Countertop geology: sheared granite

Today's countertop geology was brought to you by friends in Queensland. They have the most lovely granite table on their verandah and the fed me delicious Canadian breakfast foods while I took these photos.

This granite has been sheared and the phenocrysts are surrounded by almost sigma-shaped matrix. The picture below shows a grain that is probably at right angles to shear direction and as such hasn't rotated like some of the others.

Places I've been: Organ pipes National Park, Victoria, Australia

 

Looking east at the leading edge of the organ pipes structure.

The second stop on our Igneous Geology field trip is the Organ Pipes National Park. As this was part of our first date, I've already posted about this location so won't go into much more detail. I will however show to pictures of new things I got to see this time:

 

Basaltic columns near the edge of Jacksons Creek just past the Tesselated Pavement. Hexagonal columns are created by the contraction of the cooling lava cracking at approximately 120 degrees. These cracks propagate downward as the body of lava cools creating columns.

 

The contact between the basalt and the underlying sediments. The white line is the metamorphised sedimentary rock, and is about 5mm thick.

 

Just below the contact, something I've never noticed before!

 

A conglomerate containing rounded quartz pebbles (1cm) and large angular sandstone cobbles (up to 10cm). This indicates the rock is not the Ordovician sediments but reworked into terrestrial sediments from the Eocene.

Places I've been: You Yangs granite, Victoria, Australia

It's that time of year again - yes the end of May. This comes with the inevitable two or three weekends of teaching the 1st year geology igneous field trip. Our first stop is the You Yangs granite. One of the biggest, and closest examples of a granite to CBD Melbourne (WGS84 37° 57′ 0″ S, 144° 25′ 48″ E), it towers over the basaltic plain to the west of the city. On a clear day the views are lovely, and you can even just see the Menzies building towering over Clayton in the far distance.

The You Yangs are a small granite body, more of a stock than a batholith. The granite is rich in quartz, oligoclase and orthoclase with minor mafic minerals such as biotite and hornblende. The You Yangs are porphyritic with large K-feldspar (orthoclase) crystals up to 7cm long and a course grained groundmass. The granite also contains numerous xenoliths and enclaves.

 

A granitic tor located behind the picnic ground bathrooms.

 

An enclave in the tor - note the concentration of dark minerals and the presence of large K-feldspar crystals suggesting a similar chemistry to the granite. This is a large example of an enclave and spans 25cm. Some of the enclaves I've seen in this granite are 60cm wide.

 

A close up of the side of the granite

 

Closer still - a xenolith is bisected by two aplite veins. Xenoliths are pieces of country rock that have been surrounded by the magma during intrusion. They are then metamorphised by the heat produced by the cooling granite. The xenoliths seen in the You Yangs are typically hornfels, formed from metamorphising the Ordovician siltstones and sandstones (marine). Aplite is a quartz rich vein with small amounts of feldspar. It is thought to be the residual melt that solidifies in the cracks formed in the cooling granite.

 

Pseudofossils - 12 Apostles, Victoria

I spotted some beautiful faux fossils in the sandy-limestone used to make the underpass at the 12 Apostles while I was there with the JMSS geology class a month ago. These dendritic patterns are formed by the deposition of minerals by hydrothermal fluids. They are often mistaken for plant fossils by the geologically-uneducated. Still, they're quite pretty.

 

Sandy-limestone? in the underpass at the 12 Apostles, Victoria.

How I feel about geology

 

Medium grained, feldspathic sandstone from the Otway Basin with a tiny bit of coal.

Fancy folds - Chile

 

A fold in Rio Leon Park, Chile. (-52.740004, -72.148885) Fold is approximately 2.8km wide.

 

Parasitic folds near the hinge - wavelength of ~250m

 

Second fold to the east (400m wide), faulted off the main complex?

Counter top geology: 4/2/2015 - Galaxy 'granite', Andhra Pradesh, India

Galaxy Granite is mined in various places in India such as Chimakurthi. Though it is marketed under the term 'granite' because it is hard and dark it is actually Gabbro. Light coloured and soft rocks are almost always marketed under the term Marble. The Galaxy Granite is unusual because it contains small crystals of bronzite which give it a bright sparkly effect and as such is occasionally marketed as Star Granite. Massive shout out to Wendy Kirt and David Cook who published Graveyard Geology.  It isn't amazing in a photograph but would like great as a counter or some other decorative stone.

 

Photo from Stone Network.