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Section 18.3 – Intrusive Activity

Section 18.3 – Intrusive Activity. 8 th Grade Earth and Space Science Class Notes. Intrusive Activity. Most of Earth’s volcanism happens below the surface because not all magma emerges at the surface. Before it gets to the surface, rising magma can interact with the crust in several ways.

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Section 18.3 – Intrusive Activity

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  1. Section 18.3 – Intrusive Activity 8th Grade Earth and Space Science Class Notes

  2. Intrusive Activity • Most of Earth’s volcanism happens below the surface because not all magma emerges at the surface. • Before it gets to the surface, rising magma can interact with the crust in several ways.

  3. Plutons • General term for a type of igneous rock body that forms under the Earth • Can become exposed at the surface due to uplift and erosion • Classified based on their size, shape, and relationship to surrounding rocks

  4. Batholiths • Largest of the plutons • Irregularly shaped masses of course-grained igneous rocks • Cover at least 100 km2 • Take millions of years to form Coast Ridge Batholith in British Columbia

  5. Stocks • Small batholiths • Both batholiths and stocks cut across older rocks and generally form 5 to 30 km beneath Earth’s surface.

  6. Laccoliths • Lens-shaped pluton with a round top and flat bottom • Compared to batholiths and stocks, laccoliths are relatively small; at most, they are 16 km wide. Big Bend National Park in Texas

  7. Sill • Forms when magma intrudes parallel to layers of rock • It takes great amounts of force to lift entire layers of rock so most sills form relatively close to the surface Palasides Sill along the Hudson River

  8. Dikes • Cuts across preexisting rocks and often forms when magma invades cracks in surrounding rock

  9. Necks • When magma in a conduit solidifies Ship Rock in New Mexico

  10. Pluton Textures • Coarse-grained = deep under the ground • Fine-grained = closer to surface

  11. Plutons and Tectonics • Scientists think that some of the collisions along continental-continental convergent plate boundaries might have forced continental crust down into the upper mantle where it melted, intruded into the overlying rocks, and eventually cooled to form batholiths.

  12. Plutons and Tectonics • Plutons are also thought to form as a result of oceanic plate convergence. When an oceanic plate converges with another plate, water from the subducted plate causes the overlying mantle to melt. Plutons often form when the melted material rises but does not erupt at the surface.

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