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Tortoises and Hares:
Dissolution, Erosion and Isostasy in Landscape Evolution
MJ Simms, Earth Surface Processes and Landforms, v29, p477, 2004
Manchester Town Hall
Silicate Rocks -Sandstones
A threshold velocity exists below which erosion does not occur
Characteristic block joints in limestone enlarged when mildly acidic rainwater causes solution weathering
e.g. The Burren, Co. Clare
Streams resurging from underground in limestone country.
Surface mechanical erosion of limestone is relatively unimportant. Most of the drainage occurs underground.
Little mechanical erosion occurs for much of the time
Lowering rates seldom exceed a few mm per year.
But limestone dissolution occurs most of the time
Paragenesis occurs in completely flooded conduits where the flow rates are too slow to transport particles. Simultaneous dissolution of the roof and deposition of fine sediment demonstrates the contrast between velocity-threshold-constrained particle movement and unconstrained dissolution
Denudation of limestone occurs whenever CO2 under-saturated water is in contact with the rock. Denudation of silicate rocks require high flow velocities.
Denudation is more continuous through time on limestone than on other rocks.
Ground and near-ground cover provides the greatest capacity for energy absorption
Change from forestry to agricultural land use can increase sediment yield by up to a factor of 20.
Little water, so mechanical weathering dominates dissolution.
Weathering is mainly by occasional high runoff events.
Limestones are often mechanically stronger than silicate rocks, so they tend to form high ground.
High rainfall, so limestone dissolution important, accentuated by vegetation.
Vegetation protects silicate rocks from mechanical erosion.
Long-term surface lowering rates for limestones may exceed those for silicate rocks.
Temperate, humid climate
Limestones weather faster than silicates
Considering Ireland’s climate, we expect to see limestone outcrops corresponding to lowlands and silicate outcrops corresponding to high ground.
Munster Basin - Old Red Sanstone
Uplands underlain by silicate rock
Lowlands underlain by limestone
Uplands underlain by limestone - Problem?
Much of this topographic relief can be ascribed to relatively fast rates of dissolutional denudation on limestone outcrops in comparison with erosional denudation across silicate outcrops.
The exceptions are where limestone has only recently been exhumed from beneath a silicate cover.
Hexagonal cooling joints in basalt
e.g. Giant's Causeway