Type of Earthcache: Igneous (Volcanic) Weathering Feature.
What is Basalt?
Basalt is a common extrusive igneous (volcanic) rock formed on the surface of the Earth. By definition, basalt must be an aphanitic (individual mineral grains cannot be distinguished with the naked eye) igneous rock with less than 20% quartz and less than 10% feldspathoid by volume, and at least 65% of the feldspar is in the form of plagioclase.
Basalt is usually grey to black in colour, but rapidly weathers to brown or rust-red due to oxidation of its mafic (iron-rich) minerals into rust.
It has a fine-grained texture due to rapid cooling of the basaltic lava for large mineral crystals to develop, although it can sometimes be porphyritic. The larger crystals were formed prior to the extrusion of the lava to the surface. Basalt with a vesicular or frothy texture is called scoria, and forms when dissolved gases are forced out of solution and form vesicles as the lava decompresses as it reaches the surface.
As subaerial basaltic lava cools, significant contraction forces build up in the body of the rock mass resulting in extensive fracture network structures which are predominantly hexagonal in cross-section, but polygons with three to twelve sides can be observed similar to those seen in dried-up mud of a marsh or river-flat that has been exposed to the sun (Holmes, 1965 and Wikipedia, 2012a). The structure that is formed during the cooling of subaerial basaltic lava is called columnar jointing (Figure 1). These joints provide ideal site for weathering of the basalt to take place.
Figure 1. Formation of columnar jointing (Railsback, 2012).
Weathering of Basalt
The separation of shells of a weathered rock is called spheroidal weathering. It is best developed in well-jointed rocks which, like many basalts and dolerites, are readily decomposed. Water penetrates the intersecting joints and thus attacks each separate block from all sides at once. As the depth of weathering is greater at corners and edges than along flat surfaces, it follows that the surfaces of rupture become rounded in such positions. As each shell breaks loose, a new surface is presented to the weathering solutions, and the process is repeated again and again (Holmes, 1965).
Each successive wrapping surrounding the core becomes more nearly spheroidal than its outer neighbor, until the angular block is transformed into an onion-like structure of concentric shells of rusty and rotted residual material (Figure 2). Cores of fresher and more coherent rock may eventually stand out like boulders when their soft outer wrappings have been washed away by rain erosion (Figure 3).
By no means all basaltic rocks leave rusty shells of residual materials behind when exposed to weathering. Given the right conditions everything in the rock may be dissolved and washed away. Complete solution of basaltic minerals requires water of unusual acidity (with pH between 2.8 and 3.7), such as is available from the percolation through peat-moss or bogs, and from the drippings from dense vegetation. The waters responsible for spheroidal weathering have pH values ranging between 4 and 8.
Figure 2. Spheroidal weathering.
(Source: GC3KP4W)
Figure 3. Loose core stones (diameter = 64–256 mm) and core boulders (diameter >256 mm).
(Source: GC3KP4W)
Kuantan Basalt
The Kuantan Basalt covers a total of about 50 km2 where Pantai Batu Hitam (Black Rock Beach) is the easternmost coastal outcrop (Azman, 2009). Three distinct basaltic groups are recognised: alkali olivine basalt, basinite (limburgite) and olivine nephelinite (Chakraborty, 1980). The alkali olivine basalt, which is holocrystalline and consists of olivine, augite, and plagioclase, is the most abundant. Haile et al. (1983) dated the Kuantan Basalt using the K-Ar dating method as 1.5 to 2.5 Ma (Early Pleistocene).
The Kuantan Basalt at Black Rock Beach is exposed along the beach. It can only be studied at low tide. The study of the state of weathering of the Kuantan Basalt at Black Rock Beach is the exercise of this EarthCache.
How to claim this EarthCache?
At the listed coordinates observe the basalt fields that are exposed along the beach. Note the state of weathering of the basalt in term of the presence of columnar jointing, spheroidal weathering, and loose core stones/boulders. Note also the type of marine organisms that inhabit the surfaces of the basalt.
Send me the following;
1. The text "GC7CJG2 Pantai Batu Hitam - Basalt" on the first line.
2. The answers to the following questions;
- Note on the state of weathering of the Kuantan Basalt at Black Rock Beach.
(i) Estimate how many percent of the basalt field exhibits columnar jointing.
(ii) Estimate how many percent of the basalt field is covered by spheroidal weathering.
(iii) Estimate how many percent of the basalt field is covered with loose core stones/boulders.
- List out the sequence of weathering pattern from fresh basalt to loose core stones/boulders with the estimated percentage that you’ve estimated.
- Eventually, what will be the ultimate scenario of weathering of the basalt field at Black Rock Beach?
- Optional questions: what marine organisms inhabit the surfaces of a number of the basalt?
3. Optional task: Share your experiences and photos taken during your visit.*
Glossary
1. Augite is a common rock forming single chain inosilicate mineral with formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees (Wikipedia, 2012b).
2. Dolerite (Greek: doleros, meaning "deceptive") or diabase is a mafic, holocrystalline, hypabyssal rock equivalent to volcanic basalt or plutonic gabbro in composition. It displays distinctive bluish-black to dark grey and normally has a fine-grained texture (Wikipedia, 2012c).
3. Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust (Wikipedia, 2012d).
4. The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks (Wikipedia, 2012e).
5. Holocrystalline is a term use to describe an igneous rock having only crystalline components and no glass.
6. K–Ar dating or potassium–argon dating is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium (K) into argon (Ar) (Wikipedia, 2012f).
7. Ma is an abbreviation for megaannum which mean million years.
8. The mineral olivine (when of gem-quality, it is also called peridot and chrysolite), is a magnesium iron silicate with the formula (Mg,Fe)2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface (Wikipedia, 2012g).
9. Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series (Wikipedia, 2012h).
10. Porphyritic is an adjective used in geology, specifically for igneous rocks, for a rock that has a distinct difference in the size of the crystals, with at least one group of crystals obviously larger than another group (Wikipedia, 2012i).
11. Subaerial refers to occurring on the surface of the Earth.
12. Texture in geology refers to the physical appearance or character of a rock, such as grain size, shape, arrangement, and pattern at both the megascopic or microscopic surface feature level (Wikipedia, 2012j).
References
1 Azman, A.G., 2009. Volcanism. In: Hutchison, C.S. and Tan, D.N.K. (eds.), 2009. University of Malaya and Geological Society of Malaysia, Kuala Lumpur, 197-210. 2 Chakraborty, K.R., 1980. On the evolution of the nephelinite to hypersthenes normative alkali basaltic rock Kuantan, Pahang, Peninsular Malaysia. Geological Society of Malaysia Bulletin, 12, 79-86. 3 Haile, N.S., Beckinsale, R.D., Chakraborty, K.R., Abdul Hanif Hussein, and Hardjono, T., 1983. Palaeomagnetism, geochronology and petrology of the dolerite dykes and basaltic lavas from Kuantan, West Malaysia. Geological Society of Malaysia Bulleting, 16, 71-85. 4 Holmes, A., 1965. Principles of Physical Geology. Thomas Nelson and Sons Ltd., London, 397-400. 5 Railsback, B., 2012. An explanation of columnar jointing [online]. Available from http://www.gly.uga.edu/railsback/1121Lxr07.html [accessed on 26 Aug 2012]. 6 Wikipedia, 2012a. Basalt [online]. Available from http://en.wikipedia.org/wiki/Basalt [accessed on 24 Aug 2012]. 7 Wikipedia, 2012b. Augite [online]. Available from http://en.wikipedia.org/wiki/Augite [accessed on 26 Aug 2012]. 8 Wikipedia, 2012c. Diabase [online]. Available from http://en.wikipedia.org/wiki/Diabase [accessed on 25 Aug 2012]. 9 Wikipedia, 2012d. Feldspar [online]. Available from http://en.wikipedia.org/wiki/Feldspar [accessed on 26 Aug 2012]. 10 Wikipedia, 2012e. Feldspathoid [online]. Available from http://en.wikipedia.org/wiki/Feldspathoid [accessed on 26 Aug 2012]. 11 Wikipedia, 2012f. K-Ar dating [online]. Available from http://en.wikipedia.org/wiki/K-Ar_dating [accessed on 24 Aug 2012]. 12 Wikipedia, 2012g. Olivine [online]. Available from http://en.wikipedia.org/wiki/Olivine [accessed on 26 Aug 2012]. 13 Wikipedia, 2012h. Plagioclase [online]. Available from http://en.wikipedia.org/wiki/Plagioclase [accessed on 25 Aug 2012]. 14 Wikipedia, 2012i. Porphyritic [online]. Available from http://en.wikipedia.org/wiki/Porphyritic [accessed on 26 Aug 2012]. 15 Wikipedia, 2012j. Texture (geology) [online]. Available from http://en.wikipedia.org/wiki/Texture_(geology) [accessed on 20 Aug 2012].
* Effective immediately from 10 June 2019, photo requirements are permitted on EarthCaches. This task is not optional, it is an addition to existing logging tasks! Logs that do not meet all requirements posed will no longer be accepted.
For additional information, visit; Geosociety.org, Geocaching.com Help Center and Geocaching.com Forum.
Finding the answers to an EarthCache can often be challenging, and many people tend to shy away from these caches because of this. However, it is my opinion that geocaching is also meant to be a fun family experience that simply aims to introduce interesting and unique locations such as this one. Flexibility on logging requirements, however, can only be applied if it can be established that you have actually taken the time to visit the site. For this reason, a proper log describing your adventure accompanied by a good number of photos would be much appreciated.
Join us at Geocaching Malaysia