Lithospheric Plates Notes
The rigid outer shell of the Earth is called the lithosphere.
The lithosphere is broken into a series of moving plates.
The boundary between two plates can be a divergent boundary, a convergent boundary or a transform
boundary.
The lithosphere is composed of the crust, and some of the upper mantle.
Mature oceanic plate is approximately 80-100 km thick, of which, the upper 5-7 km is oceanic crust.
Continental plate is somewhat thicker, perhaps 150-200 km thick, and includes continental crust which
varies from 5 to 70 km in thickness.
A single plate may be part oceanic and part continental. In that case, the boundary between ocean crust
and continental crust is not a plate boundary.
The mobile part of the mantle under the lithosphere is called the asthenosphere.
Granite forms the continental crusts
Basalt forms the oceanic crusts
Plate Boundaries
The boundary between two plates can be a divergent boundary, a convergent boundary or a transform
boundary. Plate boundaries are most readily identified by seismicity, the location of significant
earthquake activity. About 95% of the world’s earthquakes occur on plate boundaries, and all
earthquakes occur in the lithosphere.
Divergent Boundaries

At divergent plate boundaries, two lithospheric plates move apart, and new plate (and crust) are created at
the boundary. The new crust is created volcanically. This process always produces ocean crust and thus
oceanic plate. Thus divergent boundaries ultimately become mid-ocean ridges.

As the lithosphere moves away from the ridge, it thickens by a process of underplating in which
asthenosphere solidifies onto the underside of the plate as the plate cools.

The thickening lithosphere subsides as it ages. The ridges are a result of this subsidence. Young
lithosphere near the ridge sits up high, while older lithosphere sits lower.

Ultimately, ocean lithosphere becomes more dense than the asthenosphere beneath it. Thus old oceanic
lithosphere will sink into the asthenosphere if it gets the chance.
Convergent Boundaries

At convergent plate boundaries, two lithospheric plates move together. The denser plate sinks in a process
called subduction.

The subduction process produces magmas through processes we will discuss later.

Only oceanic plate can subduct. Continental crust is too thick and low density to sink. It acts like a
float.

Subducting plate remains more rigid than the surrounding mantle to depths of about 700 km. This is the
maximum depth of earthquakes which only occur in rigid lithosphere. Somewhere beneath this depth, old
plates just fade into the mantle.

At some convergent boundaries, oceanic plate meets continental plate. An example of this is the west
coast of southern Mexico, Central America and South America. An oceanic trench forms offshore, and a
chain of volcanic mountains forms just inland of the coast. This type of boundary between oceanic crust
and continental crust is called an active continental margin since it is a plate boundary.

At other convergent boundaries, oceanic plate meets other oceanic plate. This results in an oceanic trench
and a chain of volcanic islands known as an island arc. Japan, the Philippines and the Aleutians are
examples of island arcs.

At some convergent boundaries, two continents collide. Neither can subduct, so the result is like a big car
crash. Both continents are crumpled, large mountain belts are formed, and their insurance rates go up.
Transform Boundaries

Transform boundaries occur when two plates move parallel to one another in opposite directions, so that
there is neither divergence or convergence. This is rare. It occurs for short intervals along mid-ocean
ridges. Thus many short transform boundaries are found along, and perpendicular to mid-ocean ridges.

Transform boundaries on continents are rare. The San Andreas fault in California is a transform
boundary. Another transform boundary cuts across the southern island of New Zealand.

No volcanism occurs at transform boundaries
What Causes Plate Tectonics

The ultimate cause of plate tectonics is the cooling of the Earth. The core and inner Earth have retained
enormous amounts of heat from the initial formation of the Earth. Since the mantle is made of rock, it is a
very good insulator.

Like a hot cup of coffee, the fastest way to cool the Earth is to stir it. Thus the material of the mantle
convects enabling the release of heat. The movement of plates is the surface manifestation of that mantle
convection.

Oceanic plates are driven by their own sinking. Thick, cold, dense oceanic lithosphere sinks at subduction
zones, pulling the rest of the ocean plate. Since plates have limited tensional strength, this "subduction
pull" cannot be the only force acting on an ocean plate. "Ridge push" is the sliding of the young thin part
of the plate downhill away from the ridge. A combination of "subduction pull" and "ridge push" move the
ocean plates along. These forces are not separate from mantle convection, but are both caused by and help
cause that convection. Continental plates are pushed around by oceanic plates.
EARTHQUAKES/SEISMOLOGY REVIEW SHEET
Earthquake Locations Where do earthquakes occur?
• Earthquakes occur along plate boundaries where two chunks of lithosphere scrape
together
• The focus is the location where the energy is released underground
• The epicenter is location on the earth’s surface directly above the focus
• Faults are cracks along which rocks break, move and slide
Seismic Waves
• Seismic Waves- during an earthquake, several types of waves are generated. The
vibrations felt are actually called seismic waves that are traveling through the Earth.
P-Waves
• Primary Wave (Compression Wave)
• Travels phastest so it arrives at a seismic stations phirst
• Push-pull wave: rock vibrates forward and backward in the same direction that the wave
travels
• Pass through solids, liquids (magma), and gasses
S-Waves
• Secondary Wave- arrives at a seismic station second.
• Slow wave- not as fast as the P-wave.
• Shake wave (Shear Wave)- vibrates side-to-side.
• Solids wave- only travels through Solids. ****P-Waves travel through solids, liquids, and gasses
Seismographs
• Instrument that detects and records seismic waves
• By studying a seismogram, we can determine a seismic wave’s distance and size
Seismic Waves Traveling Throughout the Earth
• S-Waves travel only through solids
• Seismic waves travel faster through denser material.
• Because of this, the path traveled by a seismic wave is bent towards the surface.
• Properties of the material (such as density and pressure) that the waves pass through can be inferred by
the speed and angle that the waves travel.
• The layers of the earth are determined by the jumps in velocity and “echoes” of seismic waves.
• The MOHO is a boundary between the crust and the upper mantle where the velocity of waves jumps up
sharply. This sharp increase in velocity is called a discontinuity.
• A shadow zone occurs on the opposite side of the earth from an earthquake because of the liquid outer
core.
• S-Waves are stopped all together as they are absorbed by the liquid outer core. The PWaves are
refracted (bent) as they change velocities in diferent density materials. This creates a zone in which no
waves are picked up at all.
P-Waves travel through solids, liquids, and gasses