Burial, Compaction & Cementation
1. Sediment deposits are most likely to become deeply buried and compacted if:
The area where they are deposited is slowly subsiding (sinking)
What do we call the material that holds the grains together in sedimentary rocks?
Cement
3. What is meant by the porosity of a rock?
The amount of space between the grains
4. What happens to the porosity of sediments during compaction?
Decreases
5. What happens to the porosity of sediments during cementation?
Decreases
Crystallizion of Magma
1. What difference is there between the composition of Lava and Magma?
Magma contains dissolved gases and lava does not
2. Which type of magma is most likely to erupt effusively from a shield volcano?
Magma that has already partly crystallised below ground
3. An igneous rock that is fine-grained or glassy must have:
Cooled rapidly after erupting from a volcano
4. A sheet-like intrusion formed as magma pushes in between layers of rock is called:
a Sill
5. A pyroclastic flow is formed when an explosive eruption produces a dense mixture of:
Hot gases and volcanic ash (shattered rock and pumice
Deformation & Metamorphism
1. The texture of a metamorphic rock like the one shown in the photo would be described as:
Foliated
2. Movement along faults in the Earth’s crust causes:
Earthquakes
3. Folds and thrusts usually form when the Earth’s crust undergoes:
Compression (squashing)
4. Metamorphic rocks rarely contain fossils because:
Re-crystallisation of the rock usually destroys fossils
5. When a rock is heated and re-crystallised close to an igneous intrusion, the effect is called:
Contact metamorphism
Deposition of Sedidment
1. Cross-bedded sands, like those shown in the photo, are NOT likely to have been deposited by:
Ice
2. A mixture of fine clay and larger rocks deposited by a glacier is known as:
Boulder clay
3. When a river “bursts its banks”, fine-grained sediments are deposited in the:
Floodplain
4. Calcium carbonate, dissolved in water, may be deposited as limestone by:
Living organisms
5. Large boulders of rock can be transported and deposited onto the ocean floor by:
Icebergs
Erosion & Transport
1. Mass-wasting is a process that produces:
Landslips
2. U-shaped valleys, like the one in the photo, are formed by:
Ice
3. Which of the following is not a way that rivers transport sediment?
Subduction
4. Most erosion by the sea results from the action of:
Waves
5. In which type of environment would you be most likely to observe wind erosion?
A hot desert
Igneous Rocks
1. “Rocks that have formed by cooling of molten magma are called igneous rocks. All these rocks have …………… textures.” The missing word is:
Crystalline
2. What property of granite makes it a good material to use for gravestones?
It is hard and resistant to weathering
3. What property of basalt makes it a good material to use for road surfaces?It is hard and resistant to weathering
4. What property of gabbro makes it a good material to use for worktops?
It can be polished to look attractive
5. Igneous rocks like granite are often used for street paving and kerb stones in city centres because:
They are hard and do not wear away easily
Metamorphic Rocks
Whilst the idea of a limestone re-crystallising at high temperature to form a marble (with the same essential mineralogy) is fairly simple, the changes undergone by most rocks containing silicate minerals are complex, leading not only to changes in rock texture, but also the formation of new minerals. These new minerals result from chemical reactions that take place in the solid state (i.e. not involving melting) and are isochemical.
Whilst the main controls are the temperatures & pressures that they are subjected to, reactions between minerals are greatly assisted by the presence of even the tiniest quantities of water – either present in the original rock or released by the breakdown of hydrous minerals at high temperature.
The bulk chemistry of metamorphic rocks is thus similar to their igneous or sedimentary parents, though their actual mineralogy may be completely different.
Take, for example, the changes that occur when a typical mudstone is metamorphosed.
Clay minerals in the mudstone are hydrous alumino-silicates which, when heated, react to produce micas and quartz.
Other alumino-silicates that appear at higher grades include
• garnet
• andalusite
• kyanite
• feldspar, which form at the expense of some of the mica.
As these new minerals appear, the texture of the rock also becomes gradually coarser, with an increasing tendency for minerals to separate into light and dark bands – but still the overall rock composition is chemically the same as the mudstone we started with.
This last point is vital to geologists in trying to unravel the past history of metamorphic rocks.
1. “Many metamorphic rocks re-crystallised deep in the Earth’s crust as they were heated and squashed up by immense forces. As a result, these metamorphic rocks often have ……………… textures”. The missing word is:
Foliated
2. What property of slate makes it a good material to use for roofing?
It splits easily into thin sheets
3. What property of marble makes it a good material to use for work-tops?
It can be polished to look attractive
4. What property of gneiss makes it a good material for sea-defences?
It is hard and resistant to erosion
5. What property of marble makes it vulnerable to weathering processes?
It can be dissolved by weak acids
Sedimentary Rocks
The huge variety of sedimentary rocks is due to the many ways that sediment can be derived, and the wide range of environments in which it may be deposited.
Features common to most sedimentary rocks are their occurrence in layers (beds or strata), and the occasional occurrence within them of fossils.
Sedimentary rocks are usually classified on the basis of their mineral composition and grain size, which in turn relates to the origin of the sediment:
Sediment origin:
Fragmental (or clastic) sediments are made up of particles of weathered rock, including rock fragments, residual mineral grains (especially quartz, which is both hard and chemically inert) and clay minerals.
Examples include: conglomerate, sandstone, mudstone & shale.
Organic sediments are, as the name suggests, formed largely or entirely of sediment generated by living organisms (e.g. shell or plant material). The resulting rocks include many limestones (e.g. shelly limestone, chalk); and also coal.
Chemical sediments, (or evaporites) are formed by direct chemical precipitation, typically in hot climates.
These include:
• some limestones (e.g. oolitic limestone)
• gypsum
• halite (rock salt).
Grain size:
A simple classification of coarse-, medium- and fine-grained is commonly used, with boundaries at 2mm and 1/16mm, thus effectively defining all sanstones as medium-grained, even though it is very hard to see grains in fine sandstones with the unaided eye.
Note that different boundary sizes (2mm & 5mm) apply to crystalline (igneous & metamorphic) rocks.
Limestones are often re-crystallised and it is frequently hard to decide on the original grain size of the sediment.
Mineral composition:
The mineral composition of sedimentary rocks (both grains and cement) is immensely variable, and of most use in sub-classification of the broader rock groups described above.
Some terms, such as calcareous for any rock having a significant content of calcium carbonate (grains or cement) are useful but detailed classification is beyond the scope of these web pages.
1. Which one of the following sedimentary rocks fizzes with dilute hydrochloric acid?
Limestone
2. Which of the following rocks is used for making bricks?
Mudstone
3. Which of the following rocks is used to make cement?
Limestone
4. Many sedimentary rocks are good building stones because:
They can be cut and carved to make large blocks
5. What type of soils would be treated with limestone to improve crop yields?Acid soils
Uplift & Geological Time
1. Which of the following provides the best evidence that rocks have been uplifted?
Fossil sea-creatures found in rocks on a mountain-top
2. The cliff face in a quarry is made up of layers of limestone at the top and sandstone at the bottom. Both the limestone and the sandstone are cut through by a basalt dyke (igneous intrusion). Which is the youngest rock (the most recently formed)?
Basalt
3. Which of the following can be helpful in telling us the age of a sedimentary rock?
The type of fossils that it contains
4. Conglomerate is a rock made up of rounded pebbles that have become cemented together. A sample of conglomerate from Scotland contains pebbles of granite. Which of the following is NOT true?
The granite pebbles are younger than the conglomerate
5. The age of the Earth is thought to be about:
4.6 billion years
Weathering
Biological weathering:
Some scientists prefer not to use this term as it describes both physical and chemical processes that are simply brought about, or assisted, by the presence of living organisms.
Thus, a tree root helping to prize a rock-joint apart is a physical process, whilst the action of acid secreted by a rock-boring piddock shell is a chemical process.
For all that textbooks tend to quote such large-scale examples of “biological weathering”, it is actually the biochemical action of microbes (bacteria, algae etc.) that contribute most to the breakdown of rocks and are crucial in soil-forming processes.
Chemical weathering:
To most of us, the idea of “chemistry in action” means stinks, bangs or, at the very least, colour changes that occur before our eyes.
Weathering processes are very slow however, so only their results are observable for most processes.
The exception to this rule is the dissolution of limestone by a weak solution of carbon dioxide, which may be readily shown in the lab
Hydrolysis is by far the most important (but also perhaps the most complex) weathering process, as it is by this means that most silicate minerals – apart from quartz (SiO2) – are broken down to form clay minerals and soluble salts.
Silicate minerals are the main constituents of igneous and metamorphic rocks, hence quartz and clay are by far the most common constituents of sedimentary rocks.
Physical weathering:
The simplest, of these processes is freeze-thaw weathering; its power is also easily demonstrated by freezing water in a glass bottle, sealed into a plastic bag. The bottle should burst as the water expands on freezing to form ice, with a 9% expansion.
Note that plastic bottles rarely burst and broken glass bottles should be handled with extreme caution – the old-fashioned cast-iron tubes are still best, but expensive!
Exfoliation weathering (expansion & contraction of mineral grains) can also be demonstrated using granite chips.
However, the reality of this process is actually much more complex, as it usually involves microscopic-scale chemical weathering by dew-fall that also soaks into the rock surface; evaporation then leads to the crystallisation of salts; these crystals gradually prize rock grains apart as they grow, making the surface layers of rock expand and break away.
1. Water has the power to split rocks because when it freezes, it
Expands
2. Which of the following sedimentary rocks is commonly weathered by solution?
Limestone
3. Chemical weathering is most likely to take place in environments that are:
Wet
4. In which type of environment would you be most likely to observe Biological weathering?
A rocky sea-shore
5. A common mineral group produced by chemical (hydrolysis) weathering is:
Clay
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