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Tortoises and Hares: Dissolution, Erosion and Isostasy in Landscape Evolution The Irish Landscape MJ Simms, Earth Surface Processes and Landforms, v29,

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Presentation on theme: "Tortoises and Hares: Dissolution, Erosion and Isostasy in Landscape Evolution The Irish Landscape MJ Simms, Earth Surface Processes and Landforms, v29,"— Presentation transcript:

1 Tortoises and Hares: Dissolution, Erosion and Isostasy in Landscape Evolution The Irish Landscape MJ Simms, Earth Surface Processes and Landforms, v29, p477, 2004

2 How can we explain this patttern of uplands and lowlands?

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

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5 Limestones were deposited in tropical seas Puerto Rico

6 Many Dublin buildings built from Carboniferous Limestone

7 Limestone is sandwiched between clastic silicate rocks Carb. Limestone Old Red Sandstone Millstone Grit & Coal Measures

8 Silicate Rocks - Sandstones Millstone Grit Old Red Sandstone Manchester Town Hall

9 Silicate Rocks - Granite

10 Denudation of Limestones & Silicates Slow but steady wins the race

11 Denudation of Limestones & Silicates Slow but steady wins the race

12 Denudation Mechanisms Particle Entrainment (silicates) versus Dissolution (limestones)

13 Silicate Rocks

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

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

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

18 Entrainment is a rate-limited process: A threshold velocity exists below which erosion does not occur

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

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21 Limestones

22 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

23 Tube Karren

24 Limestone Pavement: Characteristic block joints in limestone enlarged when mildly acidic rainwater causes solution weathering e.g. The Burren, Co. Clare

25 Streams disappearing underground and resurging from underground in limestone country.

26 Streams resurging from underground in limestone country. Malham Cove

27 Surface mechanical erosion of limestone is relatively unimportant. Most of the drainage occurs underground. Limestone (Karst) Scenery

28 Denudation Timescales

29 Fault Rocks Little mechanical erosion occurs for much of the time

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

31 Limestone dissolution equates to surface lowering. Lowering rates seldom exceed a few mm per year.

32 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 Paragenetic Cave Passages But limestone dissolution occurs most of the time

33 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.

34 Vegetation Cover Denudation Inhibitor (silicates) versus Denudation Enhancer (limestones)

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

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

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

38 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.

39 Typical soft Irish day in Dingle

40 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

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42 Effect of Climate

43 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. Arid Climates

44 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 Climates

45 IRELAND Temperate, humid climate Limestones weather faster than silicates

46 Implications for Landscape Development Considering Ireland’s climate, we expect to see limestone outcrops corresponding to lowlands and silicate outcrops corresponding to high ground.

47 Uplands underlain by silicate rock Galway granite Leinster granite Munster Basin - Old Red Sanstone

48 Limestone is sandwiched between clastic silicate rocks Carb. Limestone Old Red Sandstone Millstone Grit & Coal Measures

49 Lowlands underlain by limestone Central lowlands

50 Limestone is sandwiched between clastic silicate rocks Carb. Limestone Old Red Sandstone Millstone Grit & Coal Measures

51 But...

52 Uplands underlain by limestone - Problem? Burren Leitrim & Fermanagh

53 The Burren, Co. Clare - An upland region

54 Limestone is sandwiched between clastic silicate rocks Carb. Limestone Old Red Sandstone Millstone Grit & Coal Measures

55 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.

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57 Hexagonal cooling joints in basalt e.g. Giant's Causeway Ireland’s landscape


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