[EC-377] Joints and Weathering – Rocks of Nanaimo EarthCache

Joints and Weathering – Volcanic Rocks of Nanaimo

Tasks:

1. Joints:
   How many different joint directions can you identify, and in which general orientation do they run (e.g. vertical, horizontal, diagonal)?

2. Weathering:
   Describe how the rock has changed through weathering. Where do you see evidence of loose material or detached blocks?

3. Transition from Rock to Loose Material:
   Describe the difference between the massive rock at the top and the material at the base of the outcrop. What stands out to you?

Log this cache as “Found it” and send me the answers to the questions to “silka03.earthcache@gmail.com”, my profile e-mail or via Geocaching Message Center. Not in your log!

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Have fun!

Vancouver Island is part of the Wrangellia Terrane, an ancient geological block made up largely of basalts from the Karmutsen Formation. These rocks formed about 230–210 million years ago as massive lava flows on the ocean floor. Later tectonic processes uplifted and exposed them at the surface.

At this outcrop in Nanaimo, you can clearly see joints and weathering features.

Geology at the Site

a) The Rock

• Dark gray to blackish, fine-grained basalt (locally basaltic andesite).

• Originated from lava flows of the Karmutsen Formation.

• Very hard, compact volcanic rock.

b) Joints

• Joints are cracks or fractures in the rock without displacement.

• Formation here due to:

  • Cooling of lava after emplacement (contraction).

  • Tectonic forces during uplift and deformation.

• Multiple joint orientations make the rock appear blocky.

c) Weathering

• Physical weathering: the rock breaks along joints through mechanical processes (temperature fluctuations, water infiltration, stress release).

• With ongoing weathering, blocks detach and accumulate at the base of the outcrop.

• Toward the base of the slope, smaller fragments and more loose material can often be found.

d) Transition to Loose Material

• The uppermost section shows incipient loosening and breakdown of the solid rock – the first step toward loose material formation.

• Entirely geological: solid rock turning into loose debris.

Observations On Site

Standing at the outcrop you can see:

• Clear joint orientations (several directions possible).

• Detached blocks at the foot of the outcrop – a result of geological weathering.

• Transition from massive rock above to loose debris below.

https://de.wikipedia.org/wiki/Vancouver_Island

https://de.wikipedia.org/wiki/Kluft_(Geologie)

https://steinrein.com/glossar/vulkanit/?srsltid=AfmBOoo8zthrY3c8_6dmpJLYrhDHUC4CQfFihzTPFEhedf6j0mS1YDeR