Magma Chamber n Intrusive igneous rocks n magma chamber is intermittent

Idealized Ophiolite Structure n Layer numbers correspond to acoustic speeds n depths are typical

More Realistic Spreading Center Section n Note : n failed magma chambers n complexity of intrusive layers n Fractionation takes place until the last magma has solidified n the pressure where it happens determines chemistry / mineral composition

Effects of Plate Tectonics n Remember that seafloor subsides with age due to cooling and thickening of the lithosphere n same rate of subsidence in any ocean

Age vs Depth Curves n Linear when plotted vs square root n only works to 60 million years

Topography as f(spreading rate) n Faster spreading = shallower topography

Tectono-eustatic Sea Level

Formation of Guyots Happens on any plate, regardless of climate

Atolls n However, in some areas, corals grow and can keep up with subsidence n Here are the number of genera of “hermatypic” (hard) corals which are found in each area

Formation of an Atoll or Guyot n Starts off with volcanic seamount n as plate and seamount subside, coral grows upward n if coral keeps up, an atoll is formed n if not, a guyot n requires warm, clear water

Atoll / Guyot Animation

Atoll Formation n Stages of a reef n All are evident in the Hawaiian Island Chain n Lagoons are “generally” devoid of active coral growth

Atoll Structure Diego Garcia, British Indian Ocean Territory

CaCO 3 Accumulation n CaCO 3 chemistry causes it to dissolve at depth n results in a “snow line”

CaCO 3 Deposition n This pattern is evident in most oceans n depends a little on biological productivity

Hot Spots n Deep sources of extra-hot magma n rise up through mantle n are cause of intra-plate volcanism Yellowstone

Formation of the Island Chains n As plate passes over source of volcanism, a chain of islands is formed

Island Chains n For example, the Hawaiian Islands n Note ages n Ancient Hawaiians noticed this n referred to it as Pele moving from island to island

Hawaiian / Emperor Seamount Chain n All produced by the same hot spot n age progresses uniformly out the chain n plate motion changed to produce the bend in the chain

Hawaiian / Emperor Seamount Chain n Age progresses well all the way out n some error in age determination

Hot Spot Traces n The Hawaiian Islands are only one example n each hot spot under the Pacific plate produces an island chain

Iceland n Hot spot and ridge axis co- located n double dose of volcanism

“Twin Aseismic Ridges n If the hot spot becomes dormant (dies), the volcanism ceases but spreading continues n results in twin ridges

Plate Plate Positions Affect Circulation

Terminal Eocene Event n At the end of the Eocene, the Drake Passage opened up n This allowed circum-Antarctic circulation for the first time

n Circulation in the Southern Ocean was strongly affected

Deep Water Formation n This shows current conditions n impossible if no: n circulation around Antarctica n “cryosphere” (cold poles)

n Circulation around Antarctica started at around 40Ma n allowed first cold bottom water formation

Bottom Water Formation n We know this because of the “temperature” recorded by marine micro-organisms n Actually the oxygen isotopic composition n more on this later

Terminal Eocene Event n This cold, deep water production: n cooled the seafloor and the water above it n ventilated the deep ocean n lowered CO 2 levels n resulted in DROP in CCD n resulted in enhanced preservation of CaCO 3

CCD Fluctuations n The CCD has fluctuated many times over the years n due to variety of factors n sea level change has major effect

Formation of Carbon-Rich Sediments (OIL!) n All formed in shallow seas n plate motions produced and altered these

Carbonaceous Sediment Formation n Same conditions likely to form salt and oil: n restricted circulation n shallow seas n High production