Mapping Of Volcanic Series Rock Units Using Landsat Thematic Mapper Imagery,
Troodos Ophiolite Complex, Cyprus
Lower Pillow Lavas (LPL)
The lower pillow lavas are characterized by the presence of both extrusive and intrusive rocks. Extrusive rocks mainly consist of pillow lavas, non-pillowed lava flows and hyaloclastites whereas intrusive rocks which may constitute upto 50% of the outcrop, include dikes, sills and irregular intrusive masses. This lower lava unit composes of aphyric rocks with rare microphenocrysts of plagioclase, clinopyroxene, orthopyroxene and iron-titanium oxides. The rocks of this divesion consist of andesine, andesitic dacites, dacites and rhyodacites.
Similar to UPL, the rocks belonging to LPL are much decomposed at th esurface. They exhibit a greenish gary tint and the soils are more mature. However, fresh to slightly weathered outcrops are exposed in soma parts of the major river sections (e.g.: - Akalki river section). Due to the differences in composition structure and degree of chilling, an uneven but not rugged relief is observed in the LPL rocks. Numerous narrow ridges and deep gulleys, corresponding to the effects of erosion on the harder intrusives and softer pillow lava, characterize it
Individual pillow vary in size from few centimeters to nearly 40 cm across by approximately 50 cm to 3 meter in diameter. However, the diameter of many pillow fall between 1 and 2 meter range with the average width of 0.5 m. as in UPL, their shape is varied from spheroidal to oval. Most of the pillow are strongly vesicular, some vesicles
are empty whereas the others are generally infilled with zeolites, calcites, quartz and green chlorite.
The most characteristic mineral identified in the lower lava series is the green colored celadonite. Celadonite occurs as stains and open space filling in lavas and in some of the intrusions. The LL unit transitionally overlies the basal group, which consists of over 60% dikes with screens of pillow lava and underlies the UPL in which intrusive are almost absent.
Basal Group (BG)
The transition zone from the extrusive series to the sheeted dike complex is known as the Basal Group. The lower part of the BG merges into the diabase and the contact between the two is arbitrarily placed where pillow lavas are absent (Wilson, 1959). The BG consists of over 60% dikes (usually upto 90%) with screens of pillow lava. The presences of the lavas are screens results in the separation of the group from the diabase in which pillow lavas are absent. Although the whole basal Group unit is marked by a characteristic high relief, fresh exposures are hard to be seen at the surface which is mostly covered by the brownish colored weathered products.
The pillow lavas of the Basal Group have heavily intruded by dikes and the host rock recognizable only as narrow intervening screens of lavas which form a monor proportion of the unit. The fresh pillow lavas are greenish-gary in colour, possessing considerable amount of small vesicles. Vesicles are mainly filled with quartz, epidote and calcite. Generally, the harder intrusives are grayish brown in colour and are similar in character to the diabase dikes of the Sheted Dike Complex. In intrusives, vesicles are not as common as in the host pillow lava unit.
The boundary of the BG with overlying lava units is faulted in many places. A well-marked change in topography, which reflects the difference in hardness of the two units, indicates the approximate boundary between the BG and the peripheral pillow lavas. In addition to the relief, increasing number of pillow lava screens and the appearance of celadonite are excellent indicators to mark the proximity to the LPL unit. The diabase intrusive and the associated pillow lava screens of the BG form a high, rugged ground, which is oftenly, covered with pine forest, whilst the pillow lava has a low relief. The nature is responsible for the occurrence of some of the BG inliers in the study area. However, majority of BG inliers observed in the area appear to be formed due to subsequent faulting, where highly brecciaed rocks are located along the contact zone of the two units.
Digital Image processing
The main objective of applying image processing techniques in this study is to differentiate pillow lava series rock units. Since the entire pillow lava rock series is too large for a detailed study of this nature, there sub above mentioned sub scenes, a full thematic mapper scene (path 176, row 36), acquired on 3rd August 1994 was used. This image is ideal for applying image processing techniques, as it does not posses any cloud cover and also the Troodos massif is well centered. Digital image processing techniques were applied to these three sub scenes using the Integrated Land and Water Information System (ILWIS) software version 1.4, developed by ITC in 1993
Form the statistical methods, it was found that band combinations532 and 731 are the best suited sets for lithgolical interpretations in the area. Several image processing techniques were applied to Landsat Tm sub scenes using the selected band sets, for discrimination of volcanic series rock units. Out of these techniques, Saturation Enhancement, Decorrelation Stretching and Selective PCA using 3 bands are the best suited methods for litho logical discrimination.
Saturation Enhancement
By applying Saturation Enhancement technique, the visual interpretability of the sim[le flase colour composites (SFCC) can be improved. Instead of the RGB space, MMI space is applied in this technique. Although, saturation and hue can be used to describe every point within the colour triangle, a simple mathematical method which uses two orthogonal axes (M1 &M2) is usein this method. The origin of the two orthogonal axea is placed at point where Intensity axis crosses the triangle plane. M1 is arbitrarily chosen in line between origin and position of red in the triangle while M2 is orthogonal to M1, thus parallel to the line from green to blue. In this way, the data space is reduced to two dimensions and the maximum variation is limited to triangle. Within the two dimensional m1m2 space, the data cluster can be transformed in various ways (shifting, rotating and scaling)in order to spread it cover the complete triangle. In the resultant image colour is balanced and thus the image is easy to interpret.
