Soils, Sediment, Weathering and Sedimentary Rocks Sedimentary Rocks Rocks that form by the cementing of grains or fragments of pre-existing rocks, or by the precipitation of minerals out of a solution Form a veneer over basement rocks (igneous and metamorphic) Sedimentary layers (cover) over the Vishnu schist (basement) in the Grand Canyon, AZ Weathering and Erosion • Sediment – loose fragments of rocks or minerals broken off of bedrock, minerals that precipitate directly out of water, and shells of organisms. • Deposition / Sedimentation – occurs when sediment settles out as winds/water current die down, or as glaciers melt. • Lithified – sediment that has been cemented together by geologic processes to form a rock • Rocks are broken down and turned into sediments by two main processes – Physical Weathering • Plumbers snake – Chemical Weathering • Liquid Drain-O Highly weathered sandstone in Bryce Canyon N.P., UT How do Sedimentary Rocks Form? The following applies to Clastic/Detrital rocks; biochemical and chemical sed rocks are different • 1) Erosion – mobilizes particles by weathering, most commonly by rainfall & gravity. • 2) Transportation – Occurs when currents of wind, water, and moving ice (glaciers) transport particles to new locations (downhill or downstream) • 3) Deposition – sediment is deposited when the transporting current slows to the point that it can no longer carry its load. • 4) Burial – As layers of sediment accumulate, the layers accumulate in sedimentary basins. Older, previously deposited sediments are compacted. These layers remain at depth until either erosion or tectonic processes act on them. • 5) Diagenesis & Lithification – Refers to the physical and chemical changes that lithify sediment into rock. Includes pressure, heat and chemical reactions The Sedimentary Stages of the Rock Cycle Weathering Erosion carries breaks down away particles. rocks. Transportation moves particles downhill. Glacier Delta Desert Playa Deposition occurs lake when particles settle out or precipitate. Sedimentary Burial occurs rocks as layers of Metamorphic sediment rocks accumulate. Plutons Diagenesis lithifies the sediment to make sedimentary rocks. Detritus & Grain Size • Physical weathering breaks rocks into chunks called detritus • Detritus is classified by size (diameter) » Boulder > 256 mm Visible grain size » Cobble - between 64 and 256 mm Coarse-grained » Pebble - between 2 and 64 mm Medium-grained » Sand - between 1/16 and 2 mm Fine-grained » Silt - between 1/256 and 1/16 mm Microscopic grain size Fine-grained » Mud < 1/256 mm Physical/Mechanical Weathering Rocks can be physically weathered by: • Jointing – Formation of cracks in rocks. Joints – form in rocks due to, stretching, or cooling (contraction) • Exfoliation joints – when deep rocks are exposed, the removal of overburden causes sub-horizontal cracks to form causing the rock to easily peel away like layers of an onion. Also called sheet joints or unloading joints and are common in exposed batholiths. Joints in the Field • Below: Joints in sedimentary rocks in Brazil. Near vertical joints in sedimentary rocks are common. • Above: Exfoliation / Sheet / Unloading joints in the Sierra Nevada Batholith (granite) in California. Physical/Mechanical Weathering • Frost wedging: water fills cracks, freezes, expands, and forces cracks to open causing them to grow. Can lift large blocks. • Root wedging: same as frost wedging except that roots pry open the cracks. • Salt wedging: salt crystals form when evaporating water flows through rocks. The salt crystals pry open the cracks Results of Physical Weathering • Eventually, mechanical weathering processes create an apron or pile of debris at the margins of slopes called talus Chemical Weathering Chemical weathering is typically strongest in warm wet climates Types include: • Dissolution: – Primarily affects carbonates and salts…when a chemical reaction breaks down minerals into new compounds 2+ - • E.g. CaCO3 (Calcite) + H2CO3 (carbonic acid) Ca (aq) + 2HCO 3 (bicarbonate) • Hydrolysis: – Water acts to ‘loosen’ chemical bonds to break down minerals. Works faster in slightly acidic water + - • E.g. H2O (acidic) H + OH + + • E.g. H + KAlSi3O8 (K-feldspar) Al2Si2O5(OH)4 (Kaolinite) + K (aq) • Kaolinite is a clay mineral • Oxidation: – When an element loses an electron…commonly when it bonds with oxygen. 2+ 3+ • E.g. 4Fe + 3O2 2(Fe )2O3 (iron lost an electron and went up in charge) • Hydration: – Absorption of water into some minerals (mainly clays) causes them to expand Dissolution • Chemical weathering processes act on rock/mineral surfaces – Results in rounding of edges Surface Area and Weathering • All other things being equal, the ratio of surface area to volume of a material controls the rate of weathering Weathering & Roundness • Weathering tends to round off corners and leave things more smooth in shape. – Angular detritus = not very weathered – Rounded detritus = very weathered Weathering and Bowen’s Reaction Series Minerals that form early (high temp) in Bowen’s reaction series are least stable. Exceptions are calcite and halite, which are highly susceptible to chemical weathering. Typically, mafic minerals weather by oxidation, felsic minerals weather by hydrolysis, carbonates weather by dissolution, and oxides don’t weather at all. Dissolves Follow’s Bowen’s reaction series!! reaction Bowen’s Follow’s Differential Weathering Differential weathering – a primary control on the shape of our physical landscape. Under the same set of climatic conditions, not all minerals and not all rocks will weather at the same rate. Some develop more joints and fractures, some undergo faster dissolution, some remain more-or-less intact. Weak layers weather more quickly, leaving behind more resistant layers. This process occurs on small scale – such as an outcrop of rock And at a large scale, such as entire valleys and mountain ranges. e.g. El Capitán is the product of differential erosion (Show valley and ridge in GoogleEarth) El Capitán – Guadalupe Mtns (SW Texas) Soil • “Soil consists of rock and sediment that has been modified by physical and chemical interaction with organic material and rainwater, over time, to produce a substrate that can support the growth of plants.” • Soil-forming processes require long periods of time. • Soil may be easily destroyed by human activities. • Soil is a crucial natural resource in need of protection. Soil Thickness • Weathering breaks rocks down into detritus, which forms regolith • Regolith: any unconsolidated material that covers bedrock – Soil is a regolith • What controls the thickness of soil? 1- Age: The longer a surface has been exposed, the thicker the soil Soil Thickness 2- Slope: The steeper the slope the less soil will accumulate bowl-shaped areas, such as basins will accumulate thick soils Soil Thickness 3- Bedrock Type: Thick soils will form over bedrock that is easily weathered, such as carbonates, evaporites, or muds. The soil composition will partly reflect the composition of the bedorock Soil Thickness 4- Climate: Thick soils will form in warm wet climates; deserts will form little or no soil Providence Canyon, SW Georgia • Formed due to poor farming techniques in the 1800’s – Trees and natural land cover was removed for agriculture – Fast rates of soil erosion created deep gullies – Up to 150 feet deep Sedimentary Rock Types • Detrital / Clastic – Cemented fragments of pre-existing rocks. Arkose – E.g. sandstone, mudstone • Biochemical – Rocks made of cemented shells of organisms – E.g. coquina, limestone Coquina • Chemical – made from minerals that precipitate out of water solutions. – E.g. travertine, various evaporites Evaporites near a salt lake Clastic Sedimentary Rocks • A clastic/detrital rock forms in five stages: 1- Weathering/Erosion 2- Transportation 3- Deposition 4- Burial 5- Lithification and Diagenesis (compaction+cementation) Grain size is reduced as sediment is transported Classifying Clastic Sedimentary Rocks Clastic/Detrital rocks are classified by: 1. Clast size 2. Clast composition 3. Angularity and Sphericity 4. Sorting 5. Type of cement Common Clastic Rock Types • You should know most of these from lab, but if not, then make sure to know the basic characteristics of these types of clastic rocks. Biochemical and Organic Sedimentary Rocks • Biochemical Limestone – A biochemical or chemical rock made of calcite, which is made of the remains of shells of organisms that secrete calcite or aragonite (a polymorph of CaCO3) shells. – Fossiliferous limestone – lots of fossils! Limestone – Micrite – made of calcite mud – Chalk – made of plankton shells • Biochemical Chert – made of cryptocrystalline (microscopic crystals) quartz, formed from the shells of plankton that sat on the sea floor and Folded Chert Beds dissolved into a silica rich ooze. • Organic Rocks – Coal, black shale, made of organics derived from plants and animals • We’ll talk about this stuff more in Chapter 14. A Coal Seam Chemical Sedimentary Rocks • Evaporites – products of the evaporation of water – Main minerals: gypsum, halite • Travertine (chemical limestone) – limestone that precipitated out of groundwater – E.g. stalactites, stalagmites, also found at gysers & hot springs • Dolomite – chemical alteration of limestone CaCO3 into dolomite CaMg(CO3)2. – Happens when Mg bearing groundwater reacts with limestone. • Replacement Chert – When chert (SiO2) replaces minerals in a rock. – E.g. petrified wood Travertine at Mammoth Hot Springs, MT Formation of Evaporites • Evaporites