The Field Description of Igneous Rocks

Dougal Jerram, Nick Petford

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781119957171
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

The Second Edition of this unique pocket field guide has been thoroughly revised and updated to include advances in physical volcanology, emplacement of magmas and interpreting structures and textures in igneous rocks. The book integrates new field based techniques (AMS and geophysical studies of pluton shape) with new topics on magma mixing and mingling, sill emplacement and magma sediment interaction. Part of the successful Field Guide series, this book includes revised sections on granitic and basaltic rocks and for the first time a new chapter on the engineering properties of igneous rocks.

The Geological Field Guide Series is specifically designed for scientists and students to use in the field when information and resources may be more difficult to access.

Many editions have been updated for 2011 and the guides are:

• Student-friendly in design and cost
• Durable
• Lightweight
• Pocket-sized
• Reliable
• Concise

Visit the series homepage at www.wiley.com/go/geologicalfield

Early map of Arran from John Macculloch’s Western Islands of Scotland 1819 book.

1

INTRODUCTION AND OCCURRENCE

Igneous rocks occur in almost all environments at the Earth’s surface as volcanoes and their dispersed products, and within the crust and mantle as they form the pathway from which the Earth cools from it’s hot interior. The products of igneous activity not only provide key information about the evolution of the Earth through time, they can be used as key stratigraphic markers, form the basis of our understanding of the Earth’s chronology and can be found to have striking effects on our planets climate. From the beauty of diamonds, the wonders of erupting volcanoes and the polished rocks which adorn buildings all over the world, man has had a fascination with igneous rocks as far back as we can trace. The key basis from which we can understand igneous rocks and their systems is from detailed fieldwork and observation. This guide aims to provide the basic information and tools to enable earth and engineering scientists from a variety of backgrounds to investigate the wonderful world of igneous rocks in the field.

1.1 The Importance of Fieldwork

The most fundamental observations that you can make within the earth sciences are in the field. Here we are able to characterise the occurrence of earth materials in situ, in relation to their surroundings and within the context that will underpin any further investigations of the rocks in the lab. In short – Fieldwork is the basis of all geological studies. As such it is important that a person who wishes to understand rocks is proficient in the field. Whether one is studying igneous, sedimentary or metamorphic rocks the fundamentals of the fieldwork are more or less that same and indeed one may need to address all three major types of rock in the same field locality. It should be noted that there are many instances in which expensive geochemical and geophysical data have been misinterpreted through incomplete knowledge of basic field relationships. Therefore, if the appreciation of field geology is poor, then all studies based on collected samples and field measurements will be equally poor. Conversely, good appreciation of field geology forms the basis of good geological interpretation.

As a good field scientist you will need to have a background of basic skills to enable you to make the correct detailed observations that will in turn lead to clear and well thought out interpretations of the geology. The study of igneous rocks might also include petrological and mineralogical investigation, geochemical and isotopic analysis to determine the age and origin of the rocks and the use of geophysical measurements in the field to determine the distribution of rock-types below the ground. Also, many igneous rocks are associated with distinctive types of economic mineralisation and these are generally discovered and evaluated by fieldwork. Key to the success of these approaches will be the detailed understanding of the rocks in the field.

In this handbook we explain how to observe igneous rocks in the field, from the scale of outcrops down to hand specimens and to tie observations into basic interpretations of how the igneous rocks formed. Before embarking on the details of igneous rocks in the field it is valuable to consider the role of igneous rocks in a global framework and to consider the main occurrence of igneous rocks.

1.2 The Global Picture – Igneous Rocks in Relation to Regional Tectonics

Igneous rocks are materials that have solidified from molten or partially molten material, termed magma. Such rocks may be classified as extrusive rocks, which were erupted at the surface of the Earth, and intrusive rocks that crystallised beneath the surface. Igneous rocks of different compositions and field relationships exist at specific regions on the Earth associated with the plate tectonics in general. This reflects the mode of formation and emplacement of igneous rocks in the context of regional tectonic patterns. Below we briefly describe the key plate margin and within plate associations where patterns of igneous activity can be recognised.

The Earth’s crust forms the uppermost part of the outer rigid shell, or lithosphere, of the Earth and is divided into large coherent ‘plates’ that move in relation to one another. This process termed plate tectonics (continental drift) reflects our cooling planet and the convection of the mantle beneath. The plates themselves are split into two types of crust which are defined by their composition and thickness, oceanic and continental and the configuration of the plates leads to different types of plate margins (boundaries) where specific igneous associations exist.

The boundaries between plates are of four types (summarised Table 1.1, see also Figure 1.7).

1. Constructive plate margins or ocean ridges, where two plates are moving apart and the upwelling and solidification of magma forms new oceanic crust.

2. Destructive plate margins, where two plates are converging so that one plate sinks below the other and is eventually resorbed into the mantle or ‘destroyed’. This process is accompanied by formation of a range of magmas. Such plate margins may occur on oceanic (island arcs) or continental lithosphere (active continental margins).

3. Conservative plate margins, are faults where two plates slide past each other (transverse faults), so that lithosphere is neither created nor destroyed, and igneous activity is minor.

4. Collision zones, where two island arcs and/or continents have collided so that subduction of oceanic material has ceased. Such areas are characterised by widespread extrusive and intrusive igneous activity which commonly continues for a considerable time after collision.

Over 99% by volume of igneous activity occurs at constructive and destructive plate margins and at collision zones and some occurs at locations within the plates, for example volcanoes such as those of Hawaii and those associated with the East African rift system.

Igneous activity at constructive plate margins is responsible for the formation of the oceanic crust. The composition and structure of the oceanic crust is known from the study of rocks dredged from the ocean floor, from seismic studies and from studies of onshore exposures of older rocks that are believed to be fragments of the oceanic crust (ophiolites). These lines of evidence indicate that the oceanic crust consists of layers of basalt lavas, basalt/dolerite dykes, gabbro and peridotite. These rocks form a distinctive association which may be recognised in ancient orogenic belts, where it is termed the ophiolite association. The recognition of such associations is clearly of great palaeogeographic significance and the ophiolite associations are described in detail in Chapter 8.

The oceanic lithosphere moves away from the oceanic ridge by the process of sea-floor spreading and is generally returned to the mantle at a destructive plate margin within circa 200Ma. The descent of oceanic lithosphere into the mantle is accompanied by partial melting above the descending plate where water is driven off at depth. This melting in the overlying mantle forms magmas ranging in composition from basalt, through andesite to rhyolite in composition. These intrude the crust and may be erupted at the surface or emplaced at depth as gabbro, diorite and granite. In some places, the emplacement of such rocks causes melting of the lower crust and this results in the emplacement of intrusions dominantly of diorite, granodiorite and granite composition at destructive continental margins, accompanied by eruption of andesite, dacite and rhyolite. The intrusive rocks emplaced at active continental margins form linear belts of intrusive complexes of diorite-granite composition, often termed batholiths.

The composition of the continental crust broadly resembles that of the igneous rocks of andesite composition. Much of continental crust is thought to have formed as a result of igneous activity of the type seen today at island arcs and at destructive continental margins. The crust has evolved continuously as a result of magmatic and metamorphic activity, uplift, erosion and sedimentation, and hence consists largely of metamorphic and igneous rocks with a thin veneer of sedimentary rocks. Because of it’s greater age and complex geological history, the structure of the continental crust is much more varied than that of the oceanic crust. The continental crust is therefore considered to have a complex structure characterised by rapid lateral and vertical variation, and uplifted sections from which the sedimentary veneer has been eroded expose sections of a wide variety of igneous rocks emplaced at great depth within the crust.

Igneous rocks formed at locations distant from plate margins (locations within-plate, Table 1.1) may have distinctive modes of occurrence, for example as flat-lying sheets of plateau lavas, as discordant plutonic magmatic bodies within continental rifts and as concordant or discordant gabbroic intrusions. Such igneous rocks may have characteristic compositions; indeed, many magmas emplaced at locations within a plate have distinctive alkali-rich chemical compositions which may be reflected in their mineralogy.

1.3 Mode of Occurrence of Igneous Bodies

In general we can split the different types of igneous body into three main sections relating generally to their relative positions within the Earth. Volcanic rocks erupt at and onto the Earth’s surface, minor intrusions tend to occur at shallow depths within the Earth’s crust and plutonic rocks form larger intrusive bodies at a variety of depths. In the context of this field guide we will generally look at these three main modes of occurrence and different subsections of these (for example, Volcanic – lava, Volcanic – pyroclastic, Plutonic – Granite, Plutonic mafic/ultramafic). Most field areas will be dominated by rocks from one of these general levels, though many examples will exist where different styles of occurrence are found (for example, shallow intrusions with lavas) so these basic subdivisions are used as a guide to the main features of each type. Below we briefly describe these major subdivisions.

1.3.1 Volcanic rock units

Volcanic rocks are classified as lavas and pyroclastic rocks (volcaniclastic). Lava is the term for molten extrusive rock and its solidified product, and pyroclastic rocks are composed of a mix of materials (lava fragments, pumice and crystals) fragmented by explosive volcanic activity. Within the guide we will deal with lava flows in detail in Chapter 4 and pyroclastic rocks in Chapter 5. Below is a brief summary of the main occurrence of volcanic rock units.

Often lavas and pyroclastics associated with individual volcanoes are concentrated within valleys and depressions around the volcano. The most extensive pyroclastic deposits may form large-scale stratigraphic units that blanket the topography and may form plateau-like features around volcanoes, additionally pyroclastic deposits may be underestimated in volume due to a large proportion of fine ash material which can be carried great distances from the volcano. The largest lavas flows build up thick sequences (plateaus), and construct large igneous provinces of immense volume.

Volcanoes show a wide variety of forms, depending largely upon the composition of the erupted material and hence the style of eruption (cf. Chapters 4 and 5). Basaltic volcanoes such as Hawaii erupt dominantly (over 80%) lava, the dominantly andesitic products of many volcanoes in island arcs and active continental margins have less than 10% lava and over 90% pyroclastic rocks. Further, the erupted proportion of pyroclastic rocks is often underestimated from subsequent field studies because these materials are often rapidly dispersed by the wind, or are eroded after deposition more rapidly than the equivalent volume of solid lava. Hence, bear in mind that lavas might be over-represented in many island arc and continental margin volcanoes.

Many volcanoes, particularly of andesite composition are composite in the sense that they comprise both lava and pyroclastic materials and have a steep irregular conical form (for example, Figure 1.1). Such volcanoes are built by flow of lava down depressions around the volcanoes and the eruption of pyroclastic materials; they commonly have diameters of 10-40km. Volcanoes associated with mainly basaltic eruptions form relatively low relief shallow sided volcanoes known as shields due to the predominently low viscosity lava, that erupts from them. The smallest volcanic forms result from a single short-lived eruption and comprise a variety of cones and extrusions. These include pyroclastic/scoria cones comprised of material of basic and intermediate composition, and steep sided flows and domes of more viscous acid lava. Such volcanic forms are generally 1–2km in diameter and may form upon, or near to, larger volcanic forms, when they may be termed parasitic volcanoes.

Figure 1.1 Colima volcano with lava flows and pyroclastic flows which have destroyed parts of the woodland around the mountain flanks.

From this description of volcanic forms and products, it is clear that a single volcanic area may be characterised by a variety of deposits. These may be contemporaneous and, for a terrestrial composite volcano, may be interpreted in terms of variation of associations of deposits with distance from the volcano (for example, Figure 1.2). The central zone (within circa 2 km of the central vent) is characterised by lava conduits (later exposed as volcanic plugs, dykes and sills) associated with coarse, poorly-sorted pyroclastic materials which have been deposited near to the vent. The proximal zone (circa 5–15km from the central vent) has a higher proportion of lava flows, with a variety of pyroclastic flow deposits, and the distal zone (beyond circa 5–15km from the central vent, and extending beyond the volcano) is characterised by pyroclastic flow deposits associated with fine air-fall deposits dispersed by wind away from the volcano. These may be interbedded with sedimentary rocks such as lacustrine deposits and reworked volcaniclastics (epiclastic) rocks. Pyroclastic cones, flows and domes may occur within any of the three zones. Therefore, even for young volcanoes, it may be difficult to correlate individual lava flows, pyroclastic fall and flow deposits with a single eruption. Debris flows (as distinct from pyroclastic flows) formed when material collapses from volcano sides and unconsolidated deposits, may travel several kilometres around source, and can carry great masses of lava as blocks (for example, around Mount Egmont/Taranaki, in North Island New Zealand).

Figure 1.2 Schematic internal architecture of a compostie volcano. See text for descriptions of central, proximal and distal zones.

1.3.2 Intrusive rock units

Intrusions vary widely in size and relationship...

Table of contents

• Cover
• Series page
• Copyright
• Title page
• Dedication
• PREFACE
• ACKNOWLEDGEMENTS
• 1: INTRODUCTION AND OCCURRENCE
• 2: FIELD SKILLS AND OUTCROP STRUCTURES
• 3: IGNEOUS TEXTURES AND CLASSIFICATION
• 4: VOLCANICS 1 – LAVA FLOWS
• 5: VOLCANICS 2 – PYROCLASTIC ROCKS
• 6: SHALLOW-LEVEL INTRUSIONS
• 7: GRANITIC COMPLEXES
• 8: MAFIC COMPLEXES
• 9: MAGMA MIXING AND MINGLING
• 10: MINERALISATION AND GEOTECHNICAL PROPERTIES
• APPENDIX
• FURTHER READING
• INDEX