Classifying Plutons: Volcanic rock that cools and hardens covers much of Earth’s surface. Most magma, however, cools and hardens deep within the Earth. This magma forms the roots of mountain ranges and a variety of landscape features.
Classifying Plutons: Recall that magma rises through the crust toward the surface. As it rises, it may rise through fractures in the rock or force it’s way between layers of rock. The magma may form thin sheets a few centimeters thick or collect in vast pools that can be kilometers wide. All these formations have various names; dikes, sills, laccoliths and batholiths.
Structures resulting from the cooling and hardening of magma beneath Earth’s surface are called plutons. The word pluton is derived from Pluto, the Roman god of the underworld. Plutons form in continental crust wherever magma slowly crystallizes and forms intrusive igneous rock. Intrusive igneous rock = rock that forms as magma cools deep within the Earth. Extrusive igneous rock = forms once magma emerges as lava and cools above earth’s surface. Classifying Plutons:
Over millions of years, uplift and erosion can expose plutons to the world above the surface. There are several types of plutons: Sills Laccoliths Dikes Batholiths
Sills and Laccoliths: Sills and laccoliths are plutons that form when magma intrudes between rock layers close to the surface. Sills and laccoliths differ in shape and often differ in composition. A sill is a pluton that forms when magma flows between parallel layers of sedimentary rock. Horizontal sills are the most common to be found.
Sills form only at shallow depths, where the pressure from the weight of the overlying rock is low. For a sill to form, the magma must lift the overhead rock to a height equal to the thickness of the sill. While this takes energy; forcing the magma between rock layers requires less energy than forcing the magma all the way up to the surface. Exposed Sill Sills and Laccoliths:
A laccolith is a lens-shaped pluton that has pushed the overlying rock layers upward. Like sills, laccoliths form when magma intrudes between sedimentary rock layers close to the surface. The magma that forms laccoliths has a higher viscosity (thickness) than the magma that forms sills. For this reason, the magma that collects bulges upward instead of spreading out in a thin layer. Exposed laccoliths Sills and Laccoliths:
Dikes: Some plutons form when magma from a large magma chamber moves into fractures in the surrounding rocks. A dike is a pluton that forms when magma moves into fractures that cut across rock layers. Dikes are sheet-like structures that can range in thickness from less than a centimeter to more than a kilometer. Most dikes are a few meters thick and extend for no more than a few kilometers. Two examples of dikes
Batholiths: Batholiths are very large bodies of intrusive igneous rock. A batholith is a body of intrusive igneous rock that has a surface exposure of more than 100 square kilometers. Much larger than a pluton, a batholith can be hundreds of kilometers long and tens of kilometers across.
Gravity studies and seismic evidence indicate that batholiths are very thick, sometimes extending tens of kilometers downward into the crust. A body of igneous rock similar to a batholith but having an area less than 100 kilometers is called a stock. Batholiths:
How are Batholiths formed? Batholiths form from many individual plutons that begin as blobs of magma deep beneath the surface. The plutons slowly rise through the crust in the form of hot magma. They clump together forming a huge irregular mass of underground magma, molten rock.
This magma never works it’s way to the surface. Instead, it cools and hardens underground forming granitic rock. Over millions of years, uplift and erosion gradually expose the batholith at the surface. Batholiths form the core of the Earth’s great mountain ranges; from the Northern Rockies to the Sierra Nevada Mountains in southern California. How are Batholiths formed?
Examples of batholiths that have been exposed by erosion Sierra Nevada batholith Half Dome Yosemite Batholiths can be huge, extending hundreds of miles How are Batholiths formed?
Computer Lab Assignment: Use the internet to research and write 3 paragraphs about one of the following geological formations. Describe how it was formed. DO NOT COPY CUT OR PASTE: each paragraph must be a minimum of 4 sentences. The Giants Causeway in Northern Ireland The Northern Canadian Rocky Mountains Devil’s Tower, Wyoming Mount Shasta, California Crater Lake, Oregon