Understanding Intrusive Igneous Rocks

Intrusive igneous rocks, also known as plutonic rocks, are formed when molten rock (magma) cools and solidifies beneath the Earth's surface. Unlike extrusive rocks (like lava flows), intrusive rocks cool slowly because they are insulated by the surrounding country rock. This slow cooling allows for the formation of larger mineral crystals.

Magma can intrude into the existing country rock in various shapes and sizes, creating different types of intrusive igneous bodies. These bodies are classified based on their size, shape, and their relationship with the surrounding rock layers (whether they cut across or are parallel to them).

Types of Intrusive Igneous Rock Bodies

Let's look at the characteristics of the different intrusive igneous rock bodies mentioned in the options:

Laccolith Formation

A Laccolith is a lens-shaped or mushroom-shaped intrusive body that has a flat bottom and a domed top. It forms when viscous magma is injected between sedimentary layers but does not spread out far horizontally. Instead, the pressure of the magma arches the overlying layers upwards. Laccoliths are generally smaller than batholiths and form at relatively shallower depths, often near the surface or at intermediate levels within the crust.

Batholith Characteristics

A Batholith is a very large, irregular, discordant intrusive igneous body. "Discordant" means it cuts across the existing layers or structures of the country rock. Batholiths are massive, often composed of multiple merged intrusions (plutons). They form deep within the Earth's crust. Their full vertical extent is rarely seen because they crystallize at great depths and are exposed only after significant erosion removes the overlying rock. Batholiths are considered the largest types of intrusive bodies and represent solidified magma chambers or systems that fed smaller intrusions or volcanic activity closer to the surface. Because they form at such great depths and often have no discernible floor (their base is unknown or extends deeper), they are considered the lowermost or innermost of the large intrusive rock forms listed.

Lopolith Structure

A Lopolith is a large, saucer-shaped or basin-shaped intrusive body. Unlike batholiths, lopoliths are typically concordant, meaning they are injected parallel to the layering of the surrounding country rock. The central part of the lopolith sags downwards, giving it its characteristic basin shape. Lopoliths can be very large horizontally but are generally not as voluminous or deep-seated as batholiths. They are often associated with layered intrusions containing different rock types that settled out of the magma as it cooled.

Phacolith Definition

A Phacolith is a lens-shaped intrusive body that forms in the crest (anticline) or trough (syncline) of folded rock layers. They are typically concordant with the folding but their shape is controlled by the fold structure. Phacoliths are generally smaller than laccoliths, lopoliths, and batholiths, and they occur in folded orogenic belts.

Comparing Intrusive Rock Forms

Here is a summary comparing the four types of intrusive igneous rock bodies:

Identifying the Lowermost/Innermost Intrusive Rock

Based on the characteristics discussed, Batholiths are the largest intrusive bodies and form deep within the Earth's crust. They represent the main, deep-seated magma reservoirs. While other intrusive forms like laccoliths, lopoliths, and phacoliths exist, they are generally smaller and often associated with shallower or intermediate depths relative to the vastness and depth of a batholith. Therefore, the batholith fits the description of the lowermost/innermost intrusive igneous rock body among the given options, as its base is rarely reached or known, signifying its deep origin.

Revision Table: Intrusive Rock Bodies

Additional Information on Plutons

The term "pluton" is a general term for any body of intrusive igneous rock. Batholiths are essentially large composite plutons. Other common intrusive bodies not listed in the options include:

• Sill: A tabular, concordant intrusion (parallel to layering).
• Dike: A tabular, discordant intrusion (cuts across layering).
• Stock: A small, irregular, discordant intrusion, essentially a smaller batholith (less than 100 sq km in surface area).
• Volcanic Plug/Neck: A cylindrical intrusion that solidified in the vent of a volcano.

Understanding whether an intrusion is concordant (parallel to host rock layers) or discordant (cuts across host rock layers) is also a key aspect of classifying intrusive igneous rocks.