Seafloor spreading. Mid-Atlantic Ridge

Seafloor spreading

Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. When oceanic plates diverge, fractures occur in the lithosphere. Basaltic magma rises up the fractures and cools on the ocean floor to form new sea floor. Older rocks will be found further away from the spreading zone while younger rocks will be found nearer to the spreading zone.

Age of oceanic crust; youngest (red) is along spreading centres.

Mid-ocean ridges

A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges (chains), typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading centre. The uplifted seafloor results from convection currents which rise in the mantle as magma at a linear weakness (weakened zone) in the oceanic crust, and emerge as lava, creating new crust upon cooling. A mid-ocean ridge demarcates the boundary between two tectonic plates, and consequently is termed a divergent plate boundary.

The mid-ocean ridges of the world are connected and form a single global mid-oceanic ridge system that is part of every ocean, making the mid-oceanic ridge system the longest mountain range in the world. The continuous mountain range is 65,000 km (40,400 mi) long (several times longer than the Andes, the longest continental mountain range), and the total length of the oceanic ridge system is 80,000 km (49,700 mi) long.

Black Smokers

Oceanic ridge, including a black smoker

A black smoker or sea vent is a type of hydrothermal vent found on the seabed, typically in the abyssal zones. They appear as black chimney-like structures that emit a cloud of black material. The black smokers typically emit particles with high levels of sulphur-bearing minerals, or sulphides. Black smokers are formed when superheated water from below Earth's crust comes in contact with cold ocean water; many minerals precipitate, forming a black chimney-like structure around each vent. The metal sulphides that are deposited can become massive sulphide ore deposits in time.

Answer the questions:

1. What occurs in the lithosphere when tectonic plates diverge?

2. What forms the new sea-floor along the spreading zones?

3. Where are the youngest and the oldest rocks along the spreading zone?

4. What is a mid-ocean ridge?

5. Why is a mid-ocean ridge termed a divergent plate boundary?

6. What is a black smoker?

7. What do black smokers emit?

8. What is practical significance of black smokers?

Mid-Atlantic Ridge

The ridge was central in the breakup of the hypothetical Pangaea that began some 180 million years ago.

The Mid-Atlantic Ridge (MAR) is a mid-ocean ridge, a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South American Plate in the South Atlantic. Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it extend above sea level. The average spreading rate for the ridge is about 2.5 cm per year.

 

Memorize the words:

sea-floor spreading – розсування морського дна ridge – гребінь гори, гірський кряж, хребет to diverge – розходитися fracture – розлом, тріщина valley – долина rift – розколина spine – гребінь гори linear – лінійний to emerge – з’являтися,виходити to demarcate – розмежовувати, встановлювати кордон consequently – отже, тому, в результаті vent – вихідний отвір seabed – дно моря abyssal – глибинний chimney – димар, комин to emit – випускати, випромінювати sulphur – bearing – що містять сірку to precipitate – випадати в осад breakup – розвал, розпад

The location of the Mid-Atlantic Ridge

9. Which plates of the Earth’s crust does the Mid-Atlantic Ridge separate?

Lesson 8