Understanding Igneous Intrusions
* Intrusions: Igneous intrusions occur when molten rock (magma) pushes its way into existing rock layers.
* Contact Metamorphism: The intense heat from the intrusion will bake and change the surrounding rocks, a process called contact metamorphism.
The Age Relationship
* Older than the Intrusion: The rock surrounding the intrusion will almost always be older than the intrusion itself. The intrusion is, by definition, forcing its way into pre-existing rock.
Determining the Type of Surrounding Rock
While we can't definitively say what type of rock it is *without more information*, here are some possibilities and factors to consider:
1. The Original Rock: The type of rock surrounding the intrusion will depend on the original rock type that was there before the intrusion. This could be:
* Sedimentary Rocks: Commonly found at the Earth's surface (sandstone, limestone, shale)
* Metamorphic Rocks: Already altered by heat and pressure (marble, schist, gneiss)
* Older Igneous Rocks: Another igneous intrusion that was present earlier.
2. Contact Metamorphism: The heat from the intrusion will significantly alter the original rock type. The degree of metamorphism depends on the size and temperature of the intrusion. Here are examples:
* Low-grade metamorphism: Might create rocks like slate or phyllite.
* High-grade metamorphism: Could lead to the formation of rocks like marble or gneiss.
Example
Let's say we have a 50-million-year-old igneous intrusion in a region known to have a lot of sedimentary rocks. The surrounding rock might be:
* Original Rock: Sedimentary rock like sandstone or limestone.
* Metamorphosed Rock: The sedimentary rock could be metamorphosed to a rock like hornfels, a common contact metamorphic rock.
Key Takeaway
The most probable rock surrounding a 50-million-year-old igneous intrusion will be older than the intrusion, and its type will depend on the original rock type and the intensity of the contact metamorphism.
