How to Write a Rock Description for Igneous, Sedimentary and Metamorphic Rocks (Geology)

Many of us, through our time at sixth form or university, have to go through the tedious process of writing a rock description. It is, let's be honest, not the most exciting thing to do yet not something you can often escape. Might as well get it over and done with!

I have split igneous, sedimentary and metamorphic rock descriptions into separate posts to make it simpler to read. I will add links to the others as I write them:

Igneous: [this post]

Sedimentary:

Metamorphic:

At the bottom there is some more information on types of properties like crystal habit, hardness and lustre.

If requested enough, I will add each mineral's properties to the blog post too!

IGNEOUS

The general process for writing rock descriptions is to start off with a general description (general colour, grain size, texture..) , followed by identification of the minerals within the rock (the mineral assemblage), the name of the rock (deduced from the assemblage) and then finally your best guess on how and where it was formed.

(Source - http://movingtoward.com/images/igneous_canterbury.ac.nz_sm.jpg)

All this is based on igneous rocks where the mineral grains are visible. If none are (it is way too fine grained) then it is a volcanic glass, formed in a very violent eruption resulting in fast cooling magma that didn't have time to crystallise. If it is a compact glass it is an obsidian and if it is a 'frothy' glass then it is a pumice. Obsidian is much denser than pumice.

There are two different types of rock description, which either may be done together or separately. The first is a hand specimen rock description which is where you only have a rock, a hand lens and your eyes with which to try to deduce the type of rock. The second is a thin section rock description where you have a slice of the rock which is on a glass slide making it possible to view the rock under a microscope.

Hand specimen descriptions focus on physical properties of the rocks whereas the thin section descriptions looks at optical properties instead.

Then, once you have completed the description (either or both of the types of description) you use the information you have to make a decision on what type of rock it is and how it was formed.

Hand Specimen

The first thing to do is to simply look at the rock and describe what you see. In specimen, you are looking at physical properties of the rocks.

(Source: http://minerva.union.edu/hollochk/c_physicalgeology/images/Granite.jpeg)

This is a good checklist to follow:

The Japan Earthquake - Videos of the Cracks Moving! Liquefaction in action!

Hey guys, sorry about not posting any posts up in the last few weeks! By the end of the month there should be a few new posts on the blog.

To appease your appetite for geography, I'm putting up this post to highlight two videos that I find incredible. They show small fissures (cracks) in the road surfaces and parks in Japan during the earthquake widening and narrowing as the earthquake shook the area. It is reportedly the first ever time that this has been caught on video.

The earthquake also caused liquefaction on a wide scale (seen in the videos below). This caused the majority of the earthquake's damage. The image below, although not of this earthquake, does show the damage that liquefaction regularly does.

An example of buildings sinking into the ground due to Liquefaction:

http://2.bp.blogspot.com/-SbvJB...collapsed%2Bbuilding.jpg

Here is some more information on the Japan earthquake (11th March 2011):

• This earthquake released a surface energy of 1.9±0.5 ×10¹⁷ joules. This is the energy that was released to cause the shaking of the ground and lift the sea floor to cause the tsunami. That is nearly double that of the Boxing Day earthquake and tsunami in 2004 (230,000 dead).

• The earthquake lasted a STAGGERING 6 minutes. Which gave time for people to run and grab their camcorders to record the fissure movement!

• As of 06/06/2011, there were 15365 deaths recorded, 5363 injured and 8206 missing. Those still missing are likely to be dead - bringing the death toll to 23571.

• The total cost is expected to pass $300billion, making it the most expensive natural disaster ever recorded.

• The Fukushima Nuclear Plant that is STILL in danger, with radioactive liquid leaking into the environment, nearly 3 months on.

• Highest wave heights reached more than 9.3 metres. That is taller than a 3 storey building.

Here are the two videos:

For more information on this tragic earthquake and tsunami, labelled as Japan's worst disaster since the second world war, I have another post detailing the incident.

Here is the original, very well received blog post on the earthquake:
http://thegeomessenger.blogspot.com/2011/03/case-study-japan-earthquake-and-tsunami.html

The Geomessenger

Amazing Cloud Formation Facts and Pictures

As the famous phrase goes "a picture is worth a thousand words", meaning a gallery must be invaluable. Here are some phenomenal pictures I came across, while the information is from an article in 'The Telegraph' which  is linked to in the 'further reading'.

http://listsalad.com/wp-content/uploads/2011/04/SALADcloud2.jpg

Naming clouds

Clouds are classified according to their height and appearance. The 10 basic categories were first agreed by the Cloud Committee of the International Meteorological Conference in 1896 and published as the International Cloud Atlas. Their classifications were based on the pioneering work of Luke Howard (1772-1864), an English Quaker and pharmacist, who published his Essay on the Modification of Clouds in 1802. In it he gives Latin names to the four main cloud types: cirrus, "curl"; stratus, "layer"; cumulus, "heap"; and nimbus, "rain cloud". The early theorist of evolution, Jean-Baptiste Lamarck (1744-1829) had suggested an earlier system in French but it didn't catch on – his names included "hazy clouds" (en forme de voile), "massed clouds" (attroupes), "broom-like clouds" (en balayeurs). Before Howard and Lamarck, clouds were simply named after their appearance: white, black, mare's tail or mackerel.

http://cloudappreciationsociety.org/gallery/index.php?showimage=2040

Case Study: Japan Earthquake and Tsunami (11th March 2011)

The Japan earthquake occurred on the 11th March 2011. It was the largest earthquake that they have had since records began. It was originally measured as a 8.9 magnitude earthquake but this was later increased to a magnitude 9.0 as more detailed readings came in from seismographs and other equipment. This is an enormous earthquake and it is estimated that it released 10,000 times more energy than the magnitude 6.3 one in Christchurch, New Zealand, 17 days earlier - however energy released is different to magnitude. At least 124 aftershocks were bigger than a magnitude 5.

Earthquakes are measured using the Richter scale, which is a base-10 logarithmic scale. This means that a magnitude 9.0 earthquake is...

• Twice as big as a magnitude 8.9
• 10 times bigger than a magnitude 8.0
• 1,000 times bigger than a magnitude 6.0
• 1,000,000 times bigger than a magnitude 3.0

Similarly, if an earthquake is .3 bigger than another then that means that it was 3 times bigger.

Click here for videos of the cracks actually moving! Pretty incredible.
^That is reportedly the first time EVER that it has been caught on camera! (Opens in new window)

The Japan earthquake was absolutely devastating. The earthquake itself, although much bigger than usually seen in Japan, was prepared for extensively as they receive many earthquakes every year. What was not adequately prepared for was a tsunami of such scale. Many of the tsunami walls protecting coastline villages were around 7 metres high, no where near big enough to stop the 10 metre high wall of water (the height of a three storey building) that hit the coasts around 30 minutes after the earthquake. This caused many thousands of Japan earthquake victims.

The Tsunami approaching Japan:

(Source-http://static.onemansblog.com/wp-content/uploads/2011/03/Picture-of-Tsunami-Approaching-Japan-2011.jpg)

They are well prepared within 10 minutes after the earthquake it had been calculated that a tsunami had been formed and warnings were sent out using the Japan earthquake warning system. Text messages were sent to phones, alerts appeared on all TV channels, sirens went off and police alerted residents to the danger. However, people had became desensitised by so many false alarms and assumed tsunami walls could handle it, which meant that many did not evacuate.

The facts barely depict the chaos so I have interspersed them with images to show the tragedy for such a prosperous country that had been proud of their preparedness for such a disaster:

• There have been over 11,000 Japan earthquake victims so far (as of 30/03/11), with over 17,000 still missing - many of which will also be dead. The predicted death toll is 18,000 however this is likely to increase. 

(Source-http://spot2day.com/wp-content/uploads/2011/03/Japan-searching-dead-bodies-fire-fighters.jpg)

What are Geysers?

Geothermal springs are more commonly known as hot springs. Although there is no set definition, hot springs are often defined as springs where the average water temperature is above body temperature (roughly 37C).

Temperature within the Earth increases with depth, this is caused by processes like nuclear fission and the flexing of the Earth's crust producing friction (and heat) when force is applied by the convection currents of magma in the mantle. The centre of the Earth is estimated to be around 5360C and all this heat has to go somewhere!

Water Circulation in a Geothermal Reservoir:

(Source-http://blogs.kqed.org/climatewatch/files/2010/08/Geothermal_diagram.jpg)

The heat travels outwards and can heat stores of water that are deep underground. Four main types of geothermal feature exist: 

• Geysers
• Hot Springs
• Mudpots
• Fumaroles

A geyser is a type of spring that releases water that has been heated by the Earth's thermal energy. This water builds pressure under the surface in chambers which are informally known as "geyser reservoirs" and then, once the pressure exceeds that of the rock plug sealing the chamber, the superheated column of steam and water bursts out. These spouts often reach high into the air.

Castle Geyser Erupting - Yellowstone National Park:

Geysers are generally aligned along faults; however over half of the worlds 1000 geysers exist in Yellowstone National Park which is a hotspot and, as mentioned in a previous post, a supervolcano. [Link] The plumbing system along faults is made up of a intricate matrix of fractures, fissures, porous rock and sometimes chambers. For a geyser to form and not a hot pool, there must be constrictions in the system so that pressure can build up to create the periodic eruptions.

(Source-http://www.flickr.com/photos/bigskykatie/2424233856/in/set-72157604528569009)

Seeing The Northern Lights - The Aurora Borealis

Aurorae is a collective term for the Aurora Borealis and the Aurora Australis, or as they or more commonly known, the northern lights and the southern lights. They are natural light shows high in the atmosphere which are absolutely stunning to watch as they dance in slow motion across the night sky. They can vary in colour greatly, spanning the whole spectrum from red right through to blue and purple with yellow and green aurorae being the most common. They are beautiful and many people go on northern lights holidays to go see the northern lights for themselves.

The Aurora Borealis:

(Source-http://www.freewebs.com/nlbt/Northern%20Lights%20best.jpg)

It is commonly visible between latitudes of 65 and 72 north and south - which is just within the Arctic and Antarctic circles - however it is possible to see anywhere in the world at certain times. For example, on the 29th of October 2003 the Aurora Borealis was visible in the United Kingdom in the largest geomagnetic storm of the previous 20 years. Auroras have also been found on other planets and even some moons.

Aurorae on Jupiter:

(Source-http://hubblesite.org/newscenter/archive/releases/1998/04/)
However this photo is owned by NASA (in public domain)

Rivers of Ice that Plough through Mountains

Glaciers are huge masses of ice which moves over land like a very slow river. They are formed where snow accumulates over many years and through it's own weight is compressed into ice. Glaciers are so special because they aren't just packs of ice sitting at the poles, they move as a river does, flowing down towards sea level, carving valleys as they go. These valleys are stunning, and are usually much larger than river valleys.

Glacial Valley:

(Source-http://www.panoramio.com/photo/2627700)

Glacial valleys, also called glacial troughs, have flatter valley bottoms and are relatively straight compared to river valleys. These valleys have impressive, steep slopes on either side gouged out by the huge erosional forces involved. They are commonly called 'u-shaped valleys'. River valleys are instead called 'v-shaped valleys'.

Despite what you might think, glaciers are found in mountain ranges of every continent on the planet, even Africa (e.g. the Futtwangler Glacier in Tanzania) and Australasia (e.g. the Franz Josef Glacier in New Zealand). They vary greatly in size and some reach over a hundred kilometres long. 

The worlds largest glacier is the Lambert Glacier in East Antartica:

• 400 kilometres long
• 100 kilometres wide
• 2500 metres deep
• At it's fastest point it reaches a speed of 1200 metres per year.

The Supervolcano: The Most Destructive Force on Earth

Supervolcanoes are different from normal volcanoes. They aren't recognisable by a conical shape that you would imagine a volcano to be. These volcanoes form calderas which are huge craters in the ground after previous eruptions, which are so large that they are often only noticeable from the air above.


A supervolcano is the most powerful known destructive force on the planet. Only asteroids or other astronomical events are potentially powerful enough to exceed their magnitude. They have became famous due to Yellowstone National Park Volcano in the USA, so much so that there has even been a whole supervolcano dvd covering Yellowstone Park information.

Yellowstone Supervolcano Caldera: 

(Source-http://earth.imagico.de/view.php?site=yellowstone1)

Eruptions are much more powerful than a normal volcanic eruption as the magma concentrates and builds pressure just beneath the surface (as little as 5km) in massive magma chambers where the dimensions are measured in kilometres - which gives an impression of how big these things are! Normal volcanoes form where magma forces its way up through cracks in the crust along destructive plate boundaries (high energy eruptions) and along constructive boundaries (low energy eruptions), and erupt frequently in geological terms. Supervolcanoes form where this magma is stopped from reaching the surface; it melts the surrounding rock and the chamber grows in size and pressure.

"A supervolcano - a volcano capable of producing a volcanic eruption with ejecta greater than 1000 cubic kilometres. This is thousands of times larger than most historic eruptions."

Once this pressure exceeds what the thin crust can take, it breaks through in with incredible force chucking out insane volumes of ash and magma into the upper atmosphere. This causes a volcanic winter as the ash creates a screen around the Earth bouncing off sunlight, turning days to night and summers to winter. Ash covers the ground killing plants and in turn, the animals through starvation and suffocation. After the eruption, the ground collapses at the site of the eruption into the de-pressurised magma chamber and forms the huge caldera.

Introduction

Well I'm new to the whole blogging thing... but I've wanted to do this for a while and I've finally got round to it!

I'm fascinated by our planet and how us humans affect it, extraordinary landforms that have been created, the weird and wonderful and even more. I hope to bring you articles, quotes, and my own views on all these topics and any others that feel relevant.

I have set up commenting to allow everyone to comment on what I do. Please, if you enjoy it or are interested, just leave a short note saying so and feel free to suggest improvements or possible topics I could research and cover on here, maybe even become a follower of the blog! It'll be appreciated.

Here is a documentary about our planet that really gives a good view as to why I am so interested, and i would like to think that, although it's a bit slow paced, it would inspire many a person to take more of an interest into our planet:

http://www.youtube.com/watch?v=jqxENMKaeCU

Here is the first quote to have a think about:

"Now, toying with our atmosphere, we break the rhythms of the sea, nudging the climate, ignorant of whether we can adapt to the new niche we are creating. It may be an arrogant gamble"

(Great Waters, an Atlantic Passage by Deborah Cramer - Pg 133)

Hope to have some more posts coming soon, and with that some followers!

Please leave a short comment, thanks.

The GeoMessenger