Original scientific article UDC 552.51:551.3.053(450.361 Milje) Received: 2007-05-14 SHORT-TERM SURFACE CHANGES ON SANDSTONE ROCKS Stefano FURLANI & Franco CUCCHI Dipartimento di Scienze Ceologiche, Ambientali e Marine, Università degli Studi di Trieste, 1-34100 Trieste, via Weiss 2, Italy E-mail: sfurlani@units.it ABSTRACT Short-term measurements, using a traversing micro erosion meter (t-MEM) on sandstone bare surfaces in two different locations within the Peninsula of Muggia, were surveyed. These stations have been monitored since 2004, first using a MEM, then a t-MEM. Median annual lowering rates in the studied stations, resulting from a monthly surveying, is 0.002 mm/yr (MUMV1) and 0.054 mm/yr (MUSB1). Daily surface changes, concerning the relative heights of 22 coordinates, were measured three times a day. The surveying shows that there are significant daily variations on sandstone surfaces. The maximum rise was -0.018 mm, while the maximum lowering was 0.009 mm. Maximum day variation is 0.027 mm. Observations made by various authors about the phenomenon of raising, particularly on shore platforms, is confirmed also for inland sandstones. The observed values suggest that the expansion and contraction of the bedrock could be related to wetting and drying processes. Comparison between meteorological and t-MEM data seems to indicate that during the night, bedrock surface rises due to water absorption, while during the day it lowers owing to the evaporation. Key words: traversing-MEM, sandstones, surface rise, surface lowering, Muggia VARIAZIONI MICROTOPOGRAFICHE A BREVE TERMINE DELLE ARENARIE SINTESI Vengono discusse misure MEM (micro-erosion meter) e t-MEM (traversing micro-erosion meter) raccolte tre volte al giorno, in due stazioni poste su arenarie, in due diverse localita della penisola muggesana. Le stazioni (MUMV1 e MUSB1) sono monitorate mensilmente dal 2004, prima con il MEM ed in seguito con il t-MEM. Il tasso di consu-mazione media annua nelle stazioni indagate è di 0,002 mm/a (MUMV1) e 0,054 mm/a (MUSB1). Le variazioni superficial!' dell'altezza relativa di 22 punti indagine per stazione sono state misurate tre volte al giorno. Lo studio mo-stra che ci sono significative variazioni giornaliere sulla superficie delle arenarie. Il massimo innalzamento misurato è stato di -0,018 mm, mentre l'abbassamento massimo di 0,009 mm, quindi una variazione massima di 0,027 mm. Le osservazioni fatte da vari autori sul fenomeno dell'innalzamento della superficie, in particolare sulle shore platforms, è confermato anche per le arenarie dell'entroterra. I valori osservati suggeriscono che l'espansione e la con-trazione del substrato roccioso potrebbero essere collegati a processi di alterazione dovuti all'alternanza sec-co/umido tra la notte ed il giorno. La comparazione tra i dati meteorologici e t-MEM sembrano indicare che durante la notte la superficie dell'arenaria si alza a causa dell'assorbimento dell'umidita, mentre durante il giorno la superficie rilascia l'acqua attraverso l'evaporazione. Parole chiave: traversing-MEM, arenarie, innalzamento della superficie, consumazione della superficie, Muggia Stefano FURLANI & Franco CUCCHI: SHORT-TERM SURFACE CHANGES ON SANDSTONE ROCKS, 241-248 INTRODUCTION Denudation is understood to be due to chemical and physical weathering and to erosion processes. The estimation of sandstone denudation rates is very important in order to elucidate both the role of sandstones in the geological context and to evaluate the durability of the rocks used as building materials. Many authors investigated weathering processes on sandstones: Takahashi ef a/. (1994) dealt with the reason for the variability and time-dependence in sandstone erosion rates. Paradise (1997) studied the differences between physical and chemical weathering due to lichens covering on sandstones, while Fitzner ef a/. (2003) and Turkington ef a/. (2003) surveyed the effects of weathering on sandstone monuments. Moreover, many researchers investigated the lowering rates of rocks all around the world, using different field or laboratory methods. The first ones did so by repeated measurements of mass or volume loss (Forti ef a/., 1975; Gams, 1979; Forti & Stefanini, 1981; Stefanini ef a/., 1985, Plan, 2005) and surface lowering rates via the micro-erosion meter (MEM), traversing micro-erosion meter (t-MEM) or Rock Erosion Meter (REM) (Forti, 1980; Cucchi & Forti, 1986, 1988, 1989; Cucchi ef a/., 1987, 1996, 1998; Allred, 2004), while the second ones did so through laboratory investigations (Martinez & White, 1999) or through numerical models (Kaufmann & Braun, 2001). Additional surveys have provided the rates of down-cutting in the coastal sector, in particular on shore platforms (Trudgill, 1976; Kirk, 1977; Robinson, 1977; Torunski, 1979; Spencer, 1981; Trudgill ef a/., 1981; Gill & Lang. 1983; Stephenson & Kirk, 1996; Stephenson, 1997). Among them, Kirk (1977) and Mottershead (1989) measured strange phenomena of surface rises, whereas recently, Stephenson ef a/. (2004), studying short-term changes on shore platforms, found hourly variations in surface height. They used the term "swelling" to describe surface rising. Gomez-Pujol ef a/. (2007) suggest avoiding using the term "swelling", because of its confusion with surfaces that remains elevated due to erosion processes and suggest to use the term "short-term surface change". Their results encouraged us to survey daily variations also in terrestrial setting. This paper addresses these questions in the light of results from exposure trials conducted at the Muggia Peninsula (Fig. 1) since 2004 (Furlani & Cucchi, 2006). In order to compare the lowering rates of sandstones that outcrop on the Peninsula, we considered the MEM station located at the MUSB site. MUSB sites include 4 measuring laboratory-made stations, one on a micritic limestone from Borgo Grotta Gigante (Italian Karst), one on San Bartolomeo sandstone (studied herewith), one on Kastelir sandstone (studied herewith), and one on a chalk sample from Brighton (UK), collected by Dr. Robinson (University of Sussex). The aim of this paper is to investigate short-term lowering rates via MEM and t-MEM (Fig. 2) on two samples inland sandstones measured during the November 2006 - May 2007 period at the Muggia Peninsula. MATERIAL AND METHODS Study area The Muggia Peninsula is located in the north-eastern part of the Adriatic Sea (Fig. 1). The area is characterized by interbedded sandstones and marlstones belonging to the formation of Eocene Flysch of Trieste. Marlstone layer spacing varies from millimetre to centimetre, while sandstone spacing is higher. Usually, sandstones are well-sorted, 0.1 mm in diameter. The light brown sandstone can be classified as a greywacke, characterized by a relatively high hardness, light brown color and poorly-sorted, angular grains of quartz, feldspar and small rock fragments set in a compact, clay-fine matrix and car-bonatic cement. According to Vierthaler (1873) and Malaroda (1947), biotite is less abundant in Muggia, belonging to the "Muggia-Istrian" petrographic area, than in Trieste, belonging to the "Triestine" petrographic area, while density is higher in Muggia. On average, they are composed of quartz (43-53%) and flint (6-11%). The remaining part is composed of feldspar (18-26%) as pla-gioclase, mica (4-6%), carbonates (16-20% as cement or rock fragments, less abundant in Trieste than in Muggia) and other residual components (iron oxides, glau-conite, tourmaline, garnet, zircon and rutil for less than 1-3%). They lack fossils apart from very rare rehandled Globigerinae and locally Medusas. The larger grains can be sand-to-gravel sized, and matrix materials generally constitute more than 15% of the rock by volume. Geomechanical characteristics of sandstone of the Renice quarry highlight an apparent specific gravity of 2,720 kg m-3, maximum water content of 0.95%, linear thermal expansion coefficient 0.0019 mm/ml/oC, while load strength varies from 945 kg m-2 to 908 kg m-2 (before and after freezing cycles). The tribometer test shows values of 2.7 mm (http://www.ts.camcom.it/marmi/ italiano/marmi/masegno.htm). The climate in the study area is Mediterranean continental (Righini ef a/., 2001), characterized by equally distributed rainfall throughout the year, lightly rainier periods in the autumn (mean rainfall 290 mm) and less rainier in the summer (213 mm). Mean annual rainfall measured in the 1961-1990 period in Trieste (0 m a.s.l.) was 1,015 mm yr-1, whereas in the inland Karst area (Padriciano site, 300 m a.s.l.) it was 1,341 mm yr-1 (Stravisi, 2003). The minimum mean value was recorded in February, while the daily peak is 105 mm in November. Storms are more frequent at the end of the summer and in the autumn. On average, there are 130 days per year when precipitations occur. In a year there are ap- Stefano FURLANI & Franco CUCCHI: SHORT-TERM SURFACE CHANGES ON SANDSTONE ROCKS, 241-248 Fig. 1: Map of the Muggia Peninsula (Trieste, Italy) showing the location of the measuring stations (MUSB and MUMV). Sl. 1: Zemljevid Miljskega polotoka (Trst, Italija) z lokacijami merilnih postaj (MUSB in MUMV). proximately 31.5% of rainy days, 64.4% of sunny days, 3.0% of snowy days and 1.1% of days with hail (Tommasini, 1979). The hottest month is August (24°C), whereas the coldest one is January with temperatures lower than 6°C. Sampling Measurements performed at the selected sites located in the Muggia Peninsula are presented in this work. Each site contains numerous "measuring stations" and each measuring station is composed of three nails fixed on the rock. The MUSB site, located on a private property at Darsella di San Bartolomeo (Lazaret), includes 9 measuring stations. MUSB1 has been the first in operation at this site, as it was positioned on local sandstone in November 2005. MUSB2 surveys the lowering rates of a limestone collected in the Matajur area (Eastern Friuli). MUSB3 has been set to record the lowering rates of limestone collected in Borgo Grotta Gigante (Italian Karst). MUSB4 is placed on Aurisina limestone. The remaining MUSB5, MUSB6, MUSB7, MUSB8, MUSB9 include 5 different rock lithologies: a limestone from Borgo Grotta Gigante, a sandstone from San Bartolomeo, a sandstone from Kastelir Mt., a sandstone from a quarry in Muggia and one sample of chalk from Brighton. Moreover, long-term data collected in MUSB1 to MUMV1 have been compared (Fig. 3). MUMV1 is the oldest station in Muggia. The MUMV site is located in Muggia Vecchia Archaeological Park. The site includes one station positioned in June 2003 on local Eocene sandstone block. Methods Direct measurements of limestone lowering rates have been performed using a micro erosion meters (MEM), constructed by Trieste researchers since the 70's, Fig. 2: The traversing micro erosion meter used in this study. Sl. 2: Prečni mikro erozijski meter, uporabljen med to študijo. Stefano FURLANI & Franco CUCCHI: SHORT-TERM SURFACE CHANGES ON SANDSTONE ROCKS, 241-248 Fig. 3: Long-term lowering rates at MUSB1 and MUMV1. SI. 3: Dolgoročne vrednosti posedanja na postajah MUSB1 in MUMV1. following the High & Hanna (1970) specifications and a traversing micro erosion meter (t-MEM) (Fig. 2). The instrument is equipped with three specially-shaped supports, which are forced to adhere to three bolts or titanium nails, two with semi-spherical and one with flat heads, inserted into the rock. The exact re-location on the fixed bolts is possible thanks to this configuration called Kelvin Clamp Principle. The engineering dial gauge is firmly fixed to the supports, thus allowing highly accurate analyses of rock lowering rates. The lowering rates of the surfaces can be repeated in exactly the same area at pre-set time intervals, using a specially designed engineering dial gauge. Following the example of Trudgill's team, who constructed a new instrument (Trudgill ef a/., 1981), the t-MEM, capable of collecting several measurements at each site, the Department of Geological, Environmental and Marine Sciences of the University of Trieste has assembled a t-MEM built by Stefano Furlani. The instrument is equipped with a millesimal-resolution electronic dial gauge, so that readings can be directly downloaded on a laptop computer (Stephenson, 1997). This configuration allows us to obtain a large data set, up to 238 measurements at a bolt site. Due to the large amount of sites, we decided to take 22 readings at each station. A calibration steel base was constructed to periodically check the instrument and to highlight differences in measurements. The electronic dial gauge has a resolution of 0.001 mm, while the error, confirmed by the builder (Mitutoyo), is ±0.003 mm. Probe erosion was estimated using two different methodologies: (1) by repeated readings on a test block, which revealed a probe erosion of 0.003 mm after 100 measures (research performed by the authors of this article) and (2) through observation via microscope of 34 touched rock samples (70 times) and untouched ones (re- search performed by Prof. Mauro Tretiach and Dr. Paola Crisafulli, Dept. of Geobotany, University of Trieste). In any case, readings below 0.010 mm must be considered with caution (Stephenson ef a/., 2004). Readings were taken at MUSB1 station during three periods, between 11 and 12 December 2006, between 26 and 27 February 2007, and between 6 and 7 May 2007. Temperature-related error, as pointed out by several researchers (Spate ef a/., 1985; Stephenson ef a/., 2004), was tested, but it turned out to be minimal due to the proximity of sites to the laboratory, in which the instrument is normally stored. RESULTS AND DISCUSSION The maximum duration of the measurement record at the stations located at the Muggia Peninsula was exactly 530 days, corresponding to the MUSB1 station, while the MUMV1 measuring site was surveyed for 493 days. The mean annual lowering rate in MUMV1 was 0.002 mm yr-1, while in MUSB1 it was 0.054 mm yr-1 (Figs. 3, 4). In the Classical Karst, where measurements have been collected since 1979, lowering rates varied between 0.009 mm yr-1 on dolomites, 0.010-0.013 mm yr-1on sparitic limestones and 0.038 mm yr-1 on micritic limestones (Cucchi ef a/., 2006). Statistics of daily variations for sandstone stations are presented in Table 1. It shows the results between readings collected each day. It contains the date of surveying, the number of surveyed points, the number of readings, the mean lowering/raising value, the median, the maximum and minimum surveyed value, the standard deviation and variance for daily variations. Negative values indicate a rising while positive values indicate lowering. Stefano FURLANI & Franco CUCCHI: SHORT-TERM SURFACE CHANCES ON SANDSTONE ROCKS, 241-248 Tab. 1: Muggia descriptive statistics of changes (mm) between measurements. Tab. 1: Statistika sprememb (mm) med merjenji na Muljskem polotoku. Date Surveyed points Readings Mean (mm) Median (mm) Max (mm) Min (mm) Range (mm) SD Variance 11 -12/12/07 22 66 0.008 0.007 0.027 0.003 0.024 0.005 0.000 26-27/02/07 22 88 0.010 0.010 0.019 0.005 0.014 0.003 0.000 06-07/05/07 22 132 0.008 0.007 0.015 0.004 0.011 0.003 0.000 Surveying point I 0.15 1 I Aa1 P«Aa2 r n < o N v .Q .Q .a .c < < < j T-