480 SEAMOUNTS of Cretaceous seamounts is almost double that of Cenozoic seamounts Morphology Figure Seamount chains in the Rano Rahi Field adjacent to the southern East Pacific Rise The shaded relief bathymetry reveals a remarkable abundance of seamounts near the part of the spreading axis with the greatest inferred magma budget in the region contains eight submarine calderas that have erupted large (more than 100 km3) volumes of rhyolite pumice In contrast, Kick-Em-Jenny, the southernmost island in the Lesser Antilles, is an island-arc seamount that has erupted 10 times since 1939, forming lava domes of basaltic magma within the summit caldera of its very steep-sided edifice Almost every island arc has a number of seamounts that are smaller versions of the emergent volcanic islands making up the arc Mud volcanoes are a unique amagmatic type of seamount found in the fore-arc regions of several subduction zones The widely accepted explanation for mud volcanism is the upwelling of overpressured serpentinite diapirs from the subduction de´ collement through normal faults in the fore arc One of the largest and best-studied mud volcanoes is the Conical Seamount (25 km in diameter and km high), which is located in the fore arc of the Marianas Arc As more seafloor mapping is done along outer fore arcs, more examples of mud volcanoes are discovered Distribution through Time: Cretaceous Seamounts During the Cretaceous (85–120 Ma) incredibly intense seafloor volcanism occurred throughout what is now the western Pacific (Figure 1) Not only were many of the large oceanic plateaus emplaced in the Pacific during the Cretaceous (Ontong–Java, Manihiki, Shatsky), but also the topography of the seafloor reveals a striking abundance of large seamounts (Mid-Pacific Mountains, Magellan Seamounts, Wake Guyots, etc.) Both the highest density of seamounts per unit area and the highest density of large (more than 3.5 km high) seamounts occur in the Pacific Satellite altimetry data reveal that the concentration The vast majority of seamounts are predominately constructed of effusive lava flows Thus, the overall shapes of seamounts are primarily controlled by the geometry of the magma plumbing system, the eruption rate, the lava type, and the local stresses on the volcano edifice The cooling effect of water leads to the construction of much steeper edifices than those on land Most seamounts have steep (10 – 30 ) sides but nearly flat summit areas, in contrast to the low slopes of shield volcanoes (5 – 10 ) As a seamount grows, it is subject to more complex stresses and tends to develop a more complex shape (Figure 4) In map-view, seamount shape evolves from nearly circular small seamounts to the more complex stellate forms of large seamounts and ocean islands As a seamount grows larger, mass wasting from gravitational instability or wave erosion becomes more important in modifying its morphology Seamount Growth and Development Deep-water stage Basaltic magma erupted under high hydrostatic pressure produces non-explosive eruptions forming edifices built from lava flows (Figure 4A) Repeated eruptions build extrusive edifices composed of pillow lavas with minor sheet lava flows Intrusion of magma into the pelagic sediments on the seafloor produces a mixture of lava and wet sediment called peperite Small seamounts (less than km high and less than 10 km3 in volume) usually form edifices that can best be described as low lava domes Many seamounts even of this small size have relatively steep slopes and flat summits, unlike subaerial lava shields Studies of small isolated seamounts near mid-ocean ridges show that a few seamounts of this size develop complex shapes, including relatively large summit craters The seamount shape becomes generally more complex as the seamount grows When seamounts reach a somewhat larger size (ca km high and 10–100 km3 in volume) they more often have an irregular outline and an ‘overturned soup bowl’ or truncated-cone shape Shoaling stage Large seamounts (more than km high and with volumes of over 200 km3) may develop a stellate shape as a result of long rift zones concentrating lateral eruptions of lava from a central feeding system (Figure 4B) Not all large seamounts develop rift zones (e.g Galapagos), but the rift zones on some seamounts become very pronounced ridges extending for more than 100 km (e.g Hawaii) The presence of a magma chamber above the level of the seafloor and