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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