Chapter 2B. Getting around in Geology

(Rocks and the rock cycle - Key Point)

• Rocks are consolidated natural composites, or aggregates that consist of various proportions of:

Minerals

• glass, and other Amorphous Materials.
• solidified organic material. e.g. coal.

• Change to the Book:

• The book, like most books, says that there are three classes of rocks, in fact, there are four:

1. Igneous (From Latin "ignis" = Fire):

Formed from natural molten material both by eruption on the Earth’s surface and by crystallization within the Earth.

Therefore:

i) Extrusive

ii) Intrusive

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2. Hydrothermal:

(The type not considered in the book, but incredibly important especially for economic mineral deposits (e.g. copper; gold))

Deposited by natural hot aqueous fluids, for example in mineral veins, or formed by reaction of natural hot fluids with different rock types.

e.g. Economic kaobinite clay deposits in S.W. England.

Hot:

Depending on the process, temperatures typically between +80 degrees Celsius and + 700 degrees Celsius, at the high T end. Mostly: about 100 degrees Celsius - 350 degrees Celsius.

e.g. Black smokers on the sea floor: about 250 degrees Celsius to 350 degrees Celsius.

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3. Sedimentary:

Formed by sediment deposition ("sedimentation") primarily on the sea floor, but also: in lakes, river systems, ice sheets/glaciers, even on land e.g. sand dunes.

Components:

1. Rock fragments including volcanic rock fragments.
2. Primary mineral fragments.
3. Minerals formed by chemical weathering ESP. clay minerals.
4. Materials synthesized biologically by organisms e.g. coral fragments.
5. Inorganic chemical crystallization:

e.g. CaCO₃ (calcium carbonate).

CaSO₄*2H₂O (gypsum).

NaCl (Halite)

KCl (Sylvite – exceptionally important; for the potash fertilizer industry).

Fossils: Sedimentary rocks are renowned for containing fossils - which can be incredibly detailed. Fossils preserve an amazing record of the evolution of life on Earth.

All other rocks (see above), incl. pre-existing metamorphic rocks, which have been recrystallized both mineralogically and texturally by:

1. High temperature.

(E.g. marginal to an igneous intrusion; at depth in the earth).

2. High pressure
3. Especially: solid state ductile (plastic) deformation. (In many, but not all cases.)

Example:

Core zones of mountain belts (e.g. Himalayas) exposed at surface: e.g Gravenhurst – Tweed- Ottawa – Quebec area.

Geologically called: The Grenville province; 1 billion yr. old.

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All these rock types are linked by the rock cycle, which reflects an intensely active outer earth, and the chemical and physical consequences of plate tectonics.

The Rock cycle is illustrated very well in fig. 2.13, P. 35. However, we need to make three principal modifications:

1. We must add a loop to show that metamorphic rocks can be re-metamorphosed
2. We must add hydrothermal rocks.
3. On P.32, it is stated that the rock cycle is a "closed system".

This is not true of the outer part of the earth, since:

1. New magma is added by partial melting of the upper mantle.
2. Material is removed back into the mantle by the plate tectonic process of subduction (see later).

If you included the whole mantle; the rock cycle would, essentially be closed.

Note: Magma = natural molten rock.

Lava = Surface product only i.e. eruptive flows.

1. Igneous (P.32-35 of class text): key points:

• As mentioned above, there is a primary subdivision for igneous rocks between:

1. Intrusive or Plutonic:

Crystallized from molten rock (=magma) within the earth. Hence, cooled moved and therefore coarse grained. e.g. Granite.

2. Extrusive, or volcanic:

Formed at surface by various eruptive mechanisms.

Another important sub-, sub-, division between:

• Non – fragmental e.g. lava flows (e.g. basalt), domes etc.
• Fragmental, also called pyroclastic.

i.e. Fragmented by various eruptive mechanisms.

e.g. Tuff – consolidated volcanic ash.

Therefore, cooled very rapidly in air, or water; therefore, fine grained - even glassy.

Classification of igneous rocks: (e.g P.32; ESP. fig. 2.15, P. 36.)

• Based on i) Texture i.e. Grain size and grain shapes.

ii) Mineral composition, which reflects chemical composition

1. Texture:

1. Fine – grained, or aphanitic:

Very fine grained; mainly volcanics, both non – fragmental, and fragmental:

Even: Glasses

e.g. Obsidian- A beautiful natural black glass, with a beautiful glass-type conchoidal fracture, and glassy, or vitreous lustre; quite SiO₂ rich (~ 70% SiO₂)

Pumice- Also SiO₂ rich: natural pale glass, but filled with gas cavities –so light it can float. A cooled down, glass "froth".

Submarine Pillow Lavas - basaltic magma composition (not so SiO₂ rich - ~ 50% SiO₂ ). Outer few cm can be black glass from rapid cooling in water.

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N.B. Particular texture:

Porphyritic: Large crystals in a fine grained ground mass - both extrusive and intrusive.

2. Coarse-grained, or phaneritic:

Cooled slowly; time for crystals to grow.

Especially intrusive igneous rocks; interiors of some thick flows or domes.

Intrusions: small-medium size: Plutons.

• Greater than 100 km²: Batholiths.

2. Bulk Mineral/ Chemical composition: (See Fig. 2.15, p.36.)

Felsic (L.H.S.; ~ 70% SiO₂ )----> intermediate

----> Mafic (in 50% SiO₂ ) ----> ultramafic

Aphanitic (mainly extrusive), and phaneritic (mainly intrusive) name equivalents in each category.

• Must add "Komatiite" as the aphanitic equivalent of "peridotite".
• Felsic: Rich in potassium feldspars and quartz.
• Mafic: Rich in calcium feldspars, pyroxenes and olivine.

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 <-----  SiO₂ content increases.

2. Hydrothermal rocks:

As mentioned above, natural hot water related; mainly at 100 degrees Celsius – 350 degree Celsius:

E.g. (i) Mineral vein systems:

E.g. certain Au and Ag deposits in ~ 1 m wide veins.

Criss- crossing ~ 0.1-5cm vein "networks" (=stockworks) in porphyry Cu, Mo and Au systems.

-Mineral deposition in "open space", originally fluid filled veins.

(ii) Formed by hot water/rock interaction i.e. replacive .

E.g. sulphide replacement of carbonates -> mantos.

Hydrothermal alteration of granitic feldspars to kaolinite clay (e.g. S.W. England); used for ceramics; very high quality paper coating and filling.

3. Sedimentary Rocks:

Hardened (lithified) sediment:

Deposition by marine water, river systems, glaciers, wind etc.

• Key Feature: All sedimentary rocks show layering referred to as bedding or stratification, at a very large range of scales: sub-mm in shales, to kilometers in sed. sequences.

• Table 2.3, P.38 shows the prime classification into:

1. Detrital or Clastic
2. Chemical
3. Biogenic

e.g. Lithified volc. ash = Tuff

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Classification of Sedimentary Rocks

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4. Metamorphic Rocks:

Mineralogically and texturally recrystallized by high T at high P +- (ductile)  plastic deformation.

N.B. Foliation: planar alignment of minerals.

Table 2.4 describes some common metamorphic rock types, (p.40).

E.g. Foliated:

Sedimentary shale or mudstone -------> slate

coarser (can see individual mineral grains ESP. MICAS.) -----> Schist

Coarse and compositionally banded -----------------> Gneiss

Non-Foliated:

e.g. Limestone ----------� Marble (e.g. First Canadian Place).

Sandstone ----------� Quartzite

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5. Finally Structural Geology:

Extremely important for civil and the different types of geological engineering (pp. 39-40, 367-368).

1. Weak Planar structures:

i) Bedding planes in sedimentary rocks.

ii) Foliation planes in metamorphic rocks.

2. Brittle Structures:

i) Without Displacement: Joints = Brittle Fractures (no displacement).

ii) With Displacement: Faults see Fig. 4.2 P.72 for the 3 principal types:

1. Normal
2. Reverse
3. Strike slip – right and left lateral.

Click here to see the different type of faults.

Relevance to civil and geological engineering:

Simple:

The 3D i) distributions and ii) orientations of these i) weakness planes, and ii) brittle structures (joints; faults) have enormous effects on bulk rock strengths in any surface or sub-surface excavations, and in foundation engineering.

The orientations of planar features in rocks are defined by dip and strike- very well explained in Appendix 3, P. 489.

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Click here to view appendix 3

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3. Ductile Structures:

E.g. Particularly FOLDS in sedimentary rocks (and metamorphic):

E.g. Convex up in normal sediments:

The anticline; one of the most important types of oil trap.

Convex Down in normal sediments:

Intervening synclines; to remember: syncline = "saucer".

See: Fig. 13.3, P. 368

Metamorphic Rocks:

"Younging" direction not known, usually:

Convex up = Anti-form

Convex down = Syn-form

Click here to view the common structural oil traps.

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Last modified: April 08, 1999