Structural geology - 3 | Primary structures Part 3 of 3 | Geology Concepts

Summary

This video delves into primary structures found in igneous rocks, focusing on their formation and classification. It explains the difference between extrusive and intrusive igneous rocks, highlighting their cooling processes and textures. Key structures such as pillow lava, vesicles, columnar joints, and various types of lava flows (like pahoehoe and blocky lava) are discussed. The video also explores intrusive features like sills, laccoliths, and dykes, along with large-scale discordant bodies like stocks and batholiths. Lastly, it covers impact craters and their unique structural features. The content provides an in-depth look into geological formations, ideal for learners in structural geology.

Takeaways

• 😀 Igneous rocks are categorized into two types: extrusive, formed from lava on the Earth's surface, and intrusive, formed below the surface.
• 😀 Extrusive igneous rocks are fine-grained and may have a glassy texture if cooled rapidly, while intrusive rocks are more grainy due to slower cooling.
• 😀 Pillow lava forms underwater when lava cools rapidly, creating bulbous, pillow-like structures that help determine the top direction of the flow.
• 😀 Vesicles are cavities in igneous rocks formed by volatile materials escaping during cooling, and when filled with secondary minerals, they become amygdaloidal.
• 😀 Columnar joints form when igneous rocks cool and contract, creating three to six-sided columns with long axes perpendicular to the cooling surface.
• 😀 Pahoehoe lava, known for its ropey texture, is low-viscosity lava that flows easily, while blocky lava is more viscous and forms large blocks on the surface.
• 😀 Pyroclastic structures include volcanic breccia (angular fragments), agglomerates (rounded fragments), and tuff (fine particles), resulting from explosive volcanic activity.
• 😀 Sills are concordant intrusive bodies that form parallel to the existing geological layers, whereas laccoliths are similar but have a convex top and flat base.
• 😀 Dykes are discordant intrusive bodies that cut across existing rock layers, and when multiple dykes occur in one location, they form a dyke swarm.
• 😀 Impact craters are formed when high-velocity objects strike the Earth, causing circular craters and significant structural disturbances in the surrounding rocks.

Q & A

What is the difference between extrusive and intrusive igneous rocks?

Extrusive igneous rocks are formed when lava cools rapidly on the Earth's surface, resulting in fine-grained or glassy textures. Intrusive igneous rocks, on the other hand, form when magma cools slowly beneath the Earth's surface, producing coarse-grained textures.

What is pillow lava and how does it form?

Pillow lava is formed when basaltic lava erupts underwater. The rapid cooling of the lava creates a glassy surface, forming bulbous, pillow-shaped structures. This structure helps to determine the flow direction of the lava.

What are vesicles in igneous rocks?

Vesicles are small cavities formed within igneous rocks when volatile materials escape during the cooling of lava. If these cavities are filled with secondary minerals, they become amygdaloidal.

How do columnar joints form in igneous rocks?

Columnar joints form when igneous rocks cool and contract, causing the rock to fracture into vertical columns. These columns are typically 3 to 6-sided and are perpendicular to the flow boundaries and isotherms.

What is pahoehoe lava, and where is it commonly found?

Pahoehoe lava is a type of less viscous lava that flows easily, forming rope-like structures on its surface. It is commonly associated with the Hawaiian volcanic activity.

What is the difference between pahoehoe and a’a lava?

Pahoehoe lava is smooth and ropey, forming easily flowing structures, while a’a lava is more viscous and forms blocky structures as it cools rapidly. The surface of a’a lava solidifies quickly, while the interior remains molten.

What is a laccolith, and how does it differ from a lopolith?

A laccolith is a concordant intrusive structure with a flat base and a convex top, often forming dome-like features. A lopolith, in contrast, has a flat top but a concave base, creating a reverse dome shape.

What are dikes and how do they differ from sills?

Dikes are discordant intrusions that cut across pre-existing layers of rock, while sills are concordant intrusions that run parallel to the existing rock layers.

What are radial dikes, and how do they form?

Radial dikes are vertical intrusions that radiate outward from a central point, resembling spokes on a wheel. These dikes dip vertically and are often associated with volcanic centers.

What is the formation of impact craters and what features are associated with them?

Impact craters form when high-velocity objects collide with the Earth's surface, creating shockwaves that cause deformation and fracturing of the surrounding rock. Features associated with impact craters include shatter cones, circular craters, and strata that may be folded along the rim.