Magma and lava (molten materials that come to the surface) , moving as they do within and atop the earth, also form recognizable features, called landforms. Because the materials and processes which create these landforms vary, the landforms themselves differ. They are commonly classified into two categories: intrusive landforms, or plutons, emplaced below the surface; and extrusive landforms, most commonly referred to as volcanos, formed above the surface of the ground. 

Below the surface, molten material (magma) will mobilize. It moves primarily in two ways. As rocks melt, they expand and become less dense than the surrounding solid rock, As a consequence, molten materials have a tendency to rise into the overlying layers by melting upward. This process, called stopping, is commonly associated with large magma pools. Because the molten material normally occupies more space than the solid material which was melted, but is prevented from expanding by the surrounding solid rock, it is under tremendous pressure. If there is an avenue of escape, such as a break or other weakness in the overlying rocks, the molten material will be forced into these cracks. This process is called injection. 

Eventually these magma bodies cool, creating bodies of igneous rock. We commonly see these cooled intrusive bodies of rock called plutons, only when erosion exposes them at the surface. Their classification is based on two characteristics: 1) their attitude in relation to surrounding rock layers; and 2) their relative size and shape. If their attitude is such that they run with the grain of the surrounding layers, they are called concordant; but if they cut across layers they are called discordant. 

Discordant Plutons 
Satellite image of batholiths in Australia 

 
 

The largest plutons in the world are batholiths. Greater than 25 square miles in their surface expression, they are the exposed tops of cooled magma chambers. Generally granitic in composition, they can be hundreds of miles long and tens of miles wide and deep. In the western United States, an entire mountain range, the Sierra Nevadas, has been carved out of one of these batholiths. 
 
 
 
 
 

Stocks are smaller in aureole extent (up to 25 mi ), but are otherwise similar to batholiths. A classical example of a stock is Stone Mountain in Georgia. 

Dikes are still smaller than either stocks or batholiths. Usually they represent injections of molten materials into fractures in the earth and may be of any composition. 
 
 
 
 
 
 

All three of the above plutons are discordant. 
 
 
 

Concordant Plutons
Concordant plutons are usually formed by injection, but injection between layers rather than across them. Two types commonly encountered differ mainly in shape. Sills are flat and tabular in cross-section, whereas laccoliths are relatively flat on the bottom, but have blistered up the overlying layers. Most of the New Jersey side of the Hudson River across from New York City is carved into the Palisades sill; and an excellent example of a laccolith can be seen outside of Duluth, Minnesota. 

These rock bodies form in different environments. Obviously, they are associated with magmatic formation.  (More to come)