Saturday, October 17, 2009

Rocks and thier environments + Volcanoes


Rocks and Their Environments

Rocks are aggregates of minerals.
 

Igneous Rocks

  1. As magma (molten material) cools, ions arrange themselves into orderly patterns during crystallization. There are two types of crystallization:

    1. Volcanic (extrusive): Magma crystallizes quickly at spreading centers and from volcanic eruptions.
    2. Plutonic (intrusive): Magma crystallizes slowly deep below the Earth’s surface.

  2. Magma’s rate of cooling affects crystal size and mineral composition.Fast cooling results in smaller crystals, more mafic; slow cooling results in larger crystals, more felsic.

    1. Glass: No crystals. Forms when magma cools too rapidly to form crystals.
    2. Fine-grained (aphanitic): Crystals too small to distinguish individual minerals with the unaided eye. Gas bubbles leave openings or vesicles. Aphanitic rocks form quickly at Earth’s surface or in the upper crust (volcanic).
    3. Coarse-grained (phaneritic): Crystals large enough to distinguish minerals with the naked eye. Phaneritic rocks form in a slowly cooling magma chamber deep in the crust (plutonic).
    4. Porphyritic: Large crystals in a matrix of smaller crystals. Porphyritic rocks form when magma crystallizes rapidly, forming a fine-grained matrix, but then moves to a slower-cooling environment before all the melt has crystallized. The remaining melt forms large crystals.

  3. Bowen’s reaction series: The geologist N. L. Bowen (1887–1956) created a chart showing the series in which different minerals crystallize from cooling magma:

    • On the left side: Mafic minerals begin to crystallize. After each mineral crystallizes, it reacts with the remaining magma to form the next mineral in the series.
    • On the right side: Felsic, calcium-rich minerals crystallize to form early feldspars, which then react with sodium in the remaining magma to form more sodium-rich feldspars.
    • At the bottom of the series: When magma crystallization is nearly complete, the remaining magma is mostly SiO2, and quartz forms.



 
 

Volcanoes

  1. Volcanoes form where magma burns through the crust, at subduction zones, at spreading centers, or at “hot spots” like Hawaii.

    1. Successive eruptions build a cone of hardened lava. Eruptions are explosive (pyroclastic) if the magma is gas-rich and felsic, slow if the magma is gas-poor and mafic.
    2. Although volcanoes typically form at subduction zones or spreading centers, they also may form within a plate, as in the Yellowstone region of Wyoming.

  2. Volcano morphology

    1. Crater: The pit inside a volcano. A crater more than 1 km wide is called a caldera.
    2. Vent: A pipelike structure connecting the underground magma chamber to the crater.

  3. Types of volcanoes

    1. Shield volcano: A broad, slightly domed structure typically built of liquid basalt. The Hawaiian volcanoes are shield volcanoes.
    2. Composite cone (stratovolcano): A large, nearly symmetrical cone made of alternating lava flows and pyroclastic volcanic debris.
    3. Cinder cone: A generally small volcano with steep sides, built from ejected lava fragments and often in groups near larger volcanoes.

  4. Volcanic rocks

    1. Basalt: Dark green to black, fine-grained, mostly pyroxene and plagioclase feldspar, with some olivine. The ocean floor is mostly basalt.
    2. Tuff: Hardened ash from an explosive volcano.

  5. Plutons are the site of plutonic rock formation. Most magma in the Earth is deep underground, in chambers that cool slowly or rise slowly to intrude into preexisting rock.

    1. Plutonic rocks

      1. Gabbro: Has a basaltic composition (mafic) but large grain size.
      2. Granite: A phaneritic igneous rock with 25–35% quartz and more than 50% feldspar, with hornblende, muscovite and biotite.

    2. Pluton forms

      1. Batholith: A large expanse of granitic rock (more than 100 km2). Batholiths frequently form the cores of mountains, exposed only after much of the ground surface erodes.
      2. Sill: A lateral layer of igneous rock formed when fluid basaltic magma rises from a magma chamber and squeezes into horizontal strata.
      3. Dike: A vertical or angled layer of igneous rock that cuts across other rock layers, usually by injection into fractures.


 
 

Metamorphic Rocks

  1. High temperature, high pressure, or variable chemical conditions can changecountry (preexisting) rocks through the process of metamorphism.Rocks remain solid during the process.

    1. Regional metamorphism: An extensive volume of the crust is metamorphosed, usually by intensive compression at convergent boundaries.
    2. Contact metamorphism: Intruding magma heats cold country rock nearby and causes it to recrystallize.
    3. Metasomatism: Hot fluids dissolve original minerals, and then chemical reactions cause new minerals to grow.

  2. Rocks undergo both mineral and textural changes during metamorphism.

    1. Mineral changes: During metamorphism, two minerals can react, and their ions can diffuse across grain boundaries, resulting in a new mineral. Alternatively, complex minerals may break down into simpler ones.
    2. Textural changes: Rocks gain foliation (alignment) as minerals align into bands. With increasing temperature and pressure, grain size increases and texture coarsens.

  3. Classification: Metamorphic rocks are classified by strength of metamorphism. The following are listed in order from weak to strong metamorphism:

    1. Foliated rocks:

      1. Slate: A fine-grained rock, usually made of metamorphosed fine sediments.
      2. Phyllite: Similar to slate but slightly coarser-grained, and shiny due to high mica content.
      3. Schist: A coarse-textured metamorphic rock, with minerals aligned in parallel bands, containing more than 50% platy minerals (minerals with a planar, layered structure) like mica.
      4. Gneiss: Bands of abundant coarse grains, mostly feldspar and quartz, alternated with bands of flaky minerals.

    2. Nonfoliated rocks:

      1. Marble: Metamorphosed limestone with a sugary texture. Marble is composed of interlocking calcite grains.
      2. Quartzite: Metamorphosed quartz sandstone. Quartzite is very hard and is composed of interlocking quartz grains.
      3. Hornfels: Fine-grained rock altered in contact zones around igneous intrusions.


 
 

Sedimentary Rocks

  1. When weather and other forces of erosion wear away rocks, sediments form. Those sediments can be compacted, through lithification, to form sedimentary rocks.

    1. Erosion: The transport of material around Earth’s surface by a mobile agent like water or wind. Erosion and weathering form sediments and soil.

      1. Mechanical (physical) weathering: Rocks break into smaller pieces, with each piece retaining the original mineral composition.

        • Frost wedging: Water freezes and expands in a rock, breaking off fragments.
        • Unloading: Erosion removes material from above buried rock. Pieces pop off in response to the lowered pressure.
        • Biological activity: Roots wedge into and widen rock fractures, or animals burrow into soil and expose rock to the surface.

      2. Chemical weathering: Rocks break down chemically, and their constituent minerals alter during the process.

        • Oxidation: Water (H2O) is the strongest chemical weathering agent. It causes iron-rich rocks to oxidize, or rust.
        • Ionization: CO2 + H2O → carbonic acid, which breaks granite down into clay minerals.


    2. Lithification: After erosion and weathering, sediments cement to form sedimentary rocks.

  2. Sedimentary settings: Sedimentary rocks can form anywhere on or just below the Earth’s surface, in dry or wet environments.
  3. Classification: Sedimentary rocks may be classified in several different ways:

    1. Based on origin:

      1. Detrital sediments: Sediments that are fragments of broken-down rock. They are listed here in order of decreasing grain size:

        • Breccia: Lithified angular blocks of rock.
        • Conglomerate: Lithified round rock fragments, pebble-sized and larger.
        • Sandstone: Cemented sand.
        • Shale: Compacted clay, mud, or silt.

      2. Chemical and biochemical sediments: Sediments that form from minerals that precipitate from water, either physically or biologically (as organisms pull elements out of water to make their skeletons):

        • Limestone (CaCO3): Formed from cemented fragments of any size of shell.
        • Chert: Cemented shells made of silica.

      3. Evaporites: Sediments that form as water evaporates from a closed basin and the solution becomes supersaturated with certain elements, which then precipitate out as minerals like halite.
      4. Coal: An organic material that nonetheless is considered a sedimentary rock because it consists of compacted plant matter.

    2. Based on grain size and sorting:

      1. Grain size: The physical size of individual grains that make up sedimentary rock.

        • Gravel (>2 mm) forms conglomerate, breccia
        • Sand (1/16–2 mm) forms sandstone, greywacke
        • Mud (<1/16 mm) forms shale, mudstone

      2. Sorting: The degree of variety of grain size and composition within a sedimentary rock.

        • Well sorted: One grain size and type dominates the rock’s composition.
        • Poorly sorted: The rock is composed of grains of many sizes and compositions.


    3. Based on sedimentary structure:

      1. The method of sediment deposition often imparts a distinctive pattern on a package of sedimentary rocks.
      2. Bedding planes: The planes that separate strata (layers) of sedimentary rock. Often, these planes are the planes along which the rock breaks.

        • Cross-bedding: Wind or waves deposit sediments in an upsweeping pattern.
        • Graded bedding: Grain size becomes coarser or finer from the bottom to the top of a layer.





 
 

The Rock Cycle

  1. Heat, pressure, erosion, and other forces are always at work on the Earth, changing the composition of rocks: from igneous to sedimentary, sedimentary to metamorphic, igneous to metamorphic, and so on.
  2. These continual conversions from one type of rock to another are collectively termed the rock cycle.

    1. All rocks exposed at Earth’s surface are subject to weathering,which leads to the formation of sediments.
    2. All sediments are subject to burial, after which they are undergolithification (cementation and compaction) to form sedimentary rocks.
    3. All rocks can be exposed to heat and pressure, which often leads to formation of metamorphic rock.
    4. When metamorphic rocks melt into magma, the magma sometimes cools and crystallizes into igneous rock
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