IgnRx-15

Image credit: Scott Brande

Igneous Rock A15 Resources and Instructions

  • Observe sample below in the image(s) and videos. Follow guided viewing questions.
  • Evaluate sample for composition and texture.
  • Record composition and texture observations as instructed.
  • Compare features of samples to descriptions and features of igneous rock bank.
  • Enter sample composition and texture on classification table (below).
  • Identify the sample by name at the intersection of row (texture) and column (composition).
  • Review answers and correct any errors after instructor provides answer key.

Unless otherwise noted, images and videos are by Scott Brande.

Guided Viewing Questions

    • Questions below guide your observation of a sample for relevant properties.

Texture

Observe the rock sample for texture. Which texture does the rock most closely resemble?

    • crystalline - Does the sample exhibit visible (phaneritic) crystals? A mix of visible and microscopic crystals (porphyritic)? Just microscopic crystals (aphanitic)?
    • vesicular - Does the sample exhibit holes? More rock than holes? Or, more holes than rock?
    • fragmental - Does the sample exhibit a mixture of broken fragments floating in a matrix of ash? Or just fine-grained ash?
    • glassy - Does the sample exhibit conchoidal fracture?

Composition

Composition is most relevant for crystalline texture.

    • Are individual crystals visible - phaneritic - or invisibile - aphanitic?
    • Are scattered larger crystals floating in a fine-grained (aphanitic) groundmass (matrix)?
    • What is the tone (lighter or darker) of the rock?
    • Is the rock more felsic (lighter tone), mafic (darker tone), or intermediate?

Simplified Classification Table

Refer to this simplified classification table for rock names and their properties.

Click upper right corner to pop-out table to new tab.


Whole Rock Video

  • Ruler scale below is in centimeters (subdivided into millimeters)
  • Click to play YouTube video with pan-and-zoom

A15

Scale in centimeters

Image credit: Scott Brande

Whole Rock View

  • Ruler scale below is in centimeters (subdivided into millimeters)
  • Sample typically less than 5 cm in size
  • Click image to pop-out in new tab

A15

Scale in centimeters

Image credit: Scott Brande

Close-up View

  • Enlarged view for observation of detail
  • Click image to pop-out in new tab

A15

Scale subdivisions in millimeters

Image credit: Scott Brande