Post by 150FromFundy on May 17, 2009 7:36:22 GMT -5
I finally got around to slicing some of the material from my collecting at Spicer Cove and Squally Point last weekend. This is a rhyolite beach (sorry agate fans), but there is a great variety of rhyolite available along a one mile stretch of beach.
The variety is the result of contact zones (faults) between a number of bedrock formations. Each bedrock formation is the result of repeated lava flows, so there is variation within the bedrock formation itself. Along the contact zone, heat and pressure have created a “mild” metamorphosis, so there is additional variation resulting from that.
From a previous post, you might remember that granite and rhyolite are essentially the same. Only the conditions of formation differ. Granite solidifies slowly deep within the earth, so crystals tend to grow slowly, but are highly confined. Rhyolite solidifies rapidly on the surface of the earth (in lava flows), so crystals tend to grow rapidly under less confined conditions.
One of the more common rhyolites is porphyritic rhyolite, where crystals are visible. Rapidly cooling porphyritic rhyolite tends to have small fine grained crystals. Slower cooling porphyritic rhyolite tends to have larger grained crystals.
Photo 1 – Porphyritic Rhyolite
Photo 2 – Porphyritic Rhyolite
Photo 3 – Porphyritic Rhyolite
Another common rhyolite is flow banded rhyolite. Flow banded rhyolite cools so quickly that there is no time for crystal growth.
Photo 4 – Flow Banded Rhyolite
Photo 5 – Flow Banded Rhyolite
Photo 6 – Flow Banded Rhyolite
Along the contact zones, the common rhyolites above (porphyritic and flow banded) become altered by heat and pressure created along the fault producing some interesting combinations. Porphyritic rhyolite becomes re-melted and stretched out which elongates its crystals into swirly bands. The original rhyolite often becomes brecciated and the cracks and fractures fill in with new molten material.
Photo 7 – Altered Rhyolite
Photo 8 – Altered Rhyolite
Photo 9 – Altered Rhyolite
This beach has a lot of green rhyolite that I haven’t quite figured out what causes this variation, yet. The greens look like the outer portion of Mushroom Rhyolite, but the similarities stop there.
Photo 10 – Green Rhyolite
Photo 11 – Green Rhyolite
Photo 12 – Green Rhyolite
If your curious about where these rocks were found, look for A geologic Journey posted under Member Photographs.
150FromFundy
The variety is the result of contact zones (faults) between a number of bedrock formations. Each bedrock formation is the result of repeated lava flows, so there is variation within the bedrock formation itself. Along the contact zone, heat and pressure have created a “mild” metamorphosis, so there is additional variation resulting from that.
From a previous post, you might remember that granite and rhyolite are essentially the same. Only the conditions of formation differ. Granite solidifies slowly deep within the earth, so crystals tend to grow slowly, but are highly confined. Rhyolite solidifies rapidly on the surface of the earth (in lava flows), so crystals tend to grow rapidly under less confined conditions.
One of the more common rhyolites is porphyritic rhyolite, where crystals are visible. Rapidly cooling porphyritic rhyolite tends to have small fine grained crystals. Slower cooling porphyritic rhyolite tends to have larger grained crystals.
Photo 1 – Porphyritic Rhyolite
Photo 2 – Porphyritic Rhyolite
Photo 3 – Porphyritic Rhyolite
Another common rhyolite is flow banded rhyolite. Flow banded rhyolite cools so quickly that there is no time for crystal growth.
Photo 4 – Flow Banded Rhyolite
Photo 5 – Flow Banded Rhyolite
Photo 6 – Flow Banded Rhyolite
Along the contact zones, the common rhyolites above (porphyritic and flow banded) become altered by heat and pressure created along the fault producing some interesting combinations. Porphyritic rhyolite becomes re-melted and stretched out which elongates its crystals into swirly bands. The original rhyolite often becomes brecciated and the cracks and fractures fill in with new molten material.
Photo 7 – Altered Rhyolite
Photo 8 – Altered Rhyolite
Photo 9 – Altered Rhyolite
This beach has a lot of green rhyolite that I haven’t quite figured out what causes this variation, yet. The greens look like the outer portion of Mushroom Rhyolite, but the similarities stop there.
Photo 10 – Green Rhyolite
Photo 11 – Green Rhyolite
Photo 12 – Green Rhyolite
If your curious about where these rocks were found, look for A geologic Journey posted under Member Photographs.
150FromFundy