Seafloor spreading occurs at mid-ocean ridges where tectonic plates move apart, or diverge. This movement creates a gap that is subsequently filled by upwelling magma, which cools to form new oceanic crust, effectively widening the ocean basin over geological time.

Divergent plate boundaries occur when two tectonic plates move away from each other. As the plates separate, magma from the mantle rises to fill the gap, cooling to create new crust. This process is responsible for the formation of mid-ocean ridges and rift valleys. It is a fundamental concept in plate tectonics that explains the continuous renewal of the Earth's surface and the expansion of oceanic basins.

Mid-ocean ridges are continuous mountain ranges formed on the ocean floor at divergent plate boundaries. As tectonic plates move apart, magma rises from the mantle to create new oceanic crust, forming these underwater ridges.

California is home to the San Andreas Fault, which is a transform plate boundary where the Pacific Plate and the North American Plate slide horizontally past each other. This tectonic activity is responsible for frequent seismic events in the region, making it a primary site for studying transform plate movements.

The Earth's crust is in a constant state of renewal. At divergent plate boundaries, new crust is created through volcanic activity as magma rises. Conversely, at convergent boundaries, old crust is destroyed through subduction, where one plate sinks into the mantle. This continuous cycle of creation and destruction is a fundamental concept of plate tectonics.

Continental crust is primarily composed of granitic rocks, which are rich in lighter elements like silicon and aluminum (often called SIAL). In contrast, oceanic crust is composed of denser basaltic rocks, rich in magnesium and iron (often called SIMA). This difference in mineral composition and density is a key factor in plate tectonics, influencing subduction processes.

In plate tectonics, a destructive plate boundary, also known as a convergent boundary, occurs where two plates move toward each other. One plate is often forced beneath the other in a process called subduction, leading to the destruction of lithospheric material as it melts into the mantle. While subduction is a process, 'destructive' is the standard classification for these convergent margins.

A constructive plate boundary, also known as a divergent boundary, occurs where tectonic plates move apart from each other. As the plates separate, magma rises from the mantle to fill the gap, cooling to form new oceanic crust. This process is fundamental to seafloor spreading and the continuous renewal of the Earth's lithosphere.

The term 'tectonics' originates from the Greek word 'tektonikos', which is derived from 'tekton', meaning 'builder' or 'carpenter'. In geological terms, it refers to the processes that build or deform the Earth's crust, leading to the formation of large-scale structural features.

Oceanic crust is primarily composed of basalt and gabbro, which are mafic rocks rich in iron and magnesium. These elements are denser than the silica-rich, felsic rocks like granite that primarily constitute continental crust. This difference in mineral composition and elemental density is the primary reason why oceanic plates are denser and sit lower in the mantle.