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

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Presentation Transcript
slide1
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

slide7
Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks

slide8
Old Red Sandstone

Manchester Town Hall

Silicate Rocks -Sandstones

Millstone Grit

denudation of limestones silicates
Denudation of Limestones & Silicates

Slow but steady wins the race

denudation of limestones silicates11
Denudation of Limestones & Silicates

Slow but steady wins the race

denudation mechanisms particle entrainment silicates versus dissolution limestones
Denudation MechanismsParticle Entrainment (silicates)versusDissolution (limestones)
slide14
Denudation of silicate rocks occurs largely through mechanical erosion, mainly by water and sometimes by ice
slide15
Denudation of silicate rocks occurs largely through mechanical erosion, mainly by water and sometimes by ice
slide16
Dissolution contributes to fragmentation of the rock but there is no significant volume change. Without erosion, the weathered material remains in situ.
slide17
Mere contact between rock and water is not enough to effect erosion. The sediment must be entrained.
slide18
Entrainment is a rate-limited process:

A threshold velocity exists below which erosion does not occur

slide22
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

slide24
Limestone Pavement:

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

e.g. The Burren, Co. Clare

slide25
Streams disappearing underground and resurging from underground in limestone country.
slide26
Malham Cove

Streams resurging from underground in limestone country.

slide27
Limestone (Karst) Scenery

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

slide29
Fault Rocks

Little mechanical erosion occurs for much of the time

slide31
Limestone dissolution equates to surface lowering.

Lowering rates seldom exceed a few mm per year.

slide32
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 timescales33

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 cover denudation inhibitor silicates versus denudation enhancer limestones
Vegetation CoverDenudation Inhibitor (silicates)versusDenudation Enhancer (limestones)
slide35
Vegetation forms a physical barrier between flowing water and rock/regolith beneath. It also absorbs the energy of impacting raindrops.
fault populations
Fault Populations

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

slide37
Vegetation aids development of soil, which protects underlying rocks from mechanical erosion.
slide38
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.

slide40
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
slide43
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.

slide44
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.

slide45
IRELAND

Temperate, humid climate

Limestones weather faster than silicates

implications for landscape development
Implications forLandscape Development

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

slide47
Galway granite

Leinster granite

Munster Basin - Old Red Sanstone

Uplands underlain by silicate rock

slide48
Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks

slide49
Central lowlands

Lowlands underlain by limestone

slide50
Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks

slide52
Leitrim & Fermanagh

Burren

Uplands underlain by limestone - Problem?

slide54
Millstone Grit & Coal Measures

Carb. Limestone

Old Red Sandstone

Limestone is sandwiched between clastic silicate rocks

slide55
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.

slide57
Ireland’s landscape

Hexagonal cooling joints in basalt

e.g. Giant's Causeway

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