2.2.3 Climate, sediment and the mismatch of sedimentary deposits with latitude
The climate of modern Earth may be divided into different belts that have cold arctic conditions at high latitudes and hot tropical conditions at equatorial and low latitudes. The nature and style of rock weathering and erosion varies according to these climate belts, such that glaciation and freeze-thaw action predominate at present-day high latitudes, whilst chemical alteration, aeolian and/or fluvial processes are more typical of present-day low latitudes. Once a rock is weathered and eroded, each climatically controlled suite of processes gives rise to its own type of sedimentary succession and landforms:
sand dunes form in hot, dry deserts
coal and sandstone successions form in tropical swamps and river deltas
boulder clay deposits and 'U-shaped' valleys form where there are ice sheets and glaciers.
It has long been recognised that geologically ancient glacial-type features are not just restricted to the present-day, high-latitude locations, but also occur in many warm-climate continents such as Africa, India and South America. Similarly, warm-climate deposits may be found in northern Europe, Canada and even Antarctica. For instance, coal is one of our most familiar geological materials, yet the European and North American coal deposits are derived from plants that grew and decayed in hot, steamy tropical swamps 320-270 Ma ago during the late Carboniferous and early Permian Periods. Reasons for these unusual distributions are often provided by reconstructing the ancient continental areas and determining their original positions when the deposits or landforms were created.
Late Carboniferous coalfields are found in northern Britain around latitude 55°N. If these coals formed from plants that grew in the tropics between 23°N and 23°S, what is the minimum distance Britain has travelled in 300 Ma? At what rate has it travelled (in mm y−1)? (Assume the radius of the Earth is 6370 km.)
The coalfields formed from plants that grew in the tropical and sub-tropical climate belts. They must have drifted to their present positions from these latitudes either south or north of the Equator (i.e. 23°S-23°N).
This represents a latitude drift of between 23°S to 55°N (i.e. 78°) and 23°N to 55°N (i.e. 32°).
If the Earth's radius is 6370 km, its circumference must be 2 × 6370 km=40 024 km.
1° of latitue (assuming the Earth is a sphere) is therefore = 1 11 km
Therefore the minimum distance that Britain can have drifted since the late Carboniferous is 32 × 111=3552 km (i.e. about 3500 km at the level of accuracy of this information), which gives a rate of:
≈ 12 mm y−1 or 10 mm y−1 to 1 sig. fig.
The maximum distance is at least 8672 km (i.e. about 8700 km), which gives a rate of:
≈ 29 mm y−1 or 30 mm y−1 to 1 sig. fig.
Both these values could be larger if Britain drifted in terms of longitude as well - in other words, if its course was not in a straight line.