Ch. 02 Flashcards
Which rock group among igneous, sedimentary, and metamorphic is emphasized for studying the Earth’s history
Sedimentary rocks.
How are sedimentary rocks described in terms of deposition?
Deposited in layers, with the youngest at the top.
What crucial elements do sedimentary rocks contain that aid in studying past geological times?
Fossils, representing the life of past geological times.
According to James Hutton’s principle of uniformitarianism, what is the key to understanding events recorded in rocks?
The present-day activities of geological agents.
Briefly explain the principle of uniformitarianism.
‘The present is the key to the past’; events recorded in rocks can be understood by reference to present-day geological activities.
Who is regarded as the founder of modern geology and who introduced the principle of uniformitarianism?
James Hutton.
What term did Fuchsel introduce for a layer of sedimentary rock?
Stratum.
What geological principle suggests that in an undisturbed series of beds, the bottom stratum is the oldest, and successively younger beds lie upon it?
Order of superposition.
Who noted that layers of rock could be traced across the country and described them as resembling ‘superimposed layers of bread and butter’?
William Smith.
What term is used to describe layers of rock that can be traced across country with the same order of superposition?
Strata.
Explain the concept of ‘way-up’ in determining the orientation of beds in a sequence.
It involves observing internal structures, such as current-bedding, graded-bedding, included fragments, and fossils.
What is current-bedding, and how does it appear in sedimentary rocks?
Tops of beds are truncated by younger beds due to current action.
Describe graded-bedding and how it forms in sedimentary rocks.
Grains of different sizes settle at different velocities, with coarse grains at the bottom grading to finer grains at the top.
What are included fragments, and how do they contribute to understanding the sequence of beds?
Inclusions of rock, like pebbles, derived from an older formation; they indicate relative age.
How do fossils help in determining the relative age of strata?
Fossils found in strata provide clues about their relative age.
In the idea of superposition, what does the bottom stratum in an undisturbed series of beds represent?
The oldest or earliest formed.
What does the stratigraphical column represent in geological terms?
The sequence of rocks formed during geological time, listed in order of age.
Where are the oldest rocks placed in the stratigraphical column, and where are the youngest rocks found?
Oldest rocks are at the base, and the youngest rocks are at the top.
What are Periods in the stratigraphical column, and how are they named?
Periods are groupings of rocks named after areas or characteristics, such as the Cambrian (after Cambria, Wales) and the Carboniferous (referring to coal-bearing rocks).
Explain how the Carboniferous and Cretaceous Periods are named.
Carboniferous refers to coal-bearing rocks, and Cretaceous refers to rocks including chalk (Latin creta).
What are some examples of Periods named after areas where they were first studied?
Cambrian (after Cambria, Wales), Ordovician, and Silurian (after the territory of the Ordovices and the Silures, ancient tribes of Wales).
What is the Permian named after, and where is it located?
Named after Perm in Russia.
From what source are the absolute ages of many rocks derived, according to the text?
Radiometric dating.
What is an unconformity, and what does it represent in geological terms?
An unconformity is a junction where one series of strata lies upon an older series with a separation surface between them. It represents an interval of time when deposition ceased, denudation occurred, and uplift took place.
Explain the term “orogeny” and its relation to geological time.
Orogeny refers to mountain-building activity. Major periods of mountain building are indicated in geological time.
What are fold-mountain belts, and how are they characterized after denudation?
Fold-mountain belts are areas where rocks have been thrown into complex folds. After denudation, they are zones of instability in the crust, or mobile-belts.
Define epeirogenic and orogenic movements, and how are they related to the lithosphere plates?
Epeirogenic movements involve changes in the position of continents and are relatively stable. Orogenic movements are related to mountain-building. Both are linked to changes in the relative positions of plates of the lithosphere
What is a large, elongate trough of subsidence where marine sediments accumulate over a long period?
Geosyncline.
Where are marine sediments deposited on the subsiding floor of the geosyncline?
On the subsiding floor of the trough.
During the downwarping of the geosyncline, what type of sediment is poured into the trough after the marine sediments?
Coarser sediment derived from the rapid weathering of nearby land areas.
What process adds igneous material to the accumulation in the geosyncline?
Volcanic activity.
What happens to the basement of the geosyncline as volcanic activity occurs?
The basement is dragged down into hotter depths.
What geological process follows the accumulation of sediment in the geosyncline, involving the crumpling and breaking of the contents?
Thrusting.
What occurs to the sides of the geosyncline as the thrusts take place?
The sides begin to move together and are heaved up to form a range of mountains on the site of the earlier trough.
In summary, what is the entire process in the geosyncline, leading to the formation of mountain ranges?
Deposition of marine sediments, addition of coarser sediment, volcanic activity, dragging of the basement, sediment concentration, and finally, thrusting and mountain formation.
What does the ancient rock assemblage of the Precambrian represent in terms of Earth’s history?
3600 million years, comprising all rocks older than the Cambrian.
How would you illustrate the immense duration of Precambrian time in relation to the Earth’s age?
If the Earth’s age is 1 hour, then the Precambrian would occupy 52 minutes.
What types of rocks largely make up the Precambrian, including virtually undisturbed sedimentary deposits?
Igneous and metamorphic rocks, with virtually undisturbed sedimentary deposits in places.
Where can sedimentary structures similar to those formed in present-day deposits be observed within Precambrian sediments?
In Canada, Norway, South Africa, and Australia.
What does the presence of glacial deposits in Canada, Norway, South Africa, and Australia demonstrate about Precambrian times?
Glacial conditions developed more than once in Precambrian times.
Give an example of preserved varved clays in Finland and their deposition time.
Varved clays, now metamorphosed, were deposited 2800 million years ago in a glacial lake in Finland.
How were many of the metamorphic gneisses formed, and where are they now visible?
They were formed at depth below the surface under high temperature and pressure, and they are now visible in areas where uplift and erosion have exposed once deeply buried rocks.
What are the large continental areas where Precambrian rocks are close to the present-day surface called?
Shields.
What are shields, in terms of geological stability and orogenic folding?
Shields are old, stable parts of present continents that have not been subjected to orogenic folding since the end of Precambrian times.
Where are diamonds mined from the Precambrian?
Kimberley, South Africa, and in Brazil.
What is the oldest group of rocks in N.W. Scotland, named after the Hebridean island of Lewis?
Lewisian rocks.
What is the age range of the Lewisian rocks in N.W. Scotland?
3000 to 1700 million years old.
How many periods of mountain building have deformed the Lewisian rocks, and what types of rocks do they consist of?
Deformed by at least two periods of mountain building, consisting of metamorphic rocks cut by igneous intrusions (dykes).
Which series overlies the Lewisian rocks unconformably in N.W. Scotland, and where is it named after?
Torridonian series, named after Loch Torridon.
Describe the Torridonian sediments in N.W. Scotland.
Answer: They consist of brown sandstones with subordinate shales and are 800 million years old.
What geological feature fills hollows in the Precambrian topography of the Lewisian land surface in N.W. Scotland?
Boulder beds at the base of the Torridonian formation.
What group, known as Moinian, comprises upper Precambrian granulites and schists in N.W. Scotland?
The Moinian group.
