Intrusive Igneous Activity Chapter 10.2 Intrusive Igneous Activity

Plutons Most magma cools deep in the Earth and form the bottom of mountain ranges. Plutons are structures that are made from the cooling and hardening of magma deep in the Earth. Plutons can be studied after they reach the surface – plutons are classified by their shape, size, and relationship to surrounding rock.

Sills and Laccoliths These are plutons that are formed when magma is cooled close to the surface. A sill is formed when magma is injected into sedimentary rock. Laccoliths are similar but they push the rock up.

Dikes This occurs when magma is injected into fractures, and cuts across preexisting rock layers. They are sheetlike structures. Dikes will usually come out from an eroded volcanic neck.

Batholiths This is an intrusive (from under ground) igneous body that has a surface exposure of more than 100 square km.

Origin of Magma Magma occurs when solid rock, from the crust and upper mantle, partially melts from high temperatures. The geothermal gradient is how the temperature increases the deeper into the Earth you go. At 100 km deep it is estimated that the temperature is between 1400 and 1600˚C. This is still not quite hot enough to melt rock. There are a couple different ways more heat is generated – at subduction zones a lot of friction creates more heat. The second way is crustal rocks are heated as they move in the mantle by subduction.

Role of Pressure Pressure increases with depth. Therefore melting occurs at a higher temperature because of pressure. When pressure decreases you can have decompression melting – this can make magma.

Role of Water Water causes rock to melt at lower temperatures. That makes 3 ways magma can be formed – adding heat, a decrease in pressure, and lastly water lowering the melting point.