Topic 5 weathering and sedimentsPresentation Transcript
Topic 5: Weathering and Sediments Outline Introduction Weathering Mechanical Weathering Chemical Weathering Factors That Control the Rate of Weathering Erosion, Transport and Deposition Depositional Environments Lithification Soil
Introduction Sedimentary rocks are one of three main rock types. Others rock types being igneous and metamorphic rocks. Sediments are formed from the weathering of previously lithified igneous, metamorphic and sedimentary rocks. Sedimentary rocks are classified as one of two types: (a) Detrital/Clastic Sedimentary Rocks (b) Chemical/Non-Clastic Sedimentary Rocks
(a) Detrital/Clastic Sedimentary Rocks Consist of detritus. Also referred to as “clastic” sedimentary rocks. e.g. sandstone, mudstone, shale. (b) Chemical/Non-Clastic Sedimentary Rocks Originate from substances that were taken into solution. Includes biochemical sedimentary rocks. Inorganic or organic processes may remove these dissolved materials from solution. e.g. limestones, evaporites (salt), coal.
Before I begin to discuss sedimentary rocks, I will examine how the detrital or clastic sedimentary rock particles or sediments originate. Clastic sediments are formed through a series of sedimentary processes. Start with bedrock of any type: - weathering - erosion - transport - deposition - lithification (compaction and cementation) Sediments deposited in rivers (Assiniboine), lakes (Winnipeg) and oceans (Hudson Bay). Ultimate resting place of the sediment eroded from the land is in the world’s oceans.
Weathering Weathering is defined as the “physical breakdown (disintegration) and chemical alteration (decomposition) of rocks and minerals at or near the surface”. Can occur rapidly or slowly. e.g. landslide vs. waves on a calm lake. Rocks do not weather at the same rate. Leads to differential weathering of a rock surface. Differential weathering produces uneven rock surfaces. e.g. Niagara Falls (Horseshoe Falls) has more resistant rocks at the top (limestone and dolomite) and less resistant rocks beneath (shale and limestone). Lower part erodes further back. Eventually causes the upper rocks to break off and fall into the river. Interbedded limestone and lime-mudstone, Spain.
Drumheller is in central Alberta, 135 km NE of Calgary. more resistant less resistant Differential weathering . e.g. Hoodoos in Alberta. Cap rock of resistant sandstone is well cemented. Less resistant/cemented sandstone below.
Differential weathering . Spires and pillars (hoodoos) in Bryce Canyon NP, Utah.
Erosion of Deformed Sedimentary Rock Differential weathering . Resistant versus nonresistant rock layers .
A few definitions: parent material Original rock material that is being weathered. erosion Removal of the weathered material from its place of origin. Transported by water, wind or ice. soil Regolith consisting of weathered material, water, air and organic matter that can support plants. Rock materials are usually the dominant parent material for soil. Two main kinds of weathering: Mechanical Chemical
Delicate Arch. Arches National Park, Utah. Weathering and erosion.
Mechanical Weathering Breaking of rock materials by physical forces. Into smaller pieces that retain the chemical composition of the parent material. Mechanically breaks down the rock. Five main mechanical weathering processes: (i) Frost Action: Ice Wedging (ii) Pressure Release (iii) Thermal Expansion & Contraction (iv) Salt Crystal Growth (v) Plant Root Wedging First two processes (Frost Action, Pressure Release) are most significant.
(i) Frost Action: Ice Wedging Very effective type of mechanical weathering. Water in the rock will periodically freeze and thaw. In areas where the temperature fluctuates about the freezing point. As freezing occurs, additional water tends to be attracted to the ice. When water freezes to form ice, its volume increases by about 9 %. Leads to high stresses in the rock. Causes mechanical disruption. Can force apart large blocks, some weighing many tons. Frost wedging is most effective at -5° to -15° C. Therefore very common in colder climates.
Frost action/ice wedging is responsible for most of the rock debris observed on mountain slopes. e.g. Rocky Mountains, Canada. This debris forms talus cones. One of the most effective types of mechanical weathering. talus Weathered material that accumulates at the bases of slopes.
(ii) Pressure Release Rock masses that were originally buried deep beneath the Earth’s surface are subjected to very high confining pressures due to the weight of the overlying rock. As erosion wears down the upper surface, both the weight of the overlying rock and the confining pressure are reduced. A rock responds to the decrease in weight and pressure by expanding upwards. Examples are found in many granitic batholiths. e.g. Yosemite National Park, CA. Batholiths have exfoliation domes (large rounded rock domes). exfoliation “ Peeling-off” of successive shells or layers. Like the ‘skins’ of an onion, around a solid rock core. e.g. granitic batholith. Joints are also produced during the pressure release. Sheet joints and vertical joints. These are near-surface phenomena. joint A fracture along which no movement has occurred. Or where movement has been perpendicular to the fracture surface.
Small exfoliation dome in the Sierra Nevada near Donner Pass, CA.
Large exfoliation dome , Stone Mountain, Georgia.
Slabs of granitic rock bounded by sheet joints , Sierra Nevada, CA.
Sheet-joints formed by expansion in the Mt. Airy Granite in North Carolina
(iii) Thermal Expansion & Contraction Daily heating of rock in bright sunshine followed by cooling each night causes the mechanical breakdown of that rock. Common rock forming minerals expand by different amounts when heated. Also individual minerals expand differently. Most expansion is on the surface. Little expansion in the interior of the mineral. Surface temperatures over 80°C have been measured on some desert rocks. Daily temperature fluctuations of up to 40°C can occur in a desert. Not a very significant mechanical weathering process.
(iv) Salt Crystal Growth Groundwater moving slowly through fractured rocks contains ions. These ions may precipitate out of solution to form salts. Salt crystals growing within rock cavities or along grain boundaries. Can exert enormous pressure. Results in disaggregation or rupturing of rocks. Process often observed in deserts. High rates of evaporation. High temperatures. Also observed in Antarctic granite formations. Salt grows between mineral grains in the granite. Rock slowly disintegrates. Resulting debris transported by winds. Leaves an unusual landscape. Granite rocks that look like Swiss cheese or a honeycomb texture . Only observed in a few specific environments. Not widespread.
Honeycomb weathering example , Pebble Beach, CA: caused by disaggregation of granular rocks.
(v) Plant Root Wedging Seeds germinate in cracks in the rocks. Plants extend their roots further into the crack. Roots can wedge apart adjoining blocks of rock. Further widen the cracks. Analagous to sidewalk breaks and cracks. Mostly associated with the positions of trees. Not volumetrically significant. Could work in parallel with ice wedging.
Lichen (fungi and algae), from an island in the Irish Sea. Lichens derive their nutrients from the rock and contribute to chemical weathering.
Chemical Weathering Process whereby rock materials are decomposed by chemical alteration of the parent material. Chemical breakdown of rocks. Minerals in igneous and metamorphic rocks that formed at high temperatures and pressures are chemically unstable when exposed to the lower temperatures and pressures at the Earth’s surface. Such minerals break down and their components form new, more stable minerals. Ferromagnesian silicates are the most likely to react. e.g. olivine and pyroxene. e.g. host rock for diamonds (kimberlite) weathers easily. NWT diamonds located at the base of a lake/swamp. Principal agents of chemical weathering are water solutions that behave as weak acids. Chemical weathering is therefore most pronounced in regions where both temperature and precipitation are high. i.e. tropical regions. Activity of organisms also plays an important role.
Order in which silicate minerals react during weathering is the reverse of the Bowen’s reaction series. 3 main chemical weathering processes: (i) Hydrolysis (ii) Leaching or Solution (iii) Oxidation
(i) Hydrolysis As rainwater falls through the atmosphere it dissolves small quantities of carbon dioxide (CO 2 ), producing carbonic acid. Carbonic acid (H 2 CO 3 ) is slightly acidic. i.e. a weak acid. Carbonic acid breaks down in soil to form hydrogen ions (H + ) and bicarbonate ions (HCO 3 - ). Hydrogen cations are so small that they can enter a crystal and replace other ions. CO 2 + H 2 O H 2 CO 3 H + + HCO 3 -
Hydrates minerals. e.g. feldspars to clay minerals. A chemical reaction where the H + cation or OH - anion of water replace the ions of a mineral. One of the chief processes involved in the chemical breakdown of common rocks. Hydrolysis of potassium feldspar produces a clay mineral called kaolinite. i.e. changes mineralogy. 4KAlSi 3 O 8 + 4H + + 2H 2 O 4K + + Al 4 Si 4 O 10 (OH) 8 + 8SiO 2 Potassium Kaolinite Feldspar Clay Above is a balanced chemical equation (elements and charge).
Spheroidal weathering . Mostly a chemical weathering process. Spherical granite boulders.
(ii) Leaching or Solution Another common process of chemical weathering. Continued removal by water solutions of soluble matter from bedrock. Soluble substances leached from rocks during weathering are present in all surface water and groundwater. Sometimes their concentrations are high enough to give an unpleasant taste. e.g. solution of halite (NaCl) or table salt. e.g. calcite (CaCO 3 ) is insoluble in pure water. But is very soluble in the presence of a weak acid. i.e. carbonic acid H 2 CO 3 .
Headstones in a cemetery, Deerfield, Massachusetts. Both are made of sandstone. One on the right is older. Leached/weathered for a longer time interval. Ca-rich cement in the sandstone?
CaCO 3 + H 2 CO 3 Ca 2+ + 2(HCO 3 ) - Calcite + Carbonic Acid Calcium Ion + Bicarbonate Ion Karst Topography Leaches carbonate rocks. Weak acid dissolves minerals into ions. Dissolution. Dissolution of calcite in limestone rocks produces large caves and caverns. e.g. Carlsbad Caverns, New Mexico.
(iii) Oxidation Refers to reactions with oxygen to form oxide minerals. Important chemical weathering process in the alteration of ferromagnesian silicates. Such as olivine, pyroxene, amphibole and biotite. Fe in these minerals combines with oxygen to form the mineral goethite (4FeO*OH) which subsequently dehydrates to form the mineral hematite (Fe 2 O 3 ). Change minerals in the presence of oxygen. e.g. Fe-oxide minerals are generated. oxidation of Fe to form goethite: 4FeO + 2H 2 O + O 2 4FeO*OH dehydration of goethite to form hematite: 4FeO*OH 2Fe 2 O 3 + 2H 2 O
Oxidation of pyrite (FeS 2 ) in mine tailings.
Factors That Control the Rate of Weathering At least 7 main factors control the rate of weathering: (i) Particle Size (ii) Climate (iii) Parent Material or Rock Type (iv) Rock Structure and Texture (v) Local Topography (vi) Burrowing Animals (vii) Time
(i) Particle Size Relationship of surface area to volume. As a rock is reduced into smaller and smaller particle sizes, the volume remains the same but the surface area increases dramatically. Smaller particles have more surface area in proportion to their volume. More area for weak acids (chemical) or frost action (mechanical) to have an effect.
(ii) Climate Moisture and heat promote chemical reactions. Weathering more intense in a warm, moist climate. Compared to a dry, cold climate. Tropics vs. polar regions. Rate of chemical weathering .
(iii) Parent Material or Rock Type Quartz is resistant to chemical breakdown. Therefore granite is very resistant to weathering. Quartz minerals make up the bulk of granite (> 50 %). Minerals high up on the Bowen’s discontinuous reaction series are the first to weather. (iv) Rock Structure and Texture Presence of joints or fractures will speed up the weathering process. Susceptible to frost action.
Order in which silicate minerals react during mechanical and chemical weathering is the reverse of the Bowen’s reaction series.
Weathering has been concentrated along vertical fractures separating these panels of sandstone at Arches National Park, Utah.
Fluid seeps along fractures. Increases the rate of weathering.
(v) Local Topography Higher slopes lead to an accelerated rate of mechanical weathering. Solid products of weathering are washed away quickly. Exposes fresh bedrock. Rocky Mountains vs. Hudson Bay Lowlands. (vi) Burrowing Animals Bring partly decayed rock up to the Earth’s surface. Move disaggregated rock. Enormous volumes over geological time. (vii) Time More time of exposure at the Earth’s surface equals a greater exposure to weathering processes.
Erosion, Transport and Deposition
“ bridge” between solid rock (parent material) and subsequent sedimentary rock
original rock broken down into sediments
Transport and Deposition
sediments are moved by air, water, ice and gravity
sediments eventually harden to form a sedimentary rock
bedrock is weathered to become sediment
chemical and mechanical weathering
weathered material derived from pre-existing rocks
sediment is then removed from its place of weathering
process is called erosion
Transport Bridge between solid rock (parent material) and subsequent sedimentary rock. Sediments are moved by air, water, ice and gravity (transport). Clastic sediment is transported in many different ways. Main agents of transport are: - ice - wind - water - gravity Additional chapters go into more detail on transporting agents and environments. Subsequent topics in this course.
Two types of transport mechanics in water and air: Flowing water or wind carries the sediment in suspension (suspended in water or air; suspended load transport ) or by traction (bouncing along the surface; bed load transport ). Usually a flow of a certain strength is required to move the particles. Transport occurs from high areas towards low areas (generally). Sediments can be transported a considerable distance. From North Dakota all the way to the Gulf of Mexico. e.g. Mississippi River system. Vast majority of coarse grained transport occurs during flood stages of river systems. Highlights the impact and role that low frequency, high intensity events have on the geologic rock record. From Brandon to the Atlantic Ocean. Via Assiniboine River, Red River, Lake Winnipeg, Nelson River and Hudson Bay.
Mineral grains and rock particles are abraded during transport. Due to collisions with each other and the edges of the transporting medium. e.g. river channel. Abrasion reduces the size of detrital particles. Also tends to round off the corners or edges of the rock or mineral. Rounded grains indicate that the material has undergone a considerable amount of transport. More mature. Immature sediments are usually very angular.
a wide range in grain sizes
from pebble to sand…
a narrow range in grain sizes
almost all same size
Sediments can also be sorted during transport. Sorting refers to the size distribution of the sediments. A wide range in grain sizes = poorly sorted. A narrow range in grain sizes = well sorted. Sorting results from processes that selectively transport and deposit particles by size.
Deposition Bridge between solid rock (parent material) and subsequent sedimentary rock. Sediments are laid down in their final resting place (deposition). Agents of deposition are the same as those of transport. Deposition occurs when the flow is no longer strong enough to carry sediment. Therefore deposition occurs because of a drop in energy. Flowing water or wind slows down. Can no longer carry the sediment as suspended load or bed load. Sediment settles onto the bed (i.e. is deposited). Sediments are laid down in horizontal layers called beds. Horizontal Beds Book Cliffs, Utah
Depending on what was happening to the sediment as deposited, the layers may show sedimentary structures . e.g. ripple marks, graded bedding, cross bedding, mud cracks. Sedimentary structures can give some indication as to the depositional environment and strength of the transporting energy. i.e. high- or low-energy environments. Mud Cracks Ripples Cross Bedding Ripples and Graded Bedding
Depositional Environments Depositional environment is defined as “any geographic area in which sediment is deposited”. e.g. next time you are on a beach at the lake, take a look in the shallow water for ripples or ridges in the sand. These can be preserved in the rock record. These sedimentary structures indicate a relatively low energy environment in shallow water. Sediments are mainly deposited in relatively low lying areas. Most sediment is ultimately deposited in the ocean. Fairly close to the coastline. Continental margins. There are 3 general depositional settings: - continental - transitional - marine
swamp (e.g. mangrove)
shallow marine or shoreface
deep marine or abyssal plain
Lithification Bridge between solid rock (parent material) and subsequent sedimentary rock. Sediments eventually harden to form a sedimentary rock (lithification). Process by which sediment is transformed into a sedimentary rock is called lithification . From the verb lithify , meaning turning to stone. Relatively slow process. Sediments are first compacted . Pore spaces (air or water-filled pockets or voids) occur between the sediments. These are gradually reduced by the weight of overlying sediments. Pressure increases over time. Volume of the sediments decreases because of the reduction in pore space. Sand grains fit more tightly together after compaction. Muds can lose as much as 40 % of their volume. Mostly water is squeezed out during compaction of muds.
Cementation is also required for the lithification of sandstones. Most common cementing materials are calcium carbonate (CaCO 3 ) and silica (i.e. quartz, SiO 2 ). Circulating and percolating ground waters occur in the pore spaces. Substances dissolved in these waters can precipitate and cement grains together. Process called cementation.
Compaction and cementation work together on a sand in order to generate a lithified sandstone. Compaction alone can lead to the lithification of mud to form mudstone or shale.
What is Soil and How Does it Form?
Soil is a mixture of weathered rock material, water, air, and organic matter.
Sand, silt, and clay
weathered rock fragments
carbon rich decayed
develop on parent rock
Eroded and transported to
O horizon: organic matter.
A horizon: top soil, zone of leaching, intense biological activity.
B horizon: subsoil, zone of accumulation.
C horizon: partially altered parent rock, little organic matter.
Soil Profile A A
Caliche – crust of soluble calcium salts (Ca-carbonate, gypsum, Mg-carbonate).
Factors Controlling Soil Formation
topography (e.g. relief),
composition of parent rock
Three major soil types are recognized:
pedalfers (humid climates)
pedocals (arid climates)
laterites (tropical climates): reddish, rich in Fe, Al.
Laterite from Madagascar . Laterite soil is depleted within 2-5 years .
Soil degradation is a decrease in soil productivity or loss of soil. Three types are recognized:
Chemical degradation: soils overused, insufficient use of fertilizers, pollution and salinization.
Physical deterioration: when soil is compacted by weight of heavy machinery and livestock.
A large gully (erosion) has removed the soil, Rio Reventado, Costa Rica.