Plate Tectonics, Seismic Waves, and Crustal Dynamics
Seismic Waves and Earth's Interior
- P-waves (Primary Waves): Pass through both solid and liquid mediums.
- S-waves (Secondary Waves): Are stopped by liquids and arrive in second place.
- L-waves (Long / Love Waves): Only travel through the crust. Because of the large movement they generate, they produce the most damage and are the slowest.
By studying lava, scientists can determine the chemical composition of the mantle. Meteorites are samples of planetary material left over from the time of the formation of the Solar System.
Tectonic Concepts and Crustal Balance
- Plate: A solid slab of lithosphere; the outer layer of Earth is composed of distinct, separated pieces.
- Seafloor Spreading: The discovery that basalt lava erupts from mid-ocean ridges and cools to form new oceanic crust along the plate edges.
- Isostasy: The process of maintaining a state of gravitational balance in Earth's crust to keep it stable when geological events alter its distribution of weight.
Constructive Plate Boundaries
At constructive margins, new crustal material is being added while plates move apart.
Process of Seafloor Spreading
- New igneous material rises from the mantle and is added to the plate margins bordering the ridge.
- The plates on either side of the ridge move slowly outwards from it.
- Basalt lavas erupt and cool rapidly, often displaying distinctive pillow structures.
Evidence for Seafloor Spreading
- The oldest parts of oceanic crust are always found at ocean trenches above subduction zones.
- Every few hundred thousand years, Earth's magnetic field reverses polarity, leaving symmetrical magnetic stripes on the ocean floor.
- The continental fit of Africa and South America demonstrates that continents do not fracture evenly along a single straight line.
Destructive Plate Boundaries
At destructive margins, crustal material is subducted as tectonic plates move together. These occur at oceanic trenches where converging plates force one plate to descend beneath the other into a subduction zone.
Convergence Scenarios
- Oceanic Meets Oceanic: Forms a volcanic island arc system.
- Oceanic Meets Continental Edge: The dense oceanic plate is subducted, while the lighter continental crust is not dense enough to descend into the mantle. The continental margin and slope, which have accumulated thick layers of sediment, become compressed in the collision zone, leading to orogeny (mountain building).
- Continental Collision: Occurs when an entire ocean basin is closed by subduction. Continued plate movements drive extensive orogeny, characterized by rock folding, igneous intrusions, and volcanism.
Conservative Plate Boundaries
At conservative margins, plates slide past one another with no crustal material being added or subducted. Friction prevents the plate edges from sliding smoothly, causing movement to occur in sudden jerks accompanied by severe earthquakes.
Continental Breakup and Drift
If seafloor spreading takes place, it follows that continents must break apart and migrate across Earth's surface.
Supporting Evidence
- Continental Fit: Shorelines and continental shelves fit together across ocean basins.
- Geological Correlation: Matching rock formations, mountain belts, and structural geology across separated continents.
- Fossil Distribution: Identical fossilized plant and animal species found on widely separated landmasses.