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Unit 31


Unit Overview

This unit examines the two secondary rock types, sedimentary and metamorphic rocks. The main sections are as follows:

  • Sedimentary rocks
  • Metamorphic rocks
  • The rock cycle

Sedimentary rock results from the deposition and compaction of rock fragments and mineral grains derived from other rocks. Physical and chemical weathering and the subsequent erosion of the weathered material precede the lithification process. Pressure lithifies the sediment while the cementation of grains by silica and calcite begins.

Sedimentary rocks include clastic and nonclastic subtypes. Clastic rocks range in grain size from shale to conglomerates. The layering of rocks is known as stratification. The presence of entities within and between sedimentary rocks, and in addition to the arrangement of strata, reveals clues about the Earth's past environments.

Sedimentary rocks are not only layered but may also be jointed, folded, and faulted. Both igneous and sedimentary rocks are subject to metamorphism, which results from extreme heat and pressure. Metamorphic rocks tend to be more resistant to erosion than their prior sedimentary forms; nevertheless, metamorphic rocks are usually weak along their foliation planes. The formation, metamorphosis, and destruction of rocks represent a continuous process known as the rock cycle.

Unit Objectives

  • To discuss the circumstances under which sedimentary and metamorphic rocks form
  • To identify common sedimentary and metamorphic rock types
  • To discuss some observable structures within sedimentary and metamorphic rock masses

Glossary of Key Terms

Breccia In clastic sedimentary rocks when pebble-sized fragments in a conglomerate are not rounded but angular and jagged.
Cementation During the lithification process of compaction as the grains of sediments are tightly squeezed together, water in the intervening pore spaces, which contains dissolved minerals, is deposited on the grain surfaces and acts as a glue to further bond the grains together.
Clastic sedimentary rocks Sedimentary rocks made from particles of other rocks.
Compaction The lithification process whereby deposited sediments are compressed by the weight of newer, overlying sediments; this pressure will compact and consolidate lower strata, squeezing their grained sediments tightly together. Usually occurs in conjunction with cementation.
Conglomerate A composite sedimentary rock composed of gravels, pebbles, and sometimes boulders.
Contact metamorphism Metamorphic changes in rocks induce by their local contact with molten magma or lava.
Cross-bedding Consists of successive rock strata deposited not horizontally but at varying inclines; like ripple marks on sand, this usually forms on beaches and in dunes.
Foliation The unmistakable banded appearance of certain metamorphic rocks, such as gneiss and schist; bands formed by minerals realigned into parallel strips during metamorphism.
Gneiss Metamorphic rock derived from granite that usually exhibits pronounced foliation.
Limestone A nonclastic sedimentary rock mainly formed from the respiration and photosynthesis of marine organisms in which calcium carbonate is distilled from seawater; finely textured and therefore resistant to weathering when exposed on the surface, it is susceptible to solution that can produce karst landscapes both above and below the ground.
Marble Metamorphosed limestone; the hardness and density of this rock is preferred by sculptors for statues that can withstand exposure to the agents of erosion for millennia.
Nonclastic sedimentary rocks Derived not from particles of other rocks, but from chemical solution by deposition and evaporation or from organic deposition.
Quartzite A very hard metamorphic rock that resists weathering formed by the metamorphosis of sandstone (made of quartz grains and a silica cement).
Rocks Any naturally formed, firm, and consolidated aggregate mass of mineral matter, of organic or inorganic origin, that constitutes part of the planetary crust.
Rock cycle Cycle of transformation that affects all rocks and involves all parts of the Earth's crust: plutons form deep in the crust, uplift pushes them to the surface, erosion wears them down, and the sediments they produce become new mountains.
Sandstone A common sedimentary rock possessing sand-sized grains.
Schist A common metamorphic rock so altered that its previous form is impossible to determine; fine-grained, exhibits wavy bands, and breaks along parallel planes (but unevenly, as seen in slate).
Shale The soft, finest-grained of the sedimentary rocks; formed from compacted mud.
Slate Metamorphosed shale; a popular building material, it retains shale's quality of breaking along parallel planes.
Strata Layers.
Stratification Layering.
Stratigraphy The order and arrangement of rock strata.
Unconformity A gap in the geologic history of an area as found in the rock record, owing to a hiatus in deposition, followed by erosion of the surface, with further deposition continuing later; more specifically, can also refer to the contact between the eroded strata and the strata of resumed deposition.

Unit Outline

  • Sedimentary rocks
    • Formed by deposition and compaction of rock and mineral grains from other rocks
      • pressure of overlying rock removes water in the process of compaction
      • silica or calcite cements the rock together
    • Clastic and nonclastic sedimentary rocks
      • clastic rocks are formed from particles of other rocks
      • nonclastic rocks form from chemical solution or organic deposition
      • conglomerate is the coarsest type of sedimentary rock
        • if pebbles in conglomerate are jagged, called breccia
      • sandstone is usually composed from grains of quartz, highly resistant
      • shale is softer than sandstone, formed from compacted mud
      • limestone can be formed from
        • marine shell fragments
        • calcium carbonate from respiration and photosynthesis of marine life
    • Sedimentary rocks in the landscape
      • stratification or layering of rock beds
      • unconformity where stratification interrupted
    • Features of sedimentary strata
      • most strata horizontally layered
      • cross-bedding where layers deposited on varying inclines
      • folds, faults, deformations
  • Metamorphic rocks
    • Existing rocks that have been changed by heat and pressure
    • Tectonic or volcanic action
    • Metamorphic rock types
      • quartzite
      • marble
      • slate
      • schist
      • gneiss
    • Metamorphic rocks in the landscape
      • weakest along foliation points
      • The rock cycle
    • Plutons of molten magma form in crust, pushed upward eroded ¿ no beginning or end to this cycle
    • See Fig. 31.12 to trace each of the stages of cycle

Review Questions

  1. Describe the processes of compaction and cementation.
  2. What is an unconformity and how is it formed?
  3. How does cross-bedding occur?