The correct option is Deep-seated earthquake foci.
Explanation
Mid-Oceanic Ridges (MORs) represent divergent plate boundaries where tectonic plates move away from each other. This process involves the upwelling of magma from the mantle, leading to the creation of new oceanic crust (seafloor spreading).Analysis of Options:
- Intense volcanic activity Intense volcanic activity (Characteristic): As plates diverge, pressure decreases, causing the mantle to melt (decompression melting). Basaltic magma rises to the surface, resulting in continuous, though often non-explosive, volcanic eruptions along the ridge.
- Shallow earthquake foci Shallow earthquake foci (Characteristic): Earthquakes at divergent boundaries are generated by the fracturing of the thin, brittle crust as it pulls apart. These seismic events are almost exclusively shallow-focus (depths less than 70 km) because the lithosphere is hot and thin in these regions.
- Deep-seated earthquake foci Deep-seated earthquake foci (NOT Characteristic): Deep-focus earthquakes (depths up to 700 km) occur primarily at convergent boundaries (subduction zones), where a cold, dense lithospheric plate descends deep into the mantle (Benioff zone). They do not occur at Mid-Oceanic Ridges.
- Rocks with normal polarity at the crest Rocks with normal polarity at the crest (Characteristic): The rocks forming exactly at the ridge crest solidify from magma during the current magnetic epoch. Therefore, they record the Earth's current magnetic field, which is "normal" polarity. As the seafloor spreads, these rocks move away, creating symmetrical magnetic stripes.
Key Takeaway:
Mid-Oceanic Ridges are characterized by shallow-focus earthquakes, high heat flow, and basaltic volcanism. Deep-focus earthquakes are a distinctive feature of subduction zones (convergent boundaries), not divergent boundaries.