Earth Part 3 G. Yannakoulias

Understanding Earths Surface By examining its surface can better understand what is underneath crust Observing surface of land is easy Observing bottom of oceans is difficult

Early Maps of Earth’s surface Early map of Earth (1595) Does this look much like the Earth as we know it to be today?

Mapping Sea Floor cont… Initially used long ropes with weights (pre 1920’s) Sonar was used later (post 1920’s) Found that the bottom of the oceans contained many features that were also found on land—i.e. mountain ranges (ridges) and valleys (trenches)

Mid-Ocean Ridges

Mid-Ocean Ridge Features Underwater mountain range Can be very high (up to 3000 m high) Can be quite long (e.g. mid-atlantic ridge is 16,000 km long) Earthquakes and volcanic eruptions are common along these ridges Have Rift Valleys in the middle of these ridges

Rift Valleys These are valleys in the middle of mid-oceanic ridges They are cracks or rifts in the oceanic crust forming the sea floor Molten rock or magma rises from the mantle to fill these cracks

Rift Valley

Trenches Deepest parts of the ocean Are long and narrow depressions on sea floor Can be thousands of km long and many kilometers deep Marianas trench is deeper than mount Everest is tall!

Oceanic Trenches

Sea-Floor Spreading Ocean rock is younger than land (i.e. continental) rock Ocean floor near mid-ocean ridges is younger than ocean floor further away from these mid- ocean ridges Reason: new oceanic rock made at mid-ocean ridges This idea is known as sea-floor spreading

How Are These Ridges and Trenches Formed?

Sea-Floor Spreading

Mechanism of Sea-Floor Spreading Convection Currents in Upper Mantle pull apart oceanic plates at mid-oceanic ridges Gaps in ocean floor replaced by magma (molten rock from mantle fills in gaps in rift valleys along mid-ocean ridges) New rock helps to push plates away from ridge Results in undersea volcanoes and thus mid-ocean ridges

Mechanism of Sea-Floor Spreading cont…. Molten rock from a rift in a mid-ocean ridge cools, hardens forming new sea floor Convection curents in mantle continuously pushes older rock in the opposite direction on both sides of the ridge Continents can be carried by the widening ocean floor in much the same way that objects are moved by a conveyor belt

Mechanism of Sea Floor Spreading

Where Do These Oceanic Plates End Up?

Subduction Zones Ocean plates are thinner and more dense than continental plates When collide with continental plates that are less dense and lighter they slide under them These regions are known as subduction zones

Subduction Zones

How Trenches Are Formed Trenches occur where oceanic plates slide under the continental plates and there is pulling of the continental plate with the subducting oceanic plate This is due to friction This results in the formation of deep sea trenches Subducting oceanic plates are recycled in Earth’s mantle

Oceanic Plate Subducting Under Contintental Plate Notice how the oceanic plate descends down to the upper mantle and melts to become magma again Also notice how the oceanic plate pulls the edge of the continental plate forming a trench at the plate boundary

Evidence For Sea-Floor Spreading Mid-oceanic ridges and volcanic eruptions along these mid-ocean ridges

Evidence For Sea-Floor Spreading cont… Age of rock as you move from mid-ocean ridges outward

Evidence For Sea-Floor Spreading cont… Magnetic striping: Earth’s magnetic poles reverse every 200,000-300,000 years These reversals are evident in new rock formations on the sea floor as tectonic plates move apart, new rock is formed and locks in the direction of the magnetic field at the time

Magnetic Striping