Transcript Slide 1
Igneous Intrusions I. G. Kenyon Batholith Batholith is derived from the Greek ‘bathos’ (deep) and ‘lithos’ rock Large scale igneous intrusions with an exposed area over 100 km2 Silicic magma can form at depths up to 50 km. Once formed it rises towards the surface and begins to cool Formed at convergent plate margins where partial melting occurs at the base of the continental crust to form silicic magma Batholith Batholiths are silicic in composition and cool to form coarse grained plutonic rocks such as Granite, Granodiorite or Diorite Plutons Batholith Batholiths cool and solidify at depth in the crust (typically 5-12 km) and are only exposed at the surface after a long period of erosion Batholith Composed of granodiorite and dated at 87 million years old Yosemite Half Dome, Sierra Nevada Mountains, California Batholith - largest Igneous Intrusive Feature The Cornubian Batholith of South West England The Cornubian BatholithOutcrops which underlies muchand of South West England Major Granite in Devon Cornwall The 3 dimensional shape quite is similar to that of an upturned boat All the separate outcrops link up at depth to form a huge batholith of granite which comprises many thousand cubic kilometres of rock Lands End 275 – 280 Ma Bodmin Moor 287 – 288 Ma Granites dated at 270 – 300 million years Carn Brea 290 Ma St.Austell Moor 270 – 285 Ma TregonningGodolphin 280 Ma Carnmanellis 290 – 295 Ma Dartmoor 280 300 Ma At 6 km depth the granite is still at a temperature of 200°C to 225°C Granite Plutons form Moors/Uplands with Tors Plutons and Metamorphic Aureoles Heat from a pluton metamorphoses the country rock causing recrystallisation and the growth of new minerals Metamorphic aureoles can be plotted on geological maps and therefore have to be at least 50 metres wide Dykes – Discordant sheet-like Intrusions Dykes are often vertical or steeply inclined and always follow the path of least resistance along joints, faults, bedding planes or cleavages (Sills) 1.5m Dyke on the Baranof Cross Island Trail, Alaska (Dykes) Dyke intrusion results in crustal extension Put picture of dyke at Porth y Post in here …. Basaltic Dyke, Playa Blanca, Lanzarote Dykes cool more rapidly than plutons to form volcanic and hypabyssal (sub volcanic) rocks The crystal size decreases from the centre of the dyke towards the chilled margins Dykes cool and solidify over hundreds to thousands of years depending on their size Near vertical dyke 1.1 m wide Chilled margins volcanic in texture with crystals <0.5 mm, the interior may be hypabyssal with crystals up to 2mm in size Basaltic Dyke – Playa Blanca, Lanzarote Chilled margins, crystals <0.5mm Cooling joints at right angles to the margins of the dyke Baked margins in country rock Dykes often form prominent ridges as they are more resistant to erosion than the rocks into which they have been intruded Here a basaltic dyke forms a ridge 0.5 to 1.0 metre high along the beach at Kildonan on the Isle of Arran on the West coast of Scotland Dolerite Dyke on the Corrie Shore, Isle of Arran Red sandstone baked margin made of resistant metaquartzite Dolerite Dyke 1.8 metres wide Dolerite dyke is less resistant to marine erosion than the red sandstone and forms a gully Sills - Concordant Sheet-Like Intrusions Often gently dipping, do not cut across older geological structures, they intrude between and follow bedding planes for some distance The Palisades Sill was intruded at a depth of 3 km and has a layered structure-the result of multiple pulses of magma or slow cooling? Palisades Sill – New York State, USA A mafic magma intruded into Triassic sandstones, the sill trends north-south and outcrops at the surface over a distance of 80 km Dyke Sill is transgressive here The Palisades Sill dips westwards at 10°- 15° and is transgressive in places . Over 300 metres thick comprising gabbro, dolerite and basalt Drumadoon Sill on the Isle of Arran Near Vertical Cliffs showing columnar Jointing Drumadoon Sill was intruded into Triassic red marls and siltstones during the Tertiary. It is a composite sill with a basaltic top and bottom but a quartz-feldspar porphyry central section The Whin Sill of Northern England made of dolerite, a mafic, hypabyssal (sub volcanic) igneous rock The Whin Sill forms a prominent ridge Hadrian’s Wall is built along the crest of it The Whin Sill of Northern England The Whin Sill has a maximum thickness of 70 metres and is dated at 295 Ma The Whin Sill is resistant to erosion and has formed the waterfall High Force on the River Tees How to Distinguish between Sills and Lava Flows Sill Lava Flow Lava Flows Sills One (lower) chilled margin Two chilled margins One (lower) baked margin Two baked margins Xenoliths only from below Xenoliths from above and below Volcanic crystals <0.5 mm Hypabyssal crystals 0.5 – 2.0 mm Weathered/rubbly upper surface Regular upper surface Vesicles common Vesicles quite rare Most commonly made of basalt Most commonly made of dolerite aa or pahoehoe texture on surface Regular upper surface Lava fragments in rock above No sill fragments in rock above May show flow banding May show layered structure The Devil’s Tower – A Volcanic Neck/Plug in N.E. Wyoming Over 380 metres of sedimentary rock has been eroded away to leave the Devil’s Tower as a prominent feature Devil’s Tower is made of phonolite porphyry a hypabyssal silicic igneous rock The Tower rises 386 metres above the surrounding terrain Volcanic Neck/Plug This represents the conduit/feeder pipe of an ancient volcano which is much more resistant to erosion than the surrounding country rocks 482.5 m in height Exposed rock originally 750 to 1,000 metres below surface Dated at 27 Ma, it is made up of volcanic breccia and vertical dykes Ship Rock, New Mexico, USA Volcanic Neck/Plug A 10th Century church is built on this volcanic plug The plug is 82metres high and the Chapelle of St. Michel d’Aiguilhe can be accessed by a staircase of 268 steps Plug is basalt with a top diameter of 57 metres and basal circumference is 170 metres Le Puy en Velay, Southern France Stoping – magma makes its way towards the surface Stoping – Megiliggar Rocks, The Lizard, Cornwall Granitic magma penetrates joints and bedding planes, blocks of country rock fall into the magma and are melted Un-melted block of the country rock (xenolith) Process by which magma works its way towards the surface Match the igneous features listed below to the appropriate letter on the block diagram. Dyke, Lava Flow, Sill, Pluton, Batholith, Volcanic Neck/Plug, Laccolith, Cinder Cone, Feeder Pipe/Conduit, Volcano