study, in a moment of utter madness I decided to try and calculate the diameter of the meteor that had caused the K-T extinction and the energy released from the impact. I struggled through the calculations, and after a week of frustration and ludicrous answers, I finally succeeded in the math. Turns out that my knowledge of the metric system was pretty poor! The numbers show that the effects of an impact of this size were of global proportion as it produced enormous amounts of energy, molten rock and dust and caused almost incomprehensible destruction.

Here are the numbers:

Why would a meteor impact, even one as large as the Chicxulub feature, be enough to cause the extinction of the dinosaurs and a myriad of other species? We could perhaps get a better understanding of this by attempting to answer questions such as; how much material was ejected at the time of impact, how much iridium was distributed worldwide, how much did the meteor weigh and what was its diameter and how much energy was released during the impact? In an attempt to quantify these parameters I did a number of calculations using the estimated impact velocity, the average iridium content and mean thickness of the KT layer. The answers I got even with making several assumptions revealed staggering numbers. I give detail of the calculations and answers below:

(note: this crazy html text does not allow the superscript function, so I have added the up arrow ^ to denote that the number is a superscript i.e. 1 x 10^2 would be 1 times ten to the power of 2).

How much material was ejected from the site of impact?
The Iridium layer is globally present on average as a 1 cm thick layer. The surface area of the Earth is 1.3 x 10^8 km^2. Since, area x thickness = volume, then 1.3 x 10^8 km^2 x 1 x 10^-5 Km gives us the volume ejected in Km^3:

The volume of material ejected by the impact is 5.1 x 10^12 Km^3

How does this compare with other known events?

If we compare the amount of material ejected by some of the largest volcanic events in recent geological time with that of the K-T meteor impact, we see that the K-T event produced 2 million times more ejecta than the supervolcano at Yellowstone.

Krakatoa (1883) produced 21 km^3 of material
Yellowstone “Supervolcano” (100 000 years ago): 1000 km^3 of material
Yellowstone “Supervolcano”(2 million years ago): produced 2500 km^3 of material.
K-T Chicxulub impact produced: 5.10000000000 km^3

How much Iridium did the meteor contain?
Using the average density of crustal rock which is 3000 kg/m^3 and the total K-T clay mass of 1.53 x 10^16 kg dispersed around the earth and the mean iridium concentration of 0.3 parts per billion in KT layer we can calculate the metric tons of iridium in the meteor as:

4.59 x 10^6 Kg (4590 metric tons)

How much did meteor weigh?
From the average iridium abundance in chondritic meteorites, the mass of the meteor is calculated at:

4.83 x 10^14 kg (4.83 x 10^11 metric tons)

What was the diameter of meteor?

From the density of C1 Carbonaceous Chondrites (2110 Kg/m^3) the volume of the meteor is calculated at 2.30 x 10^11 m^3 (or 2.3 x 10^2 Km^3)

Assuming that the meteor was spherical, we can calculate its diameter:

Diameter = 2(3^√2.3 x 10^2 x 0.75/3.14)

Diameter = 7.6 Km

Alvarez et al used four independent methods to calculate the diameter of the impactor. The mean value they obtained was 10 Km with a standard deviation of ± 4 Km. This puts my calculation of the diameter of the meteor within the standard deviation obtained by Alvarez et al. As a size comparison Mount Everest is 8.84 Km in height.

How much energy was released on impact?

A 7.6 km diameter meteor travelling at 25 Km/second has a kinetic energy of:

Ke = ½ mv^2 = 3 x 10^23 Joules
(where Ke is Kinetic Energy, m = mass of meteor and v is the velocity of the meteor)

Energy released on impact = 3 x 10^23 Joules

Comparing the amount of energy released by the Chicxulub impactor with the Hiroshima A-bomb which produced 8.4 x 1013 Joules of energy, we can calculate that the K-T meteor impact was equivalent of 3.5 billion Hiroshima atomic bombs! That is, 0.5 atom bombs for every person living today. A megaton of TNT is 4.184 × 1015 joules, therefore the Chicxulub impactor would have produced energy equivalent to 71 million megaton of TNT.

The discovery of the K-T iridium anomaly at Gubbio is considered to be one of the most important discoveries in evolutionary science. It was also central to the recognition by scientists that occasional catastrophic events like great impacts require a rejection of strict uniformitarianism in geology.

While the impact hypothesis is now one of the strongest and most widely-accepted theories about the extinction of the dinosaurs, other ideas remain valid, and it is highly unlikely the question will be definitively answered in the near future. Vulcanism could be a source of some of the iridium, but to have provided enough of this element to give the concentrations found uniformly worldwide in boundary clays, would have involved a degree of volcanic activity far beyond human comprehension. Even the basalts of the Deccan Traps in India, proposed by some to have been a source of such iridium, contain only 0.005 ppb of this element.

What is important to note, however, is that the death of the dinosaurs was the direct cause for the rise of mammals. Dinosaurs had their chance, and lost out to a group of small, furry animals that would evolve into the forms that now have dominion over the Earth. But that doesn't mean we're exempt from the same dangers. From research of craters on the Moon and Mercury, it is estimated that the time to collision is proportional to the square of the diameter of the object . Therefore, meteors of 10 kilometers diameter will collide with Earth on average once every 100 million years, meteors of 1 kilometer in diameter every 1 million years and 100 meter diameter meteor every 10 000 years.

So pause for a moment and look up—you never know what's coming.