PLUTONIC LANDFORMS

Section 1 – Structural Landforms (≈ 1 – 20) Batholith • Stock • Dike • Sill • Laccolith • Volcanic Neck • Breccia Neck • Exposures like Stone Mountain, Yosemite, Devils Tower, etc. Section 2 – Magma Composition & Texture (≈ 21 – 30) Phaneritic vs Aphanitic • Pegmatitic • Cooling rates • Depth vs crystal size • Relation to plutonic environments. Section 3 – Practice Quiz (Plutonic Focus) (≈ 31 – end) Only the questions in your PDF tied to intrusive/plutonic concepts (e.g., “Which texture forms from slow cooling?” “Where would you find a granite batholith exposed?”).

1. Batholith – Definition

A massive body of intrusive igneous rock that forms from cooled magma deep underground; exposed after erosion removes overlying layers.

2. Batholith – Size

Largest intrusive bodies; cover hundreds to thousands of square kilometers.

3. Batholith – Composition

Typically granitic (felsic); coarse-grained texture due to slow cooling.

4. Batholith – Example

Yosemite National Park, California – part of the Sierra Nevada Batholith.

5. Stock – Definition

A smaller version of a batholith (<100 km² exposed area); irregular shape.

6. Stock – Example

Stone Mountain, Georgia – exposed granitic stock formed from slowly cooled magma.

7. Dike – Definition

A vertical or steeply inclined sheet of intrusive igneous rock that cuts across surrounding rock layers.

8. Dike – Formation

Forms when magma forces its way through fractures, cutting across preexisting rock layers.

9. Dike – Example

Dikes in the Sierra Nevada and at Shiprock, New Mexico.

10. Sill – Definition

A tabular, sheetlike intrusion that runs parallel to existing rock layers.

11. Sill – Formation

Forms when magma intrudes between sedimentary layers without cutting across them.

12. Sill – Example

Palisades Sill, New York – an example of mafic magma intrusion forming gabbro.

13. Laccolith – Definition

A lens-shaped pluton that forms when viscous magma pushes overlying strata upward.

14. Laccolith – Example

Henry Mountains, Utah – laccolithic domes formed from magma intrusions.

15. Volcanic Neck – Definition

Remnant of solidified magma within a volcano’s conduit after erosion removes the cone.

16. Volcanic Neck – Example

Devils Tower, Wyoming – columnar jointed phonolite plug.

17. Breccia Neck – Definition

A volcanic neck composed of broken angular rock fragments cemented by magma.

18. Breccia Neck – Example

Shiprock, New Mexico – formed from fragmented volcanic materials around a neck.

19. Plutonic Landform Exposure – Erosion

Deep-seated plutons become visible only after millions of years of uplift and erosion expose them.

20. Plutonic Landform Importance

Reveal the interior architecture of ancient volcanoes and magma systems.

21. Phaneritic Texture – Definition

Coarse-grained texture where individual crystals are visible; forms from slow cooling of magma deep underground.

22. Phaneritic Rocks – Examples

Granite, Diorite, Gabbro – all intrusive rocks.

23. Pegmatitic Texture – Definition

Very coarse-grained igneous texture formed by slow cooling and abundant fluids near the end of crystallization.

24. Pegmatite – Significance

Forms in late-stage crystallization zones within plutons; may contain rare minerals like tourmaline or beryl.

25. Cooling Rate vs. Crystal Size

Slow cooling → large crystals (intrusive); fast cooling → small crystals (extrusive).

26. Depth and Cooling Relationship

The deeper the magma cools, the slower the cooling rate and larger the crystal size.

27. Intrusive Rock Identification

Identified by coarse-grained (phaneritic) or pegmatitic texture.

28. Magma Chamber Solidification

As magma cools underground, early-formed minerals settle, creating layered plutonic bodies.

29. Relationship Between Magma Composition & Plutons

Felsic magmas form granite batholiths; mafic magmas form gabbroic intrusions.

30. Crystallization Sequence (Bowen’s Reaction Series)

Explains order of mineral formation; affects the composition and appearance of intrusive rocks.

31. Practice Quiz – What rock texture indicates slow cooling?

Phaneritic or Pegmatitic texture.

32. Practice Quiz – Which intrusive rock is felsic and coarse-grained?

Granite.

33. Practice Quiz – What is the difference between a dike and a sill?

Dike cuts across layers (vertical); sill runs parallel to layers (horizontal).

34. Practice Quiz – What large intrusive structure forms Stone Mountain?

A granitic stock.

35. Practice Quiz – Which intrusive body is the largest?

Batholith.

36. Practice Quiz – How does a laccolith differ from a sill?

A laccolith domes the overlying rock upward; a sill does not.

37. Practice Quiz – What causes plutonic rocks to be visible on Earth’s surface?

Erosion removes overlying layers, exposing deep intrusive bodies.

38. Practice Quiz – What texture best represents intrusive igneous rocks?

Phaneritic (coarse-grained).

39. Practice Quiz – Example of a volcanic neck?

Devils Tower, Wyoming.

40. Practice Quiz – Example of a batholith exposure?

Yosemite National Park, Sierra Nevada Batholith.