Lesson 5, lithology

What are some examples of how rocks react to chemical weathering?

Affects resistance of the rock.

Calcite, found in limestone, weathered by acidic rain, turns into solution

Quartz, found in sandstone, not affected by chemical weathering.

What is the difference between clastic and crystalline rock?

Affects resistance

Clastic-sedimentary rocks like sandstone

Crystalline- igneous and metamorphic rocks made of interlocking crystals, more resistant

What are some examples of igneous rock types?

Granite, Basalt, Dolerite

very slow erosion

Crystalline so more resistant

Have few joints and limited areas of weakness to exploit

What are some examples of metamorphic rock types?

Slate, Schist, Marble

Slow erosion

Can be crystalline

Can exhibit foliation, all crystals in the rock could be facing one direction, makes vulnerable to erosion/damage

Often folded and heavily fractured, exploited by erosion

What are some examples of sedimentary rock types?

Sandstone, limestone, Shale

Moderate to fast erosion

Most are clastic, so made of cemented sediment particles, if the cement is weak (eg calcite) then the rock will be weak.

Age is important, young rocks tend to be weaker (boulderclay newest rock as fresh deposits and the weakest rock)

Rocks like shale with bedding planes and fractures are vulnerable to erosion.

What are the Lithology EXCEPTIONS?

Some limestones are crystalline sedimentary rocks which are resistant to erosion.

Some ancient sandstones have been compressed and compacted over hundreds of millions of years, makes them very resistant

Recently erupted lava, ash and layers of tephra are easily eroded

Weakest rock is unconsolidated sediment.

What are the 3 ways lithology affects the rate of erosion?

Bedrock lithology, permeability, mineral content

Describe what bedrock lithology means

Bedrock= the rocks that make up the base of the land, these rocks are igneous and metamorphic

Igneous: cooled magma, (granite and basalts), more resistant to marine erosion and weathering than sedimentary rocks

Annual recession rate for granite is 1mm but for thinly bedded limestone it is 2.5 cm

Metamorphic: (gneiss and marble), formed from igneous and sedimentary rocks which were subjected to intense heat, so more resistant

Recession fastest on newly created volcanic islands at around 40m a year.

Also if there is glacial till and boulderclay on top erosion will be faster since boulderclay unconsolidated.

How does permeability affect erosion?

Permeability: The ability of water to pass through it. linked to porosity and perviousness

Porosity: relates to total volume of pore spaces

pervious rocks like limestone have joints and bedding planes which water can flow along

if there is porous rock above impermeable rock, the water percolating through will collect in the upper layer, leads to slumping,

(impermeable rock more resistant since water cant get in)

How does mineral content affect erosion

Some rocks contain soluble materials so vulnerable to chemical weathering like carbonation and hydrolysis.

Example: Limestone soluble in rainwater, becomes a dilute carbonic acid by absorbing CO2 from the air,

Limestone rocks contain calcium carbonate, calcium carbonate slowly dissolved into calcium bicarbonate which is carried away in solution.

seawater also absorbs carbon dioxide from the atmosphere, when the waves hit the limestone the water dissolves the limestone.

Hydrolysis, water chemically combines with some minerals, to produce clays and salts, eg feldspar in granite.