Volcanic Activity in the Crust

Plutons Magma is highly active all throughout the mantle. It does not always push its way up through the crust as a volcano. It does, however, push its way to the crust. If there is not enough pressure for the magma to push its way through the crust to form a volcano, it will instead move into cracks in the bedrock.

Plutons As magma rises, more and more makes its way into the bedrock. Over time, this magma (now out of the astenosphere) begins to cool and harden. Pluton: Any rock mass that forms when magma cools inside of other rocks. Also called an igneous intrusion.

Type of Plutons: Dikes The earths crust is not made of one rock layer, it is made of many different layers of different rocks and rock types. The layers are not always perfectly joined together, there can be cracks running through the layers due to: Faults Heat expansion/cooling Pressure Etc.

Types of Plutons: Dikes Magma will find its way into these cracks in the bedrock. When the cracks run vertically through the layers of the rocks, the magma moves into the cracks, cools and hardens. This vertical igneous rock that moves through vertical cracks is called a dike. These are very common in old volcanic regions.

Types of Plutons: Sills The cracks in the layers of the crust can also run horizontally. When magma moves into these horizontal cracks, it cools and forms a sill. Sills can be hundreds of meters thick and many kilometers long. The palisades of the Hudson River in New York are the face of a sill that was exposed by erosion and is about 50km long.

Types of Plutons: Laccoliths As magma moves through the crust, it may run into a layer either without cracks (unlikely) or the layer may be too stiff and unyielding for the magma to move through. The layer works like a dam in the path of a river.

Types of Plutons: Laccoliths Like a dam, it stops the magma from passing. Also like a dam, it causes the magma to pool. Magma takes up space though, so it pushes the blocking layer up. As that layer is pushed up, it pushes up all the layers above it. This results in a dome.

Types of Plutons: Laccoliths The magma that pushes up the layer cools and becomes dome shaped as well, this is what we call a laccolith. Examples: The Henry Mountains in Utah The Black Hills in South Dakota

Types of Plutons: Volcanic Necks At times, an active volcano will go extinct. When this happens, the magma inside the volcano cools and hardens. Over time, the rest of the volcano is eroded away. This cooled magma core is left behind, this is whats called a volcanic neck Ship Rock in Mexico is over 400m high!

Types of Plutons: Batholiths and stocks Deep in the Earths crust, a large, wide section of magma may cool before it moves upwards into the rest of the crust. This forms the largest of all plutons: the batholith. Erosion can eventually expose the batholith to the surface.

Types of Plutons: Batholiths and stocks Batholiths can often form the core of many mountain ranges. The batholiths are solid igneous rock, like all plutons. When a small area of the batholith is exposed, as in less than 100m 2, then it is called a stock.