the earth’s evolving crust. the four major steps in the formation of a sedimentary rock

The Earth’s Evolving Crust

The Four Major Steps in the Formation of a Sedimentary Rock

Clastic Sediment: is simply bits of broken rock and minerals that are moved as solid particles.

Shale

Bedform development as a function of water flow velocity

Symmetric ripples (Oscillation ripples) are formed by wave action.

Chemical Sediment: is transported in solution and deposited when the dissolved minerals are precipitated.

Coal: carbonaceous sediment composed most often of the remains of spores, algae, fine plant debris and noncarbonaceous ash.

Some terms: Stratification: results from the arrangement of sedimentary particles in layers.

Stratum: is a layer of rock or soil with internally consistent characteristics that distinguish it from other layers. (Plural of stratum is strata.)

Bed: the smallest stratigraphic unit and found within a member.

Bedding: the layered arrangement of strata in a body either of sediment or sedimentary rock.

Law of Original Horizontality: states that sediments are deposited in strata that are horizontal or nearly so and parallel to the Earth’s surface.

Conformity: a surface that separates younger strata from older rocks but along which there is no physical evidence of nondeposition.

Law of SuperpositionIn any succession of strata, not disturbed or overturned since

deposited, younger rocks lie above older rocks.

123

4

Folding 5

67

8

910

11

12

13

Unconformable contact types

Unconformity: a surface of erosion or nondeposition separating younger strata from older rocks, that represents a significant hiatus.

Angular unconformity: younger sediments rest upon the eroded surface of tilted or folded rocks.

Disconformity: contact between younger and older beds is marked by a visible, irregular or uneven erosional surface.

Paraconformity: beds above and below the unconformity contact are parallel with no discernable erosion, but whose ages are vastly different.

Nonconformity: an unconformity developed between sedimentary rock and older igneous or massive metamorphic rock that has been eroded prior to being covered by sediments.

Determining the age of an event as compared to other events

Placing geologic events in their correct chronological order

Does not involve consideration of when the events actually occurred

What is the order of events from: oldest to youngest?

E, G, L, C, tilt, H(fault), erosion, M, D, J, A, erosion, N, K, B, tilt, erosion, F

Principle of fossil succession: Fossils, and especially groups or assemblages of fossils, succeed one another through time in a regular and predictable order. (Fossils at the bottom of a sequence are older than those higher up in a sequence.)

Provides specific dates for rock units or events expressed in years before the present

Typically uses radioactive decay techniques

Geologic time scale uses both relative and absolute dating techniques

Metamorphic rock: any rock that has been changed from its original condition by heat, pressure, and the chemical activity of fluids

Metamorphism: the phenomenon of changing rocks subjected to heat, pressure, and fluids so that they are in equilibrium with a new set of environmental conditions

Index mineral: a mineral that forms within specific temperature and pressure ranges during metamorphism

Metamorphic grade: the degree to which a rock has undergone metamorphic change

Temperature: contributes to the process in two ways.  First, atoms may combine differently at different temperatures.  This means that a mineral stable at one temperature might become unstable at a higher (or lower) temperature and be converted to a different mineral with a more stable atomic structure.  This may or may not involve changing the exact elemental composition.  Second, heat makes practically all chemical reactions go faster, meaning that mineral transformations are much easier at higher temperature

Fluids: serve only to speed up other metamorphic processes, or perhaps even allow them to happen at all.  Chemical reactions require water, and most proceed much faster as the amount of water goes up.  Dissolved ions in the fluid also make those mineral transformations that require chemical changes in the minerals to occur, whether by supplying needed ions or flushing away excess ones.

Pressure: also has two effects.  As with heat, it can control which minerals or forms of minerals are stable.  Some minerals may be converted to minerals with similar composition but different atomic packing simply because pressure is increased.  The exact nature of the pressure is not important in this case, only the amount.  Thus the Confining (or lithostatic pressure) by deep burial of rocks under sediment may have this effect as well as the directed (or differential pressure ) produced by converging plates.  The second effect of pressure is to reorient minerals with linear or platy structure or to create a preferred orientation of them as they form. 

Burial Metamorphism: When sedimentary rocks are buried to depths of several hundred meters, temperatures greater than 300oC may develop in the absence of differential stress.  New minerals grow, but the rock does not appear to be metamorphosed.  The main minerals produced are often the Zeolites.  Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.

Regional Metamorphism

Regional metamorphism occurs over large areas and generally does not show any relationship to igneous bodies.  Most regional metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions.  Thus, regional metamorphism usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gneisses. 

A group of metamorphic rocks characterized by particular minerals that formed under the same broad temperature and pressure conditions

Named after most characteristic rock or mineral

Not applicable when original rocks were pure quartz sandstones or pure limestones or dolostones

Contact MetamorphismContact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion.

Craton: an old stable portion of the continental lithosphere; a core of very ancient rock.

Orogen: elongate regions of crust that have been intensely bend and fractured during continental collisions. Crust in an orogen is usually thicker than that of a craton. Orogens are the eroded roots of ancient mountain ranges that formed as a result of collisions between cratons.

Only the youngest orogens are mountainous today. Ancient orogens, now deeply eroded, reveal their history through the kinds of metamorphic rock they contain and the way the rocks are twisted and deformed.

Accreted Terrane Margins: arise from the addition of blocks of crest brought in by subduction and transform fault motions.