Geologic Field Trip 1: Urban Geology

EPSC-201-001 Understanding Planet EarthAutumn 2017

A new field trip designed for your course lectures and textbook materials.

Design by:Rowan Wollenberg, Catherina Crotty – geological observations, mineralogyJacek Scibek – digital content, text, design of field guide

photos J.Scibek, unless otherwise notedmap GoogleMapsdiagrams Marshak S. (2013) Essentials of Geology, 4th Edition, W.W. Norton & Company Inc. NY. (e-book)

Oct 2, 2017, Earth and Planetary Sciences Department, McGill University, Montreal

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ChemistryBuilding

SchulichLIbrary

Library

Map of stops For Oct 3 urban geology walk.

(Google Maps)

Stop 1 Chemical weathering of limestone.

The limestone façade of the Schulich library has been partially dissolved by acid rain. Acid precipitation affects stone primarily in two ways: dissolution and alteration into other minerals.

The dissolution causes loss of detail on carvings or holes. The alteration causes black crust in some places.

The back side of Schulich Library

altered surfaces

Sheltered areas on limestone and marble buildings and monuments show blackened crusts, primarily composed of gypsum. Gypsum is normally white, but the crystals form networks that trap particles of dirt and pollutants, so the crust looks black. (US Geological Survey, https://pubs.usgs.gov/gip/acidrain/5.html)

Stop 2 Slate rock wall.

This wall was cut into thin slabs from a rectangular block cut from a rock quarry.

Can you see the small pyrite crystals?

Slate is a fine-grained, low grade metamorphic shale.

slate

slate

Slate has distinct foliation called slaty cleavage.

At a rock quarry, slate breaks along foliation creating sheets used for roofing (here it is used on a wall for decoration).

slate

Chapter 6 Pages of Earth’s Past: Sedimentary Rocks

Clastic Sedimentary Rocks• Fine clastics are deposited in quiet water settings (floodplains, lagoons, mudflats, deltas, deep-water basins –

deep sea)• Silt, when lithified*, becomes siltstone.• Mud, when lithified (made into rock), it becomes mudstone or shale.

Chapter 7 Metamorphism: A Process of Change

Slate has distinct foliation called slaty cleavage. It develops by parallel alignment of platy clay minerals.Slaty cleavage develops perpendicular to compression.

shale

slate

Stop 3 Anorthosite rock wall with Labradorite minerals.

Anorthosite is a felsic (feldspar/silica) igneous rock that cooled in a magma chamber

The size of the crystals tells us about the evolution of magma in a magma chamber. It gives us information about the conditions, and how rapidly/slowly it cooled.

at Café java u(Sherbrooke St.)

Labradorite is a type of feldspar. It shows iridescence.

You can see the iridescent, rainbow colours changing when you view the mineral from different angles. The iridescence is due to the scattering of light by minuscule lamellae that form during crystallization of feldspar crystals .

The wall block below is the same rock, with identical mineralogy, but it is not polished smooth making it difficult to see the minerals. A fresh rock outcrop would look more like this.

Chapter 4 Up from the Inferno: Magma and Igneous Rocks

Magma chambers are located in the upper crust. Some magmas will stall in the crust, and form an igneous intrusion.

Cooling rate—how fast does a magma cool? • Deeply situated plutons tend to lose heat very slowly,

cooling slowly.• Extrusive/surface lava flows lose heat more rapidly;

cooling quickly.• Shape—spherical bodies cool slowly; tabular faster. • Groundwater—circulating water removes heat

Changes with cooling• Fractional crystallization—early crystals settle by gravity.• Melt composition changes as a result.

Stop 4 Sandstone walls.

Sandstone is a silici-clastic rock made mainly of quartz, and some other minerals. Rocks that are buried deep and compacted, can become cemented by other minerals, such as calcite, which can flow through the porous sand as a fluid, depositing to form a calcite matrix.

Weathering of Iron (Fe) causes the red colour of the sandstone. You can see bedding planes and perhaps crossbedding in some of the sandstone blocks.

Can you see the sedimentary structures?

Baie (The Bay) building

Stop 5 Fossiliferous limestone, or packstone.

Biochemical Sedimentary Rocks:

Sediments derived from the shells of living organisms. Hard mineral skeletons accumulate after death.Calcite and Aragonite (CaCO3)—limestone.

Fossiliferous limestone, or packstone, contains visible fossil shells.

columns at the KPMG courtyard, behind the church.

almost everything here is a fossil – this was a pile of small shells that got cemented together during lithification

shell imprint edge of shell

shell

Stop 6 Granite on wall Place Montreal Trust buildilng, corner of Urban Behaviour store.

Plutonic or igneous rocks.

This granite is unusual. It is called Rapakivi granite. Rapakivi is Finnish for "crumbly rock“. Because of its mineral structure, it’s crumbly.

It contains large rounded crystals with "reaction rims". In other words, it tells us that something changed in the magma chamber, such as the temperature and pressure.

Stop 7 Metamorphosed granite into gneiss (granitic gneiss)

Originally an igneous rock, then metamorphosed.

Feldspar crystals are large. The rock is not folded, or mechanically altered.

Stop 8 More strongly metamorphosed and deformed granititc gneiss. At 2220 McGill College St. beside Second Cup cafe.

On the round pillar there is a ductile shear zone and folds are also visible in many places. The rock was folded by intense metamorphism.

Can you see the structure of the shear zone?

Chapter 7 Metamorphism: A Process of Change

Foliated Metamorphic Rocks

Gneissic banding develops in several ways.• Original layering in the protolith. • Extensive high temperature shearing.• Metamorphic differentiation: minerals segregate into different layers.

Stop 9 Xenoliths in granite (rock benches beside the library).

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In front of the library there are granite benches.

A few benches show darker spots of minerals with smaller grains. These are xenoliths that fell into the magma and got trapped in the solidified granite as the magma cooled.

xenolith

granite (former magma)

Assimilation• Magma melts the wall rock it passes through.• Blocks of wall rock (xenoliths) fall into magma.• Assimilation of these rocks alters magma

composition.

Stop 10 Minerals and fossils at a geological garden of Redpath Museum.