Question

Describe two ways in which batholiths intrude into existing rock.

Short answer

Batholiths intrude by stoping, where rock blocks sink into the magma, and by assimilation, where the magma melts and incorporates surrounding rocks.

Step-by-step solution

Definition of a Batholith

A batholith is a large mass of igneous rock that forms from cooled magma deep within the Earth's crust. It is typically irregular in shape and can cover extensive areas.

Mechanism 1 - Stoping

Stoping is one way that batholiths intrude into existing rock. During stoping, blocks of the overlying country rock break off and sink into the magma, which allows the magma to rise and fill the space created by the removal of the rock blocks.

Mechanism 2 - Assimilation

Assimilation is another method by which batholiths can intrude. In this process, the magma melts and incorporates some of the surrounding rock as it moves upward. This melting and incorporation help the batholith to expand intrusively into the existing crust.

Key concepts

Stoping

Stoping is a fascinating mechanism of igneous intrusion that allows magma to make its way through the Earth's crust.
It involves the breaking and sinking of blocks of the overlying country rock into the magma.
This process can be visualized like this: imagine magma slowly pushing its way up. These blocks of country rock break off due to the immense pressure and eventually sink into the magma.
As these blocks dissolve and melt, they create voids that are immediately filled by more rising magma.
This continuous cycle ensures the magma gains space within the solid crust.

Key points about stoping include:

• Rising magma causes blocks of overlying rock to break off.
• The blocks sink into the magma and eventually dissolve.
• The resulting space allows more magma to rise and continue the process.

Assimilation

Assimilation is another important process that facilitates the intrusion of batholiths into existing rock formations.
During assimilation, the surrounding or 'country' rock melts and becomes part of the rising magma.
This occurs because the hotter magma brings temperatures high enough to melt some of the older, cold rocks it comes into contact with.
As these rocks melt, they mix with the magma, effectively changing its composition.
This assimilation process helps the magma to intrude further into the surrounding crust.
It also plays a pivotal role in magma evolution, altering chemical properties.

Key points to remember about assimilation include:

• The surrounding rock melts and gets incorporated into the magma.
• This process changes the composition of the magma.
• Assimilation helps the magma penetrate further into the crust.

Igneous Rock Formation

Igneous rock formation is the process by which magma (molten rock) cools and solidifies into rock.
These rocks are classified based on where the cooling and solidification occur.
There are two main types: intrusive (or plutonic) and extrusive (or volcanic).

Intrusive igneous rocks form when magma cools slowly beneath the Earth's surface.
Because of the slow cooling, these rocks tend to have large, visible crystals.
Batholiths, formed deep within the crust, are a classic example of intrusive rocks.

On the other hand, extrusive igneous rocks form when magma erupts onto the Earth's surface as lava and cools quickly.
The rapid cooling results in smaller, fine-grained crystals.
An example of extrusive rock would be basalt.

Key points for igneous rock formation:

• Magma cools and solidifies to form igneous rocks.
• Intrusive rocks cool slowly beneath the surface and have larger crystals.
• Extrusive rocks cool quickly on the surface and have finer crystals.