GEOL 333 #12 Plate Tectonics, Rocks and Rock Cycle

Plate tectonics theory essential to understanding origin of rocks, mineral resources/ore deposits, and Midwest geology.

I) Dynamic Earth - Earth has earthquakes (EQ's), volcanoes, and mountain ranges; produced by motion in outer portion of Earth. Plate tectonics theory (link #2) = outer portion of Earth (lithosphere) is comprised of number of discrete slabs of rock (tectonic plates, map #2) that slowly move with respect to each other. Cross-section of tectonic plate model involves 2 layers based on physical properties; lithosphere - cold, brittle outer layer including crust and upper mantle, ~100 km thick; asthenosphere - hot, plastic layer of mantle only, ~100 - 300 km deep.

 

 

7 major plates and several minor ones. Tectonics - involves reconstruction of movement of tectonic plates over geologic time and effects of those movements. See excellent summary of tectonic plate boundaries and motions by USGS.

II) Plate Boundaries - At all plate boundaries, rocks grind against each other producing EQ's. Commonly LINEAR BELTS of volcanoes, mountain ranges, and faults also occur. 3 types of plate boundaries:

A) Divergent - where plates move apart, defined by oceanic ridge (spreading center) - undersea mountain range that circles globe (e.g., mid-Atlantic ridge including Iceland), non-explosive basaltic volcanism (mafic igneous rocks), ocean crust is produced by hot rising asthenosphere (mantle melts). Crustal extension results in normal faults; minor, shallow EQ's. Cooled rock moves laterally away from ridge. Example = Mid-Atlantic ridge. Initial stage of divergence = rifting (splitting of continental plate, e.g., east Africa rift).

 

 

 

Basalt volcanoes can also occur within tectonic plate at hot spot (localized zone of upwelling mantle, e.g., Hawaii,)

B) Convergent - (subduction zone) plates move toward each other, cold oceanic crust is destroyed as it subducts into mantle (descends and melts), includes oceanic trench and mountain belt with explosive andesite volcanoes (intermediate to silicic igneous rock). Crustal compression results in reverse faults and thrust faults; major and minor EQ's, can be shallow or deep. Example = ~entire margin of Pacific Ocean.

 

 

 

C) Transform - plates move laterally past each other along transform (strike-slip) fault. No melting occurs (no vertical movements), so no volcanoes; no mountains. Get transform faults that offset oceanic ridge (and extends as fracture zone beyond seismically active region); major and minor EQ's, shallow only. Example = San Andreas Fault and offsets to oceanic ridge.

 

 

Evidence for Plate Tectonics - ocean basins provide most direct evidence (oceanic ridge, trench, magnetic anomalies, age of ocean crust, etc.), but oceans are geologically very young (and expanded view of same map showing legend better) and provide data for only ~4% of Earth history. Why?

They are also difficult to access directly. Laser and Global Positioning System satellite data of accurate positions of locations on Earth support concept of slow movement of tectonic plates (~2 - 10 cm per year). Must look at rocks on continents to see further back in Earth history, some parts of geologic past (especially very old events) can be difficult or impossible to unravel because evidence (rocks) has been obscured, buried or destroyed. See outstanding Web site with global maps showing changing nature of continents and oceans over the geologic past (up to 650 million years ago) and into the future (up to 250 million years from now).

III) Continental Margins - where continental land masses meet ocean. 4 different kinds:

A) Atlantic-type (passive) margin - present on both sides of Atlantic ocean (and Indian and Arctic oceans, map). These develop as continents rift apart to form new ocean basin; in middle of plate (not along plate boundary); broad continental shelf underlain by thick sequence of marine sediments (clastic and carbonate). Below are remnants of rifting stage (non-marine sediments, evaporites, basalt lava, and normal faults, cross-section, link #2).

B) Andean-type (convergent) margin - ocean/continent convergence, oceanic crust is subducted (e.g., Andes Mountains); abrupt changes in topography - oceanic trench to high mountains within 200 km of distance.

Trench (deep linear trough) = where oceanic plate begins subduction. Mountains = active volcanoes of andesitic composition (magmatic or volcanic arc), due to melting associated with down-going slab. Much of rising magma does not reach surface and forms intrusive bodies (large granite batholiths). Igneous activity also results in metamorphism of crustal rocks. Inner wall of trench (towards arc) consists of accretionary wedge (thrust-faulted and folded marine sediment that is scraped off downgoing slab). Complex deformation and high pressure-low temperature metamorphism are characteristics of accretionary wedge. Between accretionary wedge and volcanic arc = ~undeformed forearc basin, comprised of clastics (sedimentary rocks) shed from volcanic arc. Behind arc = fold and thrust belt, then grades to undeformed crust. Narrow or absent continental shelves.

Examples = western South America, NW USA (Cascades = magmatic arc), southern Alaska.

C) Japan-type (back-arc) margin - passive margin then small area of oceanic crust (± spreading center) then island arc (from ocean/ocean convergence). Island arc is similar to magmatic arc (mountains from andesite volcanoes, fore arc basin, accretionary wedge, and trench) except ocean crust surrounds both sides (cross-section). Examples = Japan and other parts of western Pacific (map).

D) California-type (transform) margin - strike-slip faulting (near vertical with lateral movement) results in abrupt topographic differences between continents and oceans (irregular mixture of basins and ridges as crustal fragments are broken and juxtaposed next to oceanic crust), poorly developed shelf, and deep sedimentary basins. Example = central and southern California.

IV) Collisions (Orogenesis, i.e., mountain-building)

As convergent plate boundaries evolve, down-going plate eventually carries either island arc or continent to subduction zone and it becomes collision zone. There are many different specific kinds of collision zones (Andean - Atlantic; Japan - Atlantic; Andean - Andean; etc.). Fig. 19-34 = Andean - Atlantic margin collision zone.

Example = Himalayas (India collided with and thrust under Eurasia). Important features of this kind of collision zone:

Growth of Continents - occurs as colliding island arcs or microcontinents get sutured onto main continent. These pieces added to continents = suspect (or exotic) terranes, because they formed elsewhere and were added (accreted).

Process of attaching arc or microcontinent to main continent = docking. Age and rock types in suspect terrane are usually different from those that surround it. Faults usually bound different terranes. Terranes were accreted to eastern North America during Paleozoic (Appalachian orogeny) and to western North America during Mesozoic/Cenozoic (Cordilleran orogeny).

Continental Interior = craton - relatively old area of continent that has been tectonically stable (little major faulting or folding) for long period of time (for North America since Precambrian). General features of North American craton are:

Rocks - overview

Rock = coherent and relatively hard, naturally formed mass of mineral matter. Rocks are usually characterized based on:

1) mineral content - relative abundance of minerals
2) texture - size of mineral grains (coarse-grained = visible grains vs. fine-grained microscopic grains), grain shape (rounded, angular, interlocking) and grain arrangement (preferential vs. random).

Classes of Rock (based on how they form)

A) Igneous - form by cooling and solidification of hot molten rock, typical texture = interlocking grains with random arrangement.

B) Sedimentary - form by hardening (lithification) of layers of sediment (loose grains of preexisting rock, e.g., igneous, metamorphic, or sedimentary) deposited at Earth's surface. Can also form by chemical or biological precipitation (i.e., dissolved ions extracted from water to form minerals). Typical feature = layering, horizontal bands formed by settling of grains in water (or land). Texture = compacted and granular ± cemented (grains compressed together with or without mineral cement).

C) Metamorphic - form when preexisting rock (igneous, metamorphic, or sedimentary) changes (shape or mineral content) due to intense heat and pressure. Metamorphism usually occurs deep below Earth's surface (e.g., during mountain building). One typical texture for metamorphic rock = foliated, metamorphic banding.

Rock Cycle (link #2) - depicts how igneous, metamorphic, and sedimentary rocks are related to each other; Each type of rock forms from pre-existing rock, all rocks at Earth's surface undergo weathering, producing dissolved ions and sediment; sedimentary rocks form near Earth's surface; metamorphic rocks form at great depths (high T and P); molten rock occurs at various depths, forming igneous rock when it cools. Continuous burial will typically produce first sedimentary, then metamorphic, and then igneous rock. Rock cycle can be interrupted at any stage due to uplift. Continuous recycling of rock due to burial and uplift.

Convergent plate boundary (animation) = example of rock cycle in context of plate tectonics. Igneous rock is produced from volcanic eruptions and igneous intrusions due to melting of down-going slab. Igneous intrusions cause metamorphism of crust. Exposed volcanic rock at surface gets weathered to form sediment, which is transported and accumulates into basins on both sides, eventually getting buried to form sedimentary rock. Some of this rock gets subducted and remelted, continuing rock cycle.