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CHAPTER 12 Rock formation and deformation Introduction

Earth’s surface evolution has been considered so far as the product of a global machine – the supercontinental cycle – driven by tectonic processes which assemble, fragment and relocate crustal plates and the continents and ocean basins which they support. We turn now to the detailed operational processes of this cycle which require the formation, alteration and recycling of rock material. The conceptual framework for this development is provided by a model rock cycle, which is introduced as consisting of primary and secondary loops, corresponding to conventional distinctions between igneous and sedimentary rocks.

It is important that the treatment of geological and geomorphological processes should retain the dynamic sense of planetary evolution, spatial and temporal change running through Chapters 10 and 11. To this end the rock cycle is considered in parallel with geochemical and supercontinental cycles and the influence of tectonic processes and environments. Particular rock types are closely associated with continuing geochemical fractionation and specific locations and stages in crustal evolution. The principal rock-forming minerals and processes are introduced as the key to this holistic approach and also to the coverage of weathering and soil-forming processes in later chapters. It is also asserted that human societies extend geological fractionation and cycling processes by refining and eventually discarding minerals and other geological derivatives.

The formation of primary minerals from magma and the importance of silicates lead naturally into igneous processes and landforms. Alteration and resorption of oceanic crust represent the shortest route through the rock cycle, and there are also strong associations between magma emplacement, subduction and metamorphism. Metamorphic processes are dealt with next for these reasons, although it is recognized that sedimentary rocks are also subjected to metamorphism. The sedimentary stage is then outlined, prior to a more detailed review of oceanic sedimentary processes later in this chapter and of terrestrial sedimentary processes in parts of Chapters 13–17. The oceanic stage of the cycle is completed with a review of hydrothermal circulation and metasomatism. The chapter concludes by outlining the major processes of rock deformation associated with tectonic activity and the elevation of crust into the endogenetic environment.

Chapter Summary

Rock-forming minerals and processes

• Minerals are the building blocks of the rocks which form Earth’s lithosphere and crust, derived from the raw elemental fractionates of the underlying mantle. The lithology of rock is determined by the specific range and abundance of its mineral assemblage.

• Of well over 2000 known minerals, fewer than twenty-five form over 95 per cent of the crust. Oxygen and silicon form 75 per cent of lithospheric minerals by mass; and aluminium, iron, calcium, sodium, potassium, magnesium and titanium form a further 24 per cent.

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• Minerals possesses their own distinctive crystalline structure, based on the geometric arrangement of constituent cations and anions and their particular form of bonding.

• Each mineral structure is unique, although some may form a solid solution, with one ion substituting for another, and others occurring in more than one form are said to be polymorphic. Minerals also possesses their own distinctive geochemistry.

• Silicate minerals based on the silicate tetrahedron SiO44– dominate the rock-forming

minerals. They occur either in pure form (quartz) or with other mineral cations, and in varying structures as single, ring, chain or framework silicates. These minerals are primarily magmatic in origin.

• Other mineral groups are based on simple oxides, sulphides and chlorides with oxygen, sulphur, chlorine or fluorine anions; and carbonate, sulphate, phosphate and hydroxyl anion complexes. These minerals are not primarily magmatic in origin.

• Magmatic minerals form by fractional crystallization, as solid minerals form and settle out at successively lower temperatures. High-temperature, basaltic or mafic minerals form first, followed by andesitic, intermediate minerals and finally by low-temperature, granitic or felsic minerals.

• As magma temperature falls, silicate content and viscosity increase and the remaining more acidic magma flows more slowly.

The rock cycle

• Igneous processes and landforms are associated with subduction zones and hot-spots where magma is intruded into existing rocks or extruded and erupted at Earth’s surface. Their distinctive landforms vary according to magma mineralogy and emplacement style.

• Intrusive magma solidifies before reaching the surface and is usually therefore dominated by the silicate-rich (felsic), granitic portion of a melt. It forms an underground network of magma reservoirs, sheets or columns which may feed surface extrusions or be restricted to thermal diapirs.

• Extrusive magma solidifies after reaching the surface as silicate-poor (mafic), basaltic and often effusive eruptions forming shield volcanoes or basalt plateaux; or as intermediate, andesitic and often explosive eruptions forming stratovolcanoes.

• All rocks may be subjected to alteration due to changes in temperature or pressure, ranging from minor syngenesis or diagenesis to almost complete melting or migmatization.

• Metamorphism lies between these extremes and involves permanent textural or mineralogical alteration without leaving the solid phase. Metamorphism proceeds by textural alteration and/or solid-state recrystallization of constituent minerals under subsurface high temperature and pressure conditions.

• Metamorphism is therefore commonly associated with subduction zones, transform plate boundaries and igneous rock emplacement, and metamorphic rocks comprise up to 70 per cent of continental crust.

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• The infusion of high-temperature fluids and gases associated with magma emplacement or contact metamorphism causes metasomatism, or the minero-chemical alteration of surrounding rock.

• Sediments are the unconsolidated fragments and dissolved debris of other rocks and organisms and are normally transported from their point of origin, during which time their initial characteristics undergo substantial reworking and alteration.

• Sediments accumulate by debris deposition and are lithified to form clastic (particulate), chemical (precipitate) and biogenic sedimentary rocks.

• Each sediment parcel forms a distinctive facies, which reflects its mode of deposition, and bundles of facies with common genetic origins represent a sediment environment.

• Plate tectonics and landsurface processes determine the global location of sedimentary basins, which accumulate the detrital products and signature of prevailing environmental conditions.

The oceanic rock cycle

• Oceans provide unique sites for the formation of new rocks from both magmatic and sedimentary sources. Cold sea water itself weathers sea-bed rocks by hydration or oxidation, forming new mineral species.

• Sea water is also recycled through oceanic crust by hydrothermal circulation associated with mantle convection and mid-ocean ridges, altering the condition of sea water and sea-bed rocks by metasomatism en route.

• Ocean basins provide a major sink for the collection, reworking and recycling of terrigenous sediments, mostly on continental shelf–slope systems.

• Long-distance transport of suspended, ice-rafted and aeolian sediment augments the primarily biogenic muds which accumulate on deep abyssal plains.

Rock deformation: folding and faulting

• Tectonic activity sets up huge stresses in rocks, which result in strain or deformation when they exceed internal resistance. Movement then depends on rock rheology or ability to flow and occurs along planar structures.

• Plastic deformation occurs when the strain rate is low, and leads to micro-scale foliation and large-scale folding affecting the whole of the rock mass.

• Brittle failure takes over when the strain rate exceeds the plastic deformation rate, leading to faulting; intervening rock mass usually retains its integrity.

• Deformation is sometimes so severe as to cause shear faulting and the formation of nappes in folded strata, or to cause significant changes in material properties leading to detachment or decollement through the altered material.

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CASE STUDY : Morphotectonics in Northwest England Aims and Objectives Chapters 10-13 focus on geological and oceanic environments and related processes of rock formation, deformation and denudation. The geological history of terranes in northwest England, representing two of Britain’s three Phanerozoic orogenic events of the past 500 Ma, were featured in the 3rd edition (Chapter 10, 2001). We set out the region’s early geological development here, linked to website Case Studies supporting Chapters 11 and 13. These Cases can also be read in association with sections of Chapter 1, describing Wharfedale, Chapter 10 describing the geological development of Wales and the Geological Evolution of Britain ~ and compared with the website Case Study supporting Chapter 10. Rock Formation The appearance of the older, Caledonian terrane of the south-east Lake District contrasts clearly with the Variscan terrane of the western Yorkshire Dales in the Landsat image (Plate 1 and Figure 1), for reasons outlined here and developed in the Chapter 13 Case Study.

Plate 1 and Figure 1 Southern Lake District and western Yorkshire Dales of north-west England. Borrowdale Volcanic Group (map dark blue) and Windermere Supergroup (light blue) form mountainous areas. They were domed by thermal diapirism, including emplacement of the now-exposed Shap granite pluton (S) during the Caledonian orogeny. Part of the radial pattern of glacial rock basins holding Wastwater (WW), Coniston Water (CW) and Windermere (WM) punctuate the mountains, showing in black on the image (white on map). Upper Palaeozoic strata (brown) form the Variscan orogen and terrane’s Askrigg fault block (AB) of the western Yorkshire Dales and Bowland Forest fault block (BF). Carboniferous limestone benches in Wensleydale (WD), Upper Wharfedale (UW) and the monadnock peaks (▲) of Whernside (W), Ingleborough (I) and Pen-y-ghent (P) are visible. Drumlin swarms (dimpled micro-relief) mark the path of Late Quaternary glaciers sweeping out of the Lake District and through the Dales. Barrier and estuarine coasts fringe Morecombe Bay lowlands (MB) on Mesozoic strata (green). Landsat Thematic Mapper Image 90 km E→W and 80 km N→S, in bands 4, 5 & 7. Moorland and montane vegetation (green) contrasts with plantation, pastoral and arable farmland (brown) and estuarine sediments (cyan) on the false-colour image. Credit: British Geological Survey

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Central Lake District rocks of mid–late Lower Palaeozoic age (c. 460–400 Ma ago) reflect the later, closing stages of the Iapetus Ocean, when ‘southern Britain’ migrated from 60o south towards c. 30o south of the equator. During earlier oceanic-plate subduction on the outer continental margin of Gondwanaland, trench turbidites formed the Skiddaw Group, beyond the northern edge of the Landsat image. Next, a 6·5 km thick accumulation of eruptive rocks, the Borrowdale Volcanic Group of Ordovician age, formed in the maturing fore-arc basin. The lower 2·5 km are andesite lavas, erupted in an oceanic setting, whilst the upper 4 km are mostly intermediate-acid volcanic ashes (Plate 2) probably accumulated around terrestrial calderas as the volcanic focus migrated onshore. Volcanic island-arcs in the western Pacific/southeast Asia region provide suitable modern analogues of these environments.

Plate 2 The peaks of the Helvellyn range (maximum 925 m OD) carved by glaciers on rocks of the Borrowdale Volcanic Group form some of the highest and most rugged relief in the Lake District. Photo: Ken Addison This volcanic episode was short but intense, probably lasting less than 10 Ma c. 455 Ma ago, followed by rapid erosion and caldera collapse as the heat (and associated thermal dome) went out of the eruptions, causing crustal subsidence. This coincided with global sea-level rise, as Gondwana’s south polar ice sheet melted, re-flooding the basin. Five kilometres of marine sediments, dominated by turbidite muds and sourced from a land-mass to the north, eventually accumulated there during the Silurian period, and were compressed c. 400 Ma ago as the basin accreted onto the continent, becoming the younger Windermere Supergroup (Bannisdale Slates and Coniston Grits) of the southern Lake District . A short time afterwards, a swarm of intrusive granite batholiths were emplaced 7–10 km below the landsurface by c. 395 Ma in the early Devonian period. Two of these outcrop at the surface at Eskdale and Shap (Plate 3), on the western and eastern margins of the southern Lake District respectively, and another underlies the western Yorkshire Dales.

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Plate 3 Shap granite, exposed in recent centuries by quarrying for its attractive decorative masonry and crushed rock aggregates value on Shap Fell ~ but only after 7-10 km of younger rocks were stripped off by erosion at various times since its emplacement c. 395 Ma ago. Photo: Ken Addison Further tectonic reorganization of the Gondwanaland margin accompanied its equatorwards drift, during which time erosion reduced the extent of the Caledonian orogen. The Lake District terrane was fringed but not buried by Devonian sediments and in due course formed a basement for younger, Upper Palaeozoic terranes. It found itself flanked and partially-submerged by an marine basin of Carboniferous age 355–310 Ma ago, opening in response to the trench suction force of a subduction zone 1,000 km further south. Fault-blocks dipped slowly and intermittently northwards, whilst their southern margin reared up simultaneously to form more abrupt scarp fronts, creating a series of shallow marine basins with deeper troughs between blocks. The western Yorkshire Dales today reflect the control of tectonics and equatorial climates on sedimentary environments during the Carboniferous period. Carbonate platforms and coral reefs 300–500 m deep (Great Scar Limestone Group) developed in the shallow basins c. 340-327 Ma ago, much as they do in the Caribbean region today, whilst a 2-4 km thick sequence of marine muds infilled the deeper troughs, locally the Craven Basin. Intermittent rotation of the rigid blocks and changing sea levels meant that sedimentation was not continuous but cyclical. Moreover, marine conditions gradually gave way to advancing deltaic and then estuarine environments, as the sea closed and terrestrial sources of sediment began to dominate c. 327-315 Ma ago. The result was a sequence of horizontally bedded limestones, overlain progressively by mudstones and then coarse grits of the Yoredale Group which dominate the Dales’ landscape today (Plates 4, 5 and 6). The sequence ended with the accumulation of coal seams derived from equatorial coastal swamp forests ~ similar to the Amazon delta region today. Subsequent erosion mostly restricts their modern outcrop to the Pennine margins but tiny, downfaulted outliers are found in some places. The erosional history of the landscape underpinned by our two terranes, leading to their modern landforms, is told in the Case Study to Chapter 13.

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Plate 4 Carboniferous limestone exposed as karst pavement by Cenozoic erosion and, more recently, Quaternary glaciation and (probably) human-induced erosion. Photo: Ken Addison

Plate 5 Smoother and usually vegetated slopes form the steepest part of the west flank of Pen-y-ghent on shales of the Carboniferous Yoredale Group.

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Plate 6 Cross-bedding structures in Millstone Grit, the uppermost formation of member of the Yoredale Group exposed fringing the summit plateau of Ingleborough, provide evidence of their origin as deltaic sediments. Photo: Ken Addison Rock Deformation During the 100 Ma separating the youngest Lake District Silurian from the oldest Carboniferous rocks, first the ancient Iapetus and then the Rheic Oceans progressively closed (see the website study supporting Chapter 11). Resultant intercontinental collisions formed the global-scale Caledonian and Variscan cordilleran mountain systems respectively and set the structural framework for our terranes. Lake District rocks caught up in the tremendous orogenic compression and uplift of the Caledonian mountains were steeply folded and strongly metamorphosed. NE-SW “Caledonian” lineaments or structures paralleling the Iapetus Suture are visible at two scales on the Landsat image. The boundary between the Borrowdale Volcanic Group and Windermere Supergroup, identified first on the map, runs as a major structure across the northern shores of Coniston Water and Lake Windermere. Subsidiary NE-SW Caledonian structures ~ primarily folds and regional cleavage ~ can be seen intermittently on the Borrowdale Volcanic rocks and in profusion on the hills to the south, especially in a belt running across the lakes before disappearing under recent valley-infill sediments. A second and distinctive element in the structural evolution of the Lake District terrane is associated with the late-Caledonian emplacement of granite batholiths some 7-10 km below its surface during later stages of intercontinental collision in the early Devonian period. This thermal diapirism created a dome, imprinted across existing tectonic structures. Further regional uplift is thought to have occurred during the Cenozoic era, although ridge-push associated with Atlantic Ocean widening was a more likely cause.

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The structural character of the western Yorkshire Dales is quite different and results from Variscan crustal extension to the south causing one-sided or half-graben rifting of its eroded Caledonian basement ~ aided by continuing buoyancy of the Wensleydale granite batholith. Crustal slabs tens of kilometres long behaved like huge rotational landslides, for which active Basin-Range structures in Nevada provide a modern analogue (see Chapter 13 of the main text), although the cause of the latter’s extension (uplift) and terrestrial (rather than continental-margin) setting are different. One such, the Askrigg block, underlies the western Yorkshire Dales and its fault-controlled south-west and north-west boundaries ~ the Craven (NW-SE) and Dent fault (NE-SW) systems respectively ~ are clearly visible on the satellite image. The upper, hanging-wall dip slope is reflected today by the same gentle northeastwards dip of formerly horizontally-bedded shelf sediments, compared with the marine sediments draped across the steeper faulted foot-wall southwards into deeper Craven Basin ~ which separates the Askrigg block from the less-pronounced Bowland Block to the south. This account of the Palaeozoic geological and environmental history of Northern England continues with their subsequent geomorphic development in the website Case Study supporting Chapter 13. Learning Objectives

• appreciate the processes linking the stages and geographical locations of the supercontinental cycle with the Rock Cycle

• understand the principal rock-forming processes, and the relationship between

rock properties acquired during these processes with diagnostic rock characterisation and future performance

• explain how and why rocks are altered and later deformed after their initial

formation, and how their modified character may influence subsequent geomorphic processes

Essay titles

1 Describe the process of fractional crystallization in magma and outline how it aids an understanding of magmatic processes at different types of plate boundary.

2 Explain the processes by which solid-state metamorphism occurs in rocks and outline associations which exist between metamorphic facies and tectonic environments.

3 Explain how single clasts and bulk properties of sediments are transformed during transport and deposition.

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Discussion Topics

1 Identify the stages, processes and landforms associated with the life cycle of a stratovolcano.

2 Explain how humans act as agents in the geochemical and rock cycles and identify a number of examples.

3 Consider which sites you would include in a short illustrated field guide to the igneous landsystems of Britain.

Further Reading Bell, F. G. (1998) Environmental Geology: principles and practice, Oxford: Blackwell.

Another fine example of the author’s readable blend of pure and applied Earth science, varying our perspective on geological, geomorphic and pedological processes in twelve chapters in which human–environment impacts are never far away, before concluding with four more applied chapters.

Leeder, M. (1999) Sedimentology and Sedimentary Basins: from turbulence to tectonics, Oxford: Blackwell. This new text commences with a useful review of sedimentary properties and processes as a prelude to comprehensive cover of a full range of the sedimentary environments. Good tectonic and climate contexts are provided and the whole is well illustrated.

Park, R. B. (1997) Foundations of Structural Geology, third edition, Cheltenham: Thornes. This extensively revised edition focuses on deformation and geological structures with good, if rather technical, explanations of folding, faulting, etc. It concludes with modern interpretations of plate tectonic roles in deformation processes and zones.

Sigurdsson, H., ed. (2000) Encyclopedia of Volcanoes, San Diego, CA: Academic Press. Encyclopaedic in size and extent, although with a conventional article-based structure, the 1400 pages of this splendid text provide state-of-the-art and extensively illustrated coverage of volcanic processes, landforms, hazards and applications.

References Allen, J. R. L. (1968) Current Ripples: their relations to patterns of water and sediment motion, Amsterdam: North Holland Bell, F. G. (1998) Environmental Geology: principles and practice, Oxford: Hjulström, F. (1935) ‘Studies of the morphological activity of rivers as illustrated by the river Fyris’, Bull. Geol. Inst. Univ. Uppsala 25, 221–527 Howells, M. F., Leveridge, B. E. and Reedman, A. J. (1981) Snowdonia, London: Allen & Unwin Howells, M. F., Reedman, A. J. and Campbell, S. D. G. (1991) Ordovician (Caradoc) Marginal Basin Volcanism in Snowdonia (North-west Wales), London: HMSO for the British Geological Survey Newton, C. and Laporte, L. (1989) Ancient Environments, third edition, Englewood

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Cliffs, NJ: Prentice-Hall Reading, H. G., ed. (1996) Sedimentary Environments and Facies, third edition, Oxford: Blackwell Selby, M. J. (1993) Hillslope Materials and Processes, second edition, Oxford and New York: Oxford University Press Skinner, B. J. and Porter, S. C. (1995) The Dynamic Earth, sixth edition, New York: Wiley Smith, D. G., ed. (1982) The Cambridge Encyclopedia of Earth Sciences, Cambridge: Cambridge University Press Web resources http://edsserver.ucsd.edu/visualizingearth The Visualizing Earth project reviews the technical opportunities provided by remote sensing, GIS (Geographic Information Systems) and visualization for the study of Earth and Geographic sciences and provides a gateway to visualization through its many illustrations, access to further websites, reports, resources and valuable image databases. http://geolsoc.org.uk/index/html The UK’s oldest and leading professional geological organization, providing information and direct access to relevant Educational and Career services, Publications (some available online), Library and Information services, events and regional groupings of professional geologists and the geology of Britain. The website also includes direct links to the Society’s websites and other similar organizations, such as the Geologists’ Association. http://www.nasa.gov and http://nai.nasa.gov Both websites relate to the work of the US National Aeronautical & Space Administration with access to plenty of terrestrial, as well as astronomic and other planetary, materials. They are hyperlinked into NASA’s various US specialist laboratories, through which current news, data, information, video clips and access to published materials and images. Specifically geological material needs to be searched for but usually the results are rewarding. The following websites provide access to a range of different aspects and interpretations of the geology of the Lake District and Yorkshire Dales: http://groups.msn.com/YorkshireGeology/homepage http://www.yorksgeolsoc.org.uk/ http://www.english-nature.org.uk/special/geological/sites/area_ID41.asp http://www.rocksafoot.com/lake_district.htm http://www.lake-district.gov.uk/index/understanding/specialqualities.htm http://www.lake-district.gov.uk/index/understanding/geology.htm