Rocks of different origins and ages occur in three fundamentally different geological provinces Mountain belts Cratons or shield areas Rift systems –Have paired deep valleys and flanking mountain ranges –Boundaries between valleys and flanking ranges are steeply-inclined faults –Faults accommodate crustal elongation –Rifts experience extensive igneous activity –Volcanoes in rift systems are rarely explosive; volcanism is quiescent

Rocks of different origins and ages occur in three fundamentally different geological provinces Mountain belts Cratons or shield areas Rift systems –Consist of linked collections of deep valleys and moderately high flanking mountain ranges –Steeply-inclined faults mark the boundaries between deep valleys and flanking ranges –Faults accommodate crustal elongation –Valleys and flanking ranges are often the locus of extensive igneous activity –Active volcanoes in rift systems are rarely explosive; volcanism is quiescent

Bulk chemical composition of continents difficult to determine accurately, but we estimate: Continental rock is relatively enriched in –SiO 2 Al 2 O 3 K 2 O Na 2 O CaO Continental rock is relatively depleted in – FeO MgO

Taking the elevation of 1 km below sea level as edge of continents, roughly 40% of earth’s crust is continental

Ocean basins Four ocean basins on earth at present Ocean floors have a restricted suite of rock types –Unconsolidated sediments (clays, oozes, etc.) –Fine-grained sedimentary rocks –Extrusive igneous rocks - basalts –Intrusive igneous rocks - gabbros –Foundation is serpentinized peridotites, which are part of the earth’s mantle

Ocean basins Four ocean basins on earth at present –Atlantic, Pacific, Indian, & Arctic Find a relatively restricted suite of rock types on ocean floors –Unconsolidated sediments –Fine-grained sedimentary rocks (clays, oozes, etc.) –Extrusive igneous rocks of a particular chemical composition - basalts –Intrusive igneous rocks of the same particular chemical composition - gabbros –All are underlain by serpentinized peridotites, which are part of the earth’s mantle

Ocean floors are so dominated by basalts that the bulk chemical composition of oceanic rock is relatively easy to estimate: Ocean floor rock is relatively enriched in – Al 2 O 3 CaO FeO MgO Ocean floor rock is relatively depleted in – SiO 2 K 2 O Na 2 O

Oceanic rocks are uniformly younger than 200 Ma old While this sounds like an immense length of time, it is only about 5% of earth’s history

Unique geomorphologic features in ocean basins Continental shelves –70-80 km wide regions where water depth <200 m –Underlain by thick accumulations of unconsolidated sediments and young sedimentary rocks –Sediment derived from adjacent continent –Strata lie on a basement of continental rock –Are the submerged fringes of continents

Unique geomorphologic features in ocean basins Continental shelves –Typically km wide; water depth is typically <200 m –Often have very thick accumulations of unconsolidated sediments and young sedimentary rocks –Detritus or sediment is derived from the adjacent continent –Sedimentary rocks lie on a basement of regular, continental rock. Basement may be igneous, metamorphic, or old sedimentary rocks, depending upon what crops out above shoreline. –Continental shelves are the submerged fringes of continents Continental slopes Continental rises Abyssal plains

Unique geomorphologic features in ocean basins Continental shelves Continental slopes –Seaward of continental shelf, water depths increase from 200 m to 3-4 km –Average surface slope is 4-5 –Underlain by sedimentary rocks continuous with those in continental shelves –Basement still is typical continental rock

Unique geomorphologic features in ocean basins Continental shelves Continental slopes –Continental slopes are found seaward of continental shelves –Water depths increase from 200 m to 3-4 km (5-10 km along ‘active’ continental margins) –Average surface slope is 4-5°; slope is very gentle but is still 20x average dip of continental shelves –Underlain by young sedimentary rocks that are continuous with those found in continental shelves –Basement still is typical continental rock Continental rises Abyssal plains

Unique geomorphologic features in ocean basins Continental shelves Continental slope Continental rises –Seaward of continental rise –Water depths usually about 3-4 km –Average surface slope is ~1° –Underlain by strata continuous with those in continental slopes –Basement can be either granites or basalts and gabbros

Unique geomorphologic features in ocean basins Continental shelves Continental slope Continental rises –Found seaward of continental rise –Water depths usually about 3-4 km (5-10 km along ‘active’ continental margins) –Average surface slope is ~1° –Underlain by young sedimentary rocks that are continuous with those found in continental shelves –Basement may be either continental rock (granites) or oceanic rock (basalts and gabbros) Abyssal plains

Unique geomorphologic features in ocean basins Continental shelves Continental slopes Continental rises Abyssal plains –Lie beneath km of water –Nearly level, often featureless plains Abyssal hills rise <1 km above abyssal plains Seamounts rise >1 km above abyssal plains –Sea floor sedimentary rocks are distinct from those found near continents –Basement is basalts or gabbros

Unique geomorphologic features in ocean basins Continental shelves Continental slopes Continental rises Abyssal plains –Nearly level floors of ocean basins lie beneath km of water –Largely featureless, but with some local relief –Abyssal hills rise < 1km above abyssal plains –Seamounts rise > 1 km above abyssal plains –Underlain by young sedimentary rocks that are distinct from those found fringing the continents –Basement is oceanic rock (basalts or gabbros)

Continental shelves, slopes and rises are dissected by submarine canyons Deep, V-shaped valleys Some submarine canyons abut or connect with rivers that flow off continents –Turbidity currents derived from rivers erode a continuation of the river Some submarine canyons are ‘headless’ –Headless canyons often have periodic spacing –inferred to where escaping from sediment weakens sediments and sedimentary rocks, leading to slope failure

Continental shelves, slopes and rises are dissected by submarine canyons Deep, V-shaped valleys eroded into shelves, slopes and, to a lesser degree, rises Some submarine canyons abut or connect with rivers that flow off continents –Sediment-charged river water in turbidity currents flows down canyon, eroding a continuation of the river Some submarine canyons are ‘headless’ –Headless canyons often have periodic spacing along the continental margin –Are inferred form where a high water table leads to weakening of sediments and sedimentary rocks, which causes slope failure

The remaining geomorphic features in ocean basins (found in the abyssal plains) correlate with and correspond to three distinct geological provinces

Mid-ocean ridges (MORs) Stand km above abyssal plains, and may be ~1000 km across Axial valleys are 600 m – 2 km lower than ridge mountains Axial valley/ridge mountains abut oceanic fracture zones Numerous volcanoes have quiescent eruptions Axial valleys & ridge mountains composed of basalts in distinctive pillow forms

Mid-ocean ridges (MORs) Stand km above abyssal plains, and may be ~1000 km across Characterized by high relief; steep-sided axial valleys or troughs are 600 m – 2 km lower than adjacent ridge mountains In plan, straight sections of axial valley/ridge mountains abut against oceanic fracture zones Axial valleys and ridge mountains dotted by active volcanoes that erupt often, with a quiescent eruption style Axial valleys and ridge mountains composed of volcanic rock - basalts in distinctive pillow forms

Mid-ocean ridges (MORs), continued Near axial trough, sediments and sedimentary rocks are thin or absent Sediment thickness increases with distance from axial trough, but never exceeds 1.3 km Age of oldest sediments increases with distance from axial trough Sedimentary rocks are cut by faults, but MORs are not like continental mountains

Mid-ocean ridges (MORs), continued In vicinity of ridge mountains or axial valleys, rare unconsolidated sediments and sedimentary rocks occur in thin, flat-lying layers Thickness of sediment cover increases with distance from axial trough, but never exceeds 1.3 km Sedimentary rocks may be cut by faults but are not deformed like sedimentary rocks in continental mountains Age of the oldest sedimentary rock (lying on basalt) is greater at increasing distance from axial trough