Limestone and Its Formation

Composition and Characteristics

• Hard, grey sedimentary rock
• Largely composed of calcium carbonate
• Formed under the sea from remains of organic matter (e.g., seashells, plants)

Structure

• Characterized by:
  • Vertical cracks (joints)
  • Horizontal cracks (bedding planes)
• Pervious rock: Allows water to flow through joints and bedding planes

Weathering Process: Carbonation

Carbonation Overview

• Main process that weathers limestone
• Reaction involves:
  • Calcium carbonate + carbon dioxide (from rainwater)
  • Forms carbonic acid
  • Carbonic acid reacts with limestone to produce calcium bicarbonate
• Calcium bicarbonate is washed away, gradually weathering limestone

Conditions Affecting Carbonation

• Higher concentration of carbon dioxide in soil than in the atmosphere due to soil microbes
• Rainwater percolating through soil increases carbonic acid production
• Ideal conditions for carbonation:
  • High rainfall
  • Warm temperatures
  • Lush vegetation

Karst Landscapes

Regions of Interest

• Best developed in:
  • Tropical Southeast Asia
  • Subtropical Southern China
• Example: South China Karst
  • Covers 600,000 km²
  • Features: pinnacle karst, large cave systems, spectacular tower karst
  • Requires:
    • Minimum 120 cm precipitation/year
    • Average temperature of 18°C

Definition

• Karstification: Process of water dissolving rocks like limestone, producing a karst landscape

Global Locations of Karst Landscapes

• Varying altitudes
• Example locations:
  • Pancake rocks at Pernikaiki, New Zealand (sea level)
  • High altitude karst in Sichuan, China

Features of Karst Landscapes

Caves

• Hollowed out by underground streams and carbonation
• Characterized by formations known as speleothems:
  • Stalactites: Hang down from cave ceiling
  • Stalagmites: Grow from cave floor up
  • Flowstone: Forms where water runs down cave walls or floors
  • Rimstone pools: Uniquely formed in alpine valleys

Underground Water Flow

• Rivers and streams often disappear underground, flowing through limestone
• Resurgence: Where underground stream re-emerges at the surface

Sinkholes (Dolines)

• Topographic depressions in limestone
• Vary in size (2-100 m deep; 10 m to 1 km in diameter)
• Formed by:
  • Cave ceiling collapse
  • Enlargement of natural fractures by solution

Polders

• Large basins with flat floors and steep walls but no surface outflow
• Formed by the union of several sinkholes
• Example: Lake Disappear, New Zealand (drained by an underground stream)

Gorges and Cliffs

• Ceiling collapse can create steep-sided gorges
• Cliffs (scars): Edge of limestone area, typically with a 90-degree angle

Karin Features

• General term for various small-scale solution features in limestone
• Common on limestone pavement (large exposed limestone rock areas)
  • Divided into clints (slabs) and grogues (vertical fissures)
  • Runnels: Channels eroded from limestone surface, draining into grogues
  • Pits formed by solution processes on cliff tops.