the axis into segments. One hypothesis for different along-axis depths is variations in magma supply to the spreading center. Ultra-slow spreading ridges form both magmatic and amagmatic (currently lack volcanic activity) ridge segments without transform faults. Volcanism Mid-ocean ridges exhibit active volcanism and seismicity. The oceanic crust is in a constant state of 'renewal' at the mid-ocean ridges by the processes of seafloor spreading and plate tectonics. New magma steadily emerges onto the ocean floor and intrudes into the existing ocean crust at and near rifts along the ridge axes. The rocks making up the crust below the seafloor are youngest along the axis of the ridge and age with increasing distance from that axis. New magma of basalt composition emerges at and near the axis because of decompression melting in the underlying Earth's mantle. The isentropic upwelling solid mantle material exceeds the solidus temperature and melts. The crystallized magma forms a new crust of basalt known as MORB for mid-ocean ridge basalt, and gabbro below it in the lower oceanic crust. Mid-ocean ridge basalt is a tholeiitic basalt and is low in incompatible elements. Hydrothermal vents fueled by magmatic and volcanic heat are a common feature at oceanic spreading centers. A feature of the elevated ridges is their relatively high heat flow values, ranging from between 1 μcal/cm2s to about 10 μcal/cm2s. (Micro calories per centimeter squared per second) Most crust in the ocean basins is less than 200 million years old, which is much younger than the 4.54 billion year age of the Earth. This fact reflects the process of lithosphere recycling into the Earth's mantle during subduction. As the oceanic crust and lithosphere moves away from the ridge axis, the peridotite in the underlying mantle lithosphere cools and becomes more rigid. The crust and the relatively rigid peridotite below it make up the oceanic lithosphere, which sits above the less rigid and viscous asthenosphere. Driving mechanisms The oceanic lithosphere is formed at an oceanic ridge, while the lithosphere is subducted back into the asthenosphere at ocean trenches. Two processes, ridge-push and slab pull, are thought to be responsible for spreading at mid-ocean ridges. Ridge push refers to the gravitation sliding of the ocean plate that is raised above the hotter asthenosphere, thus creating a body force causing sliding of the plate downslope. In slab pull the weight of a tectonic plate being subducted (pulled) below an overlying