Figure 10-4 - Crustal Spreading

Figure 10-4 - Crustal Spreading

Where giant heat convection currents rise in the low rigidity rocks of the earth’s mantle and spread (A), rigid crustal plates are set in motion, causing them to split and spread apart. In the zone of spreading (B), forces are extensional and pressure is lessened on the underlying mantle rock, allowing some minerals to melt as each mineral in the mantle rock reaches its melting point, at that lower pressure. The resulting magma is relatively hot and fluid and is under pressure, causing it to rise to the surface along opening fissures, usually on the ocean floor. Fluid basaltic lavas pour onto the ocean floor forming pillow lava structure (Figure 10-12). In the zone of spreading, temperatures are high and rocks of the ocean crust experience thermal expansion, resulting in a mid-ocean rise along the spreading plate boundary. Basalt, filling the opening fissures, forms new ocean crust. If the spreading plate boundary divides a continent, ocean crust will form in the widening gap between the continental parts, and give birth to a new ocean (e.g., Red Sea).

The simplified drawing represents an ocean basin, with a spreading plate boundary at its center and continents on the right and on the left. The continental margin on the west (C) represents the passive continental margin where there is no subduction and the continental margin on the east represents an active continental margin where there is active crustal subduction and force is compressional.

The continent of the left is moving west as part of the western plate which is getting larger with plate movement. The western edge of the left continent, beyond the drawing, is probably an active margin with subduction of ocean crust beneath the continent, like we see on the right. If this is so, there is a mountain range on the west edge of the left continent and the entire continent may drain to the ocean on its eastern margin (e.g., South America) and huge amounts of sediment are deposited at the continental margin (C), depressing the crust by isostasy. This thick sequence of sediments deflects the mantle convection downward and becomes the folded sedimentary layers of the next mountain range at the new subduction margin.

The continent on the right is moving west against the eastward moving ocean crust (D). Compressional forces fold a thick sequence of coastal sediments to form a mountain range parallel to the subducting plate boundary, and partial melting of the descending, mafic ocean crust provides viscous felsic magmas for violent stratovolcanoes and giant batholiths which form at depth beneath the mountain range.