Third order Variscan folds
Location: Garretstown, Co. Cork, Ireland
Description: In the Late Palaeozoic (approximately 250 million years ago), uplift and compression during a prehistoric mountain building event called the Variscan orogeny deformed sedimentary successions into a series of folds. The Variscan orogenic belt in southern Munster is characterised by folds occurring on all scales. First order folds have wavelengths in excess of 10 km, second order folds typically have wavelengths less than 5 km, while third order folds have metre scale wavelengths. The photograph shows an example of asymmetrical third order folds which verge to the west (left on the photograph). In structural geology, vergence can be observed and recorded in folds to help geologists determine characteristics of larger (second or first order) folds, which are abundant in southern Munster.
Lewis Overthrust in the Rocky Mountains
Location: Glacier National Park, Montana, USA
Description: The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains found within the bordering national parks of Glacier in Montana, United States and Waterton Lakes in Alberta, Canada. The Lewis Overthrust was created due to the collision of tectonic plates about 170 million years ago. Due the nature of this prehistoric mountain building event, the 600 million year old Precambrian rocks shown at the top of the mountain on the photograph are 400-500 million years older than the Cretaceous rocks at the base of the mountain.
Tongariro and Ngāuruhoe volcanoes
Location: Tongariro National Park, North Island New Zealand
Description: Mount Ngāuruhoe is a volcanic cone in New Zealand. It is the youngest volcanic vent in the Tongariro stratovolcano complex on the Central Plateau of the North Island and first erupted about 2,500 years ago. Although often regarded as a separate mountain, geologically, it is a secondary cone of Mount Tongariro. The photograph was taken from the crater of Mount Tongariro looking toward Mount Ngāuruhoe.
Megacrystic Hornblende Diorite, New Zealand
Location: Fiordland, South Island New Zealand
Description: A megacryst is a crystal that is considerably larger than the encircling matrix. Hornblende is a rock-forming mineral that is an important constituent in igneous rocks. A diorite is a coarse-grained igneous rock which is intermediate in chemical composition between felsic and mafic. The photograph shows rare, very large, dark-coloured hornblende crystals in a light-coloured plagioclase-rich matrix. Its large crystals indicate that it cooled very slowly, likely over hundreds or thousands of years, which only occurs where molten rock is insulated from the surface by overlying layers of solid rock.
