The Ocean floor and Coast Tectonic Processes and the ocean basin. The littoral zone

The Continental Drift Hypothesis Proposed by Alfred Wegener in 1915 – revised in 1960 (to be called theory of plate tectonics) Supercontinent Pangaea started to break up about 200 million years ago. Continents "drifted" to their present positions. Continents "plowed" through the ocean crust.

Outer crust of earth (lithosphere) consists of a number of separate plates which ‘float’ on the underlying layer (asthenosphere). Plates move slowly relative to each other, Where plates move in relation to each other, there are three different types of boundaries between them. Earthquakes, mountain building, volcanic activity, and the formation of the ocean trenches all occur along these plate boundaries.

Convection Currents

Continental Drift: Evidence Geographic fit of South America and Africa Fossils match across oceans Rock types and structures match across oceans Ancient glacial features

Evidence Tight fit of the continents, especially using continental shelves.

Evidence Fossil critters and plants

Evidence Correlation of mountains with nearly identical rocks and structures

Evidence Glacial features of the same age restore to a tight polar distribution.

The Rise of Plate Tectonics WW II and the Cold War: Military Spending U.S. Navy mapped seafloor with echo sounding (sonar) to find and hide submarines. Generalized maps showed: oceanic ridges—submerged mountain ranges fracture zones—cracks perpendicular to ridges trenches—narrow, deep gashes abyssal plains—vast flat areas seamounts—drowned undersea islands Dredged rocks of the seafloor included only basalt, gabbro, and serpentinite—no continental materials.

Bands of seismicity—chiefly at trenches and oceanic ridges

These plates move slowly relative to each other These plates move slowly relative to each other. Where plates move in relation to another, there are three different types of boundaries between them known as convergent, divergent and transform boundaries. Earthquakes, mountain building, volcanic activity and the formation of ocean trenches all occur along these plate boundaries.

Most earthquakes and volcanic eruptions happen at plate boundaries. Earth’s outer shell is broken into thin, curved plates that move laterally atop a weaker underlying layer. Most earthquakes and volcanic eruptions happen at plate boundaries. Three types of relative motions between plates: divergent convergent transform

Active Margins Continental margins with intense geological activity. Earthquakes and volcanoes are characteristic here. Have steep, rocky shorelines, narrow continental shelves, steep continental slopes, and lack a developed continental rise.

Passive Margin Inactive geologically. Flat, coastal plains with wide continental shelves and gradual slopes, leading to a thick continental rise.

(b) Relate tectonic processes to the production of ocean trenches, mid-ocean ridges, hydrothermal vents, abyssal plains, volcanoes, earthquakes and tsunamis Ocean trenches are formed along tectonic plate boundaries where one plate is forced under another plate (subduction). Ocean trenches are long and narrow and are the deepest part of the ocean floor, for example, the Challenger Deep in the western Pacific Ocean is approximately 10 000 m below sea level.

(b) Relate tectonic processes to the production of ocean trenches, mid-ocean ridges, hydrothermal vents, abyssal plains, volcanoes, earthquakes and tsunamis Mid-ocean ridges are underwater mountain ranges, formed by upward movement and spreading of the underlying magma, which cools and solidifies as it emerges. This process occurs at a divergent tectonic plate boundary and is responsible for seafloor spreading.

(b) Relate tectonic processes to the production of ocean trenches, mid-ocean ridges, hydrothermal vents, abyssal plains, volcanoes, earthquakes and tsunamis Hydrothermal vents occur in the deep ocean, typically along mid-ocean ridges where two tectonic plates are diverging. Sea water which seeps into cracks in the ocean floor (and water from the upwelling magma) is released from the hot magma. Hydrothermal vents occur at depths of about 2100 m below sea level.

(b) Relate tectonic processes to the production of ocean trenches, mid-ocean ridges, hydrothermal vents, abyssal plains, volcanoes, earthquakes and tsunamis Abyssal plain: relatively flat areas between ocean trenches and continental rises. Formed by the upward movement of molten material from oceanic crust (mantle convection) Plain arises as the uneven rock surface becomes covered with fine grained sediment

(b) Relate tectonic processes to the production of ocean trenches, mid-ocean ridges, hydrothermal vents, abyssal plains, volcanoes, earthquakes and tsunamis A volcano is formed where there is an opening in the Earth’s crust, allowing hot gases and molten rock to escape from beneath the surface. Volcanoes can form where there is thinning of the crust at tectonic plate boundaries, for example at divergent plate boundaries. Volcanoes can also be formed where tectonic plates are moving towards each other, or converging. Most divergent tectonic plate boundaries are under the sea and therefore most volcanic activity is submarine. This results in the formation of new seafloor. Divergent tectonic plate boundaries also give rise to midocean ridges; where these rise above sea-level, volcanic islands may be formed, such as Iceland.

Volcanoes Underwater Lava Flow 2

Volcanoes Time lapse of the eruption

Volcano locations

(b) Relate tectonic processes to the production of ocean trenches, mid-ocean ridges, hydrothermal vents, abyssal plains, volcanoes, earthquakes and tsunamis An earthquake occurs when there is a sudden release of energy in the Earth’s crust, creating seismic waves. Earthquakes can arise from convergent tectonic plate boundaries. If the two plates are unable to slip past each other, they may lock. Continued plate movement results in strain building up until the plates suddenly move again, releasing stored energy and creating an earthquake.

Earthquakes Earthquakes are a result of motion within the earth. This only occurs where the earth is solid and therefore can only occur within about 100 miles of the surface Earthquakes provide the best evidence regarding the interior structure of the Earth.

Earthquakes