Layers of the Earth – from

The United States Geological Survey (USGS) provides a basic tutorial on Plate Tectonics  (much of this material is from their page – for additional information, please visit their page).

The Earth – It is <very simply> composed of a rigid (hard) outer layer, a ductile (flowable) middle layer, and a solid sphere at the center.  The rigid outer layer is called the crust, the ductile middle layer is called the mantle, and the inner solid sphere is called the core.

The crust varies in thickness from ~5 km beneath oceans.  Under continents the crust is, on average, 30 km but can be up to 100 km thick.

The crust and the upper part of the mantle (which is also rigid) is called the  lithosphere.  The underlying mantle that is ductile and flows is called the asthenosphere (it is solid, but close to its melting temperature)

The lithosphere is broken up into plates that move towards, away from, or past one another on the order of centimeters per year (about the rate that your fingernails grow).  Nasa has published a map with plates and plate boundaries that includes direction and rate of movement.

John Weier: Nasa:

Converging Plates

Plates that move towards one another are called converging plates.  In the example shown below, an oceanic plate is converging with a continental plate (black arrows indicate relative plate motion).  The oceanic crust is more dense (contains more mafic minerals or heavy minerals) and therefore is subducted beneath the continental plate.  At the oceanic plate is subducted, compressional forces act on both plates.  The stress is conveyed inboard and produces a fold and thrust belt (mountain belt).  The  loading of the crust in the fold and thrust belt weighs down the crust and results in a subsiding basin (in this case, called a foreland basin).  The foreland basin is filled with sediment and sedimentary rock.  Additionally, the subducting oceanic plate carries extra water or hydration into the upper asthenosphere.  The water acts to facilitate melting at the lithosphere-asthenosphere boundary.  The melt ascends and can produce a volcanic arc that parallels the ocean – continent boundary.  The volcanic arc is composed of volcanoes overlying igneous intrusions.  Metamorphic rocks are produced around the magma chamber, in the center, at depth of the fold and thrust belt as well as potentially at the base of the foreland basin.


Diverging Plates

Plates that move away from one another are called diverging plates or a rift zone (Black arrows indicated relative plate motion).  Extensional forces act to separate the lithosphere; space is created; and the ductile Asthenosphere ascends to fill the space.  The rising asthenosphere undergoes decompression melting and produces partial melting.  The melt rises upwards and results in volcanoes at the Earth’s surface.  At depth intrusive igneous rocks accumulate.  Normal or extensional faults offset the crust.  Highlands (mountains) are produced by rift flank and block uplift relative to subsiding rift. Sedimentary rocks are produced in subsiding basins where sufficient sediment is deposited, for example, the central rift.  Contact metamorphism is common around rising melt and results in a halo of metamorphic rocks.



Ancient Plates, Boundaries, Motions

How do geologists reconstruct past geological environments?  A useful technique involves understanding the modern Earth environments including plate tectonic setting and types of rock or sediment that are being produced around that plate tectonic setting.  The above cartoons are drawn from modern plate boundary settings.  For instance, a ‘classic’ ocean-continental subduction zone is the Pacific Northwest of North America where the Nazca oceanic plate subducts beneath North America.  The Cascade Mountains, including Mt Rainier and Mt St Helens, are part of the volcanic arc  formed by partial melting and magma upwelling.  In Washington and Oregon, sediment is delivered by rivers to the coast and ocean.  Some of the sediment is transported into deepwater as turbidity currents into deposits called turbidites.  The turbidites (layers of sand and mud) are then incorporated into the accretionary wedge (see subduction zone diagram above).  Originally deposited as  horizontal layers, the turbidites become inclined and near vertical in the thrust faulting processes related to subduction.