Introduction of boulders to the R. Ribble: a short geomorphological evaluation

The placement of artificial in-channel structures (including large cobbles / boulders) into any fluvial system needs to be considered carefully. This brief report summarises considerations for boulder introductions into the River Ribble from a geomorphological perspective. This includes boulder placement and shape. Bagnold's Excess Stream Power Method is used to calculate the boulder sizing.

Geomorphological considerations for the introduction of boulders and groynes for fisheries enhancement: assessment of River Eden at Carhead

This report seeks to briefly provide geomorphological guidance over the introduction of 'habitat enhancement' features for fisheries improvement which is an expanding area of interest throughout the region. This document is a primary attempt to draw together the salient details from the large quantity of literature published in recent years with reference to boulder introduction and to groyne/deflector placement in NW rivers. Secondly the potential introduction of such features is assessed, with reference to the proposed works at Carhead on the Eden.

Size-frequency distribution of boulders ≥7 m on comet 67P/Churyumov-Gerasimenko

Aims. We derive for the first time the size-frequency distribution of boulders on a comet, 67P/Churyumov-Gerasimenko (67P), computed from the images taken by the Rosetta/OSIRIS imaging system. We highlight the possible physical processes that lead to these boulder size distributions. Methods. We used images acquired by the OSIRIS Narrow Angle Camera, NAC, on 5 and 6 August 2014. The scale of these images (2.44−2.03 m/px) is such that boulders ≥7 m can be identified and manually extracted from the datasets with the software ArcGIS. We derived both global and localized size-frequency distributions. The three-pixel sampling detection, coupled with the favorable shadowing of the surface (observation phase angle ranging from 48° to 53°), enables unequivocally detecting boulders scattered all over the illuminated side of 67P. Results. We identify 3546 boulders larger than 7 m on the imaged surface (36.4 km2), with a global number density of nearly 100/km2 and a cumulative size-frequency distribution represented by a power-law with index of −3.6 +0.2/−0.3. The two lobes of 67P appear to have slightly different distributions, with an index of −3.5 +0.2/−0.3 for the main lobe (body) and −4.0 +0.3/−0.2 for the small lobe (head). The steeper distribution of the small lobe might be due to a more pervasive fracturing. The difference of the distribution for the connecting region (neck) is much more significant, with an index value of −2.2 +0.2/−0.2. We propose that the boulder field located in the neck area is the result of blocks falling from the contiguous Hathor cliff. The lower slope of the size-frequency distribution we see today in the neck area might be due to the concurrent processes acting on the smallest boulders, such as i) disintegration or fragmentation and vanishing through sublimation; ii) uplifting by gas drag and consequent redistribution; and iii) burial beneath a debris blanket. We also derived the cumulative size-frequency distribution per km2 of localized areas on 67P. By comparing the cumulative size-frequency distributions of similar geomorphological settings, we derived similar power-law index values. This suggests that despite the selected locations on different and often opposite sides of the comet, similar sublimation or activity processes, pit formation or collapses, as well as thermal stresses or fracturing events occurred on multiple areas of the comet, shaping its surface into the appearance we see today.

40Ar/39Ar ages of boulders drilled at DSDP Hole 57-439

The 40Ar/39Ar stepheating dating method was applied to parts of three boulders recovered at Site 439, DSDP Leg 57. All the samples gave a well-defined isochron. The isochron ages agree with each other within the experimental uncertainties and give a mean value of 21.4 ± 1.0 Ma.

Distribution of basalt boulders and ice rafted debris in the region of the Iceland-Faroe-Ridge (Fig. 6-12)

Psephitic particles in the region of the Iceland-Faeroe-Ridge have been transported and deposited by means of a complex interplay of glacier movements and drifting icebergs. The composition of the particle association is controlled by the sedimentation of basaltic rock particles derived from the ridge itself and, in addition to that and in southern parts of the ridge, from the Faeroe Islands, the Faeroe-Bank and the Bill Baileys-Bank. Besides, there are crystalline and sedimentary dropstones showing a very varied petrography and a wide range of particle sizes. Their percentage becomes greater as the distance from the ridge increases. The association of dropstones is relatively homogeneous in the region of the ridge and only at greater distances from the ridge it becomes more differentiated. Owing to their composition and distribution, as well as on the basis of characteristic fossils and rock types, the drop-stones are derived from Scandianvia and Great Britain. During periods of maximum glaciation, the Icland-Faeroe-Ridge, th eFaeroe-Bank and the Bill Baileys-Bank were under ice.

Overlay of large, placed quarrystone and boulders to increase riprap stability /

"December 1976."

A lava park in Spain

The lava park is surrounded by the volcanic mountains of Les Preses, revealed as the edges of a vast caldera and repeated at a human scale with low walls made up of small volcanic boulders. These walls are evidence of how successive communities have gradually worked amongst this lava flow to create arable land, supported by rich soils. The people saw the land prosper and learned how to maximise its productivity. Boulders that had come to the surface during agricultural cultivation were moved with human labour to create "artigas“, the characteristic pilings of volcanic stone. They have been used to raise and lower areas, to create shelter and exposure for their crops and to make caves for storage. Amongst all this, paths weave and cross. The whole place is made up of grey and black rocks with a constant cover of green crops or grass.

Submarine pyroclastic deposits formed at the Soufrière Hills volcano, Montserrat (1995–2003) : what happens when pyroclastic flows enter the ocean?

The Soufrière Hills volcano, Montserrat, West Indies, has undergone a series of dome growth and collapse events since the eruption began in 1995. Over 90% of the pyroclastic material produced has been deposited into the ocean. Sampling of these submarine deposits reveals that the pyroclastic flows mix rapidly and violently with the water as they enter the sea. The coarse components (pebbles to boulders) are deposited proximally from dense basal slurries to form steep-sided, near-linear ridges that intercalate to form a submarine fan. The finer ash-grade components are mixed into the overlying water column to form turbidity currents that flow over distances >30 km from the source. The total volume of pyroclastic material off the east coast of Montserrat exceeds 280 × 106 m3, with 65% deposited in proximal lobes and 35% deposited as distal turbidites.