Gr 11: Geomorphology- Massive igneous rocks

Igneous Rocks: Definition and Intrusion Basics
📌 Igneous rocks are solid, uniform rocks without layers, formed when magma (underground) or lava (above ground) cools.
🌋 Intrusions refer to igneous rock bodies that form *below* the Earth's surface.
💎 A Pluton (or plutonic rock) is a body of intrusive igneous rock of varying sizes found beneath the surface.

Types of Igneous Intrusions
🍄 The Batholith is the largest intrusion, typically massive and more or less mushroom-shaped, covering at least $100 \text{ km}^2$ up to thousands of $\text{km}^2$.
📉 The Laccolith is a smaller, dome-shaped intrusion (less than $16 \text{ km}$ in diameter) that forces overlying sedimentary strata to bend upwards.
🍮 The Lopolith is a saucer-shaped intrusion where the overlying sedimentary strata bend downwards, forming a depression upon cooling.
↔️ A Sill is a horizontal intrusion that forms when magma cools between the horizontal bedding planes of sedimentary rock layers.
🧱 A Dyke is a more or less vertical intrusion formed when magma fills cracks in the overlying rock strata and subsequently cools.

Joints in Igneous Rocks
🌬️ Unloading Joints form near the surface during uplift and erosion when the compressive stress/pressure previously held by overlying rocks is released.
😖 Contractual Joints form as the magma cools and contracts; fractures occur when this contraction exceeds the rock's tensile strength (maximum load support before fracturing).

Igneous Landforms Exposed at the Surface
⛰️ Granite Domes (e.g., exposed Batholiths) are large dome-shaped landforms resulting from the erosion of overlying layers, which decreases pressure, leading to exfoliation (sheets peeling off like an onion).
🪨 Tors form when chemical weathering, particularly spheroidal weathering, occurs along joints in granite underground, rounding the rock masses before overlying softer material erodes, exposing mounds of rounded boulders.

Key Points & Insights
➡️ Understanding intrusive igneous rocks requires visualizing their formation below the surface before they are exposed as landforms through erosion.
➡️ Key intrusive features are differentiated by their orientation (vertical like a Dykes, horizontal like a Sill) and size/shape (Batholiths are massive; Laccoliths are dome-shaped; Lopoliths are saucer-shaped).
➡️ Landform identification relies on shape: Granite Domes are large, smooth domes formed by exfoliation, whereas Tors are mounds of more rounded, weathered blocks.
➡️ Always prioritize the most obvious differences when answering comparison questions (e.g., Dome shape vs. Mound of rounded rocks) in assessments.

📸 Video summarized with SummaryTube.com on Oct 29, 2025, 16:24 UTC