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Chapter 6: Sedimentary and Metamorphic Rocks

Ch. 6.1 Formation of Sedimentary Rocks

Sediments produced by weathering and erosion form sedimentary rocks through the process of lithification.

Weathering and Erosion

Rocks are broken down by weathering- a set of physical and chemical processes that breaks rock into smaller pieces. Sediments- small pieces of rock that are moved and deposited by water, wind, glaciers and gravity. Sediments become glued together to form sedimentary rocks.

Weathering

Weathering produces rock & mineral fragments (sediments). Chemical weathering dissolves or chemically changes minerals. More resistant minerals break off as chemical weather occurs.

Erosion

The removal and transport of sediment. Four main agents: wind, moving water, gravity, and glaciers.

Rock fragments and sediments are often transported to new locations. Eroded material is almost always carried downhill.

Deposition

Sediments are deposited in layers with largest grains at the bottom.

Energy of Transporting Agents

Fast moving water can transport larger particles better than slow moving water. As water slows, larger particles settle out first, sorting particles into layers. Wind is similar but only moves small particles.

Glaciers and landslide do not sort sediments.

Lithification

Sediments collect in low areas. As more layers build, the lower levels are exposed to increasing pressure and temperature causing lithification. Lithification- the physical and chemical process that transform sediments into sedimentary rocks.

Compaction

The weight of overlying sediments forces the sediment grains closer together. Water is squeezed out as grains come together. Sand doesn't compact as well due to quartz content. Open spaces between grains often contain groundwater, oil, and natural gas.

Cementation

Occurs when mineral growth glues sediment grains together into solid rock. Calcite (CaCo3) or iron oxide (Fe2O3) form as minerals precipitate out of ground water.

Sedimentary Features

Sedimentary rocks tell geologists how they formed and the history of the area in which they formed

Bedding

Primary feature of sedimentary rock; horizontal layers formed from the way sediment settles out of water or wind. Two types, depending on method of transport.

Graded Bedding: Bedding in which layers get finer the higher the bed. Common with marine sediments (mudslides)

Cross-Bedding: Formed as inclined layers of sediment are deposited across a horizontal surface. Preserved once lithification occurs. Large scale with dunes, small with beaches, streams, etc.

Ripple Marks

When sediment is moved into small ridges by wind or wave action or by a river current. Waves form symmetrical ripples due to back-and-forth motion. Current flowing in one direction produces asymmetrical ripple marks.

Angular vs. Rounded

When a rock breaks the fragments are angular, and jagged. As they travel away from source they become rounded.

Evidence of Past Life

Fossils are found in sedimentary rock. Fossils are evidence of once-living organism. As lithification occurs, parts of the organism can be replaced by minerals and turned into rock. Fossils tell us: types of organisms, environments, and how organisms have changed over time.

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Ch. 6.2 Types of Sedimentary Rocks

Sedimentary rocks are classified by how they are formed.

Clastic Sedimentary Rocks

The most common; formed from the abundant deposits of loose sediments that are formed from deposits of loose sediments. Clastic- meaning broken. Further classified by grain size.

Coarse-grained Rocks

Consist of gravel-sized rock and mineral fragments.

  1. Conglomerates- have rounded, gravel-sized particles.
  2. Breccias- composed of angular, gravel-sized particles.

Medium-grained Rocks

Consist of sand-sized rock and mineral fragments. Sandstone used by geologists to map ancient stream and river channels. Porosity- the percentage of open spaces between grains in a rock. Fluids can move through these spaces of sandstones. Can have reservoirs of oil, natural gas, and groundwater.

Fine-grained Rocks

Consist of silt and clay sized particles (siltstone, shale). Form from slow water in which sediments settle to the bottom. Have low porosity; forms barriers for movement of groundwater and oil.

Chemical and Biochemical Sedimentary Rocks

Involves evaporation and precipitation. As waters evaporate, dissolved minerals can increase in concentration.

Chemical Sedimentary Rocks

Evaporites- at saturation crystals that form precipitate and settle to the bottom and can form layers. Primarily form in arid regions, drainage basins on continents that have low water flow, and coastal settings.

Biochemical Sedimentary Rocks

Formed from remains of once-living organisms (limestone). Many types of limestone contain many fossils. Shells made of calcium carbonate.

Silica shells form siliceous ooze which is lithified into chert.

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Ch. 6.3 Metamorphic Rocks

Form when preexisting rocks are exposed to increases in temperature and pressure and to hydrothermal solutions.

Recognizing Metamorphic Rock

Metamorphism occurs when a rock changes form while remaining solid. This is due to increase in temperature and pressure. The temperature change is from Earth's internal heat; from deep burial or nearby igneous intrusions. The pressures come from deep burial or from compression during mountain building.

Metamorphic Minerals

Bowen's reaction series and fractional crystallization allow for minerals to change- solid state alterations.

Metamorphic Textures

Classified into two groups: foliated and nonfoliated.

Foliated Rocks: Layers and bands of minerals; high pressure causes minerals with flat crystals to form with their long axes perpendicular to the pressure.

Nonfoliated Rocks: composed mainly of minerals that form with blocky crystal shapes (Quartzite, Marble).

Under certain conditions, new metamorphic minerals can grow large while the surrounding minerals remain small (Garnet).

Grades of Metamorphism

Different combinations of temperature and pressure result in different grades of metamorphism. Low grade: low temp & pressure, High grade: high temp & pressure, Intermediate: between high and low.

Types of Metamorphism

  1. Regional: When temperature and pressure affect large regions
  2. Contact Metamorphism: When molten material comes in contact with solid rock; igneous intrusion. Metamorphic effects decrease with distance since temperature decreases with distance.
  3. Hydrothermal Metamorphism: When very hot water reacts with rock and alters its chemical and mineral composition. Gold, copper, zinc, tungsten, and lead.

Economic Importance of Metamorphic Rocks and Minerals

  1. Metallic Mineral Resources: Precipitated out of hydrothermal solutions, in veins or spread throughout rock mass (Gold, Silver, Copper).
  2. Nonmetallic Mineral Resources: Metamorphism of ultrabasic igneous rock (Graphite, Talc, Asbestos).

The Rock Cycle

Igneous crystallize from magma or lava, Sedimentary from cemented or precipitated minerals and sediments, metamorphic from changes in temperature and pressure.

Any rock can be changed into another type of rock. Rock cycle- continuous changing and remaking of rocks.

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Page last updated August 16, 2017.