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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 • • • • 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