Download Intrusive Landforms

Intrusive Landforms
Intrusive Landforms:
Intrusive Landforms are formed by magma
rising towards the surface but cooling and
solidifying before being extruded.
This is likely to be the case if…
• If magma is rising slowly.
• If there is a great thickness of crust to pass
through.
• Few weaknesses in the crust through which
it can flow out.
Why does the magma cool so slowly?
• Because it is not exposed to the air…
• …and so mineral crystals (quartz in
granite) grow to a large size.
Types of Intrusive Landforms
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Batholiths
Bosses
Sills
Dykes / Dikes
Batholiths
• Large masses of intrusive rock.
• Cause a general doming up of the surface as they
are forming.
• Only exposed after general weathering and erosion
of less resistant overlying ‘country rock’.
• Weathering is facilitated, (helped), by the fractures
and cracks that develop due to the tensional forces
that develop the surface experiences as it is
stretched during uplift.
• Similar but smaller features are known as Bosses
(Shap in Cumbria).
Batholiths continued….
• The heat / pressure exerted on the country rock
causes metamorphic rock to be produced around
the intruding magma.
• An example of this is sandstone being
metamorphosed into schists and limestone into
marble.
Metamorphic Rock = Rocks that have been
changed from their original form by heat or
pressure beneath the surface of the earth.
Sills
• Intrusions formed parallel to bedding planes in
country rock, often, but not always, lying
horizontally.
• The bedding planes provide a line of weakness
along which the magma will flow before cooling
and solidifying.
• The magma contracts as it cools, producing cracks
in the resultant rock.
• When the overlying rock is weathered and eroded
the sill is exposed.
• Sometimes these exposed sills form steep coastal
cliffs or rock outcrops, including cap rocks on
waterfalls.
Dykes (Dikes)
• Dykes cut across the bedding planes of country
rock often vertically.
• Magma flows through cracks and weaknesses but
again cools and solidifies before reaching the
surface.
• Contraction joints develop parallel to the surface
as the magma solidifies.
• Once exposed, Dykes can appear as linear
outcrops of resistant rock.
An Exposed Dike in New Mexico