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The Irish Landscape. Tortoises and Hares: Dissolution, Erosion and Isostasy in Landscape Evolution. MJ Simms, Earth Surface Processes and Landforms , v29, p477, 2004 http://www.habitas.org.uk/staff_pdf_files/Tortoises.pdf. How can we explain this patttern of uplands and lowlands?.

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Tortoises and Hares: Dissolution, Erosion and Isostasy in Landscape Evolution

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The Irish Landscape

Tortoises and Hares:

Dissolution, Erosion and Isostasy in Landscape Evolution

MJ Simms, Earth Surface Processes and Landforms, v29, p477, 2004

http://www.habitas.org.uk/staff_pdf_files/Tortoises.pdf


How can we explain this patttern of uplands and lowlands?


Carboniferous Limestone underlies c. 40% of Ireland’s surface


Limestones were deposited in tropical seas

Puerto Rico


Many Dublin buildings built from Carboniferous Limestone


Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks


Old Red Sandstone

Manchester Town Hall

Silicate Rocks -Sandstones

Millstone Grit


Silicate Rocks - Granite


Denudation of Limestones & Silicates

Slow but steady wins the race


Denudation of Limestones & Silicates

Slow but steady wins the race


Denudation MechanismsParticle Entrainment (silicates)versusDissolution (limestones)


Silicate Rocks


Denudation of silicate rocks occurs largely through mechanical erosion, mainly by water and sometimes by ice


Denudation of silicate rocks occurs largely through mechanical erosion, mainly by water and sometimes by ice


Dissolution contributes to fragmentation of the rock but there is no significant volume change. Without erosion, the weathered material remains in situ.


Mere contact between rock and water is not enough to effect erosion. The sediment must be entrained.


Entrainment is a rate-limited process:

A threshold velocity exists below which erosion does not occur


Once entrained, sediment can be transported at relatively low speeds.


Limestones


Limestone is dissolved slowly by water. This process forms many different types of solutional sculpting, together called karren. Since water moves downwards under gravity, the limestone is dissolved downwards to form solution pits, solution runnels, the familiar grikes of limestone pavements, and even vast potholes sometimes hundreds of metres deep. 

Solution Runnels


Tube Karren


Limestone Pavement:

Characteristic block joints in limestone enlarged when mildly acidic rainwater causes solution weathering

e.g. The Burren, Co. Clare


Streams disappearing underground and resurging from underground in limestone country.


Malham Cove

Streams resurging from underground in limestone country.


Limestone (Karst) Scenery

Surface mechanical erosion of limestone is relatively unimportant. Most of the drainage occurs underground.


Denudation Timescales


Fault Rocks

Little mechanical erosion occurs for much of the time


But when flow speeds are high, several metres may be eroded in a single event.


Limestone dissolution equates to surface lowering.

Lowering rates seldom exceed a few mm per year.


Paragenetic Cave Passages

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 Timescales

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.


Vegetation CoverDenudation Inhibitor (silicates)versusDenudation Enhancer (limestones)


Vegetation forms a physical barrier between flowing water and rock/regolith beneath. It also absorbs the energy of impacting raindrops.


Fault Populations

Ground and near-ground cover provides the greatest capacity for energy absorption


Vegetation aids development of soil, which protects underlying rocks from mechanical erosion.


Effect of deforestation and cultivation illustrates importance of soil.

Change from forestry to agricultural land use can increase sediment yield by up to a factor of 20.


Typical soft Irish day in Dingle


Stalagtites & stalagmites of redeposited calcite in cave passages at shallow depth beneath soil and vegetation covered limestone shows that water percolating through the soil still dissolves the limestone


Effect of Climate


Arid Climates

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.


Temperate Humid Climates

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.


IRELAND

Temperate, humid climate

Limestones weather faster than silicates


Implications forLandscape Development

Considering Ireland’s climate, we expect to see limestone outcrops corresponding to lowlands and silicate outcrops corresponding to high ground.


Galway granite

Leinster granite

Munster Basin - Old Red Sanstone

Uplands underlain by silicate rock


Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks


Central lowlands

Lowlands underlain by limestone


Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks


But . . .


Leitrim & Fermanagh

Burren

Uplands underlain by limestone - Problem?


The Burren, Co. Clare - An upland region


Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks


Almost all of Ireland’s high ground corresponds with sillicate outcrops.

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.


Ireland’s landscape

Hexagonal cooling joints in basalt

e.g. Giant's Causeway


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