IgnRx-08

Image credit: Scott Brande

Igneous Rock A08 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?

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

A08

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

A08

Scale in centimeters

Image credit: Scott Brande

Close-up View

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

A08

Scale subdivisions in millimeters

Image credit: Scott Brande