Earth Materials- Hefferan/ O'Brien

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Earth Materials (abbr. E.Ms)

Minerals (abbr. mnrl) are defined by five properties:

Solid

Naturally occurring

Minerals usually ford by inorganic processes (Some solid E.Ms form by both inorganic and organic processes- ex. calcite CaCO3)

Each mnrl. species has a specific chemical composition- In addition, some types of atoms may substitute freely for one another when a mineral forms, generating a well-defined range of chemical compositions (Mg and Fe my substitute freely for one another in the mineral olivine)

Every mnrl. species possesses a long range, geometric arrangement of constituent atoms + ions- mrnls. are not randomly arranged. This pattern is called its crystal structures.

Solid materials that lack a long-range crystal structure are amorphous materials, meaning w/o long-range geometric order.

Naturally-occurring, solid materials that lack one of the properties described ^ are commonly referred to as mineraloids (abbr. mnrlds.)

A rock (abbr. rck.) is an aggregate of mnrl. crystals and/or mnrlds.

A monominerallic rck. consist of multiple crystals of a single mineral.

Ex. sedimentary rck. quartz sandstone, consists of quartz held together by quartz cement

Most rcks. are polyminerallic; composed of many types of mineral crystals .

Ex. granite commonly contains quartz, potassium feldspar, plagioclase, hornblende, and biotite

Compositional Layers

The layers within the Earth are defined largely on the basis of chemical composition.

These include: (1) the crust, which is subdivided into the continental and oceanic crust, (2) the mantle, annd (3) the core.

Mechanical layers

The layers that are defined principally on the basis of mechanical properties include: (1) a strong lithosphere that include all of the crust and the upper part of the mantle; (2) a weaker asthenosphere including a transition zone; and a (3) mesosphere or lower mantle. The core is divided into a liquid outer core and a solid inner core.

Earth's Crust

The crust is separated from the underlying Mohorovicic (Moho) discontinuity. Two major types of crust occur.

Oceanic Crust (abbrv. O.C)

Composed largely of dark colored, mafic rcks. enriched in oxides of magnesium, iron, and calcium.

The elevated iron content is responsible for both the dark color + the elevated density of the O.C.

The elevated density + small thickness of oceanic crust cause it to be less buoyant than continental crust.

O.C consists consists principally of rcks. such as basalt and gabbro, composed largely on minrls. pyroxene and calcic plagioclase.

O.C is young relative to the age of the Earth (~4.55 Ga= 4550 Ma).

Older O.C has largely been destroyed by subduction

Continental Crust (abbr. C.C)

C.C can be generalized as "granitic" in composition.

Although igneous and metamorphic rocks of granitic composition are common in the upper portion of C.C, lower portions contain more rcks. of dioritic and/or gabbroic composition.

C.C is generally much thicker than O.C; the depth of the Moho avg. ~30km

Greater thickness + lower density of C.C makes it more buoyant than O.C

The oldest well-documented C.C includes 4.03 Ga rcks. from the Northwest Territories of Canada

Crust is suggested to have began forming within 300 million years of Earth's birth

In concrast to O.C, C.C is largely preserved as its density is too low for it to be readily subducted.

Earth's Mantle

The mantle is thick relative to the Earth's radius and constitutes ~83% of Earth's total volume.

Mantle is distinguished from the crust by being very rich in MgO and FeO, it contains an average of ~40-45% SiO2, which means it has an ultrabasic composition.

In the upper mantle the silicate minrls olivine + pyroxene are dominant; spinel, plagioclase and garnet are locally common.

These mnrls combine to produce dark-colored ultramafic rcks. such as peridotite, the dominant group of rcks. in the upper mantle

Upper mantle and transition zone

The uppermost part of the mantle and the crust together constitute the relatively rigid lithosphere.

A discrete low velocity zone (LVZ) occurs within the upper mantle; the top of the LVZ marks the contact btwn. the strong lithosphere and the weak asthenosphere .

The asthenosphere is more plastic and flows slowly, rather than rupturing when subjected to stress.

The low P-wave velocity of the LVZ has been explained by small amnts. of partial melting.

Below the LVZ, seismic wave velocities increase indicating that the materials are more rigid solids. These materials are still a part of the weak asthenosphere which extends to the base of the transition zone, between the upper and lower mantle.

Olivine -> Wadleysite -> Ringwoodite -> Perovskite

These mnrl. changes are inferred to be largely responsible for the seismic wave velocity that occur at 410 and 660 km

Lower mantle (mesosphere)

Perovskite, periclase, magnesiowustite, stishovite, ilmenite, and ferrite are thought to be the major minerals in the lower mantle/mesosphere.

D" Layer

Anything else...?

Earth's core

Earth's core consists primarily of iron (~85%) with smaller, but significant amnts. of nickel (~5%) and light elements of (~8-10%) such as oxygen, sulfur and/or hydrogen.

B/c S-waves are not transmitted by non-rigid transmitted by non-rigid substances such as fluids, the outer core is inferred to be liquid.

Circulating molten iron in Earth's outer core is responsible for the production of most of Earth's magnetic field.

The outer/inner core boundary, the Lehman discontinuity, is marked by a rapid increase in P-wave velocity and the occurrence of low velocity S-waves.

Density + magnetic studies suggest that Earth's inner core also consists largely of iron, with less nickel and oxygen, sulfur and/or hydrogen than in the outer core.

Global Tectonics

The fundamental tenet of plate tectonics is that the lithosphere is broken along major fault systems into large pieces called plates that move relative to one another.

The fact that plates overlie a weak, slowly flowing asthenosphere permits them to move.

Each plate is separated from adjacent plates by plate boundary segments ending in triple junctions, where three plates are in contact.

The relative movement of plates with respect to the boundary that separates them defines tree major types of plate boundary segments and two hybrids: (1) divergent plate boundaries, (2) convergent plate boundaries, (3) transform plate boundaries, and (4) divergent-transform and convergent-transform hybrids.

Divergent Plate Boundaries

Occur when two major plates are moving apart relative to their boundary.

Such areas are characterized by horizontal extension and vertical thinning of the litphosphere .

Horizontal extension is marked by continental rift systems + the ocean ridge system

Continental Rifts

Continental rift systems form where horizontal extension occurs in continental lithosphere.

This stretching occurs by brittle, normal faulting near the cooler surface and by ductile flow at deeper, warmer levels.

Extension is accompanied by uplift of the surface as the hot asthenosphere rises under the thin lithosphere.

If horizontal extension and vertical thinning occur for a sufficient period of time, the continental lithosphere may be completely rifted into two separated continents.

Oceanic ridge systems

The oceanic ridge system is Earth's largest mountain range and covers roughly 20% of Earth's surface.

O.C and lithosphere form along the axis of the ridge system, then spreads away from it in two directions, causing ocean basins to grow through time.

As the lithosphere is thinned, the asthenosphere rises toward the surface generating basaltic-gabbroic melts.

B/c most oceanic lithosphere is produced along divergent plate boundaries marked by the ridge systems, they are also called constructive plate boundaries.

As the sea floor spreads away from the ridge axis, the crust thickens from above by the accumulation of additional marine sediments and the lithosphere thickens from below by a process called underplating , which occurs as the solid, unmelted portion of the asthenosphere spreads laterally and cools.

As the denser entire lithosphere cools, it contracts, becomes denser and sinks so that the floors of the ocean gradually deepen away from the thermally heated ridge axis.

If the density of the ocean lithosphere exceeds that of the underlying asthenosphere, subduction occurs.

Chptr 1- earth materials and the geosphere