On 17/02/2011 ajfclark wrote:
>On 17/02/2011 evanbb wrote:
>>The volcanic rock is too soluble and doesn't form proper canyons.
>
>Do you mean insoluble?
Nope. Weathering in sandstone is almost entirely mechanical. Weathering in volcanics is chemical; hence the trenchy cracks in granite. It's much younger rock, which fractures while cooling. Water flows down the surface so the whole thing dissolves. In sandstone the weathering only occurs at the bottom of the crack where the sand is tumbling along in a stream. Hence, it saws straight down.