Core Physical Rocks &weathering 3.1 Plate tectonics

• The Earth is composed of layers, including a thin crust, a large mantle (82% of Earth's volume), and a dense core.
• These layers increase in density towards the center due to temperature and pressure effects.

The Lithosphere

• The lithosphere is the Earth's upper surface layer, consisting of the crust and the upper mantle.
• It extends to a depth of about 70 kilometers.
• The Earth's crust is divided into continental crust and oceanic crust.
• Continental crust is characterized by silica and aluminum, often forming granitic rocks when combined with oxygen.
• Oceanic crust is mainly basaltic rock, rich in silica, iron, and magnesium.

Comparison of Oceanic and Continental Crust

• Continental crust has a thickness of 35-70 km and is very old, mainly over 1500 million years.
• Oceanic crust has a thickness of 6-10 km and is very young, mainly under 200 million years.
• Continental crust is lighter in color and has an average density of 2.6, while oceanic crust is darker and denser, with an average density of 3.0.
• Continental crust consists of numerous rock types, often containing silica and oxygen. Granitic rock is most common.
• Oceanic crust consists of few rock types, mainly basaltic.

Continental Drift

• In 1912, Alfred Wegener proposed the idea of continental drift.
• Wegener suggested that continents were once joined in a supercontinent called Pangaea, which broke apart about 250 million years ago.
• Initial debate surrounding Wegener's theory lacked a mechanism to drive continental drift.

Sea Floor Spreading

• In the mid-20th century, Harry Hess proposed that convection currents drive magma up to crack the crust.
• Research in the 1960s on rock magnetism supported Hess's theory.
• Rocks at the Mid-Atlantic Ridge showed alternating magnetic orientations, and rocks farther from the ridge were older.

Plate Tectonics Theory

• In 1965, J. Wilson linked continental drift and seafloor spreading into the theory of plate tectonics.
• Evidence for plate tectonics includes the distribution of earthquakes, changes in Earth's magnetic field, and the fit of continents like a jigsaw puzzle.
• Fossil and geological matches across continents support the theory.

Plate Boundaries

• Earthquake zones help define major and minor plates.
• Plate boundaries are divided into divergent (constructive), convergent (destructive and collision), and conservative types.
• Divergent boundaries, where new crust forms, are commonly found in mid-ocean ridges and feature shallow earthquakes.
• Convergent boundaries cause deep-sea trenches to form when one plate is subducted, and deep earthquakes are common.
• Transform faults occur where plates slide past each other, causing shallow earthquakes (e.g., San Andreas Fault).
• Continent-continent collisions cause crushing and folding, leading to mountain formation.

Movement of Plates

• The convection current theory states hot magma rises to the surface, spreads at mid-ocean ridges, then cools and sinks back into the Earth's interior.
• The dragging theory states older, cold, and heavy plate edges are dragged or subducted.
• Hotspots, or plumes of lava, rise vertically through the mantle and can cause movement by exerting a drag force on the plates.

Sea-Floor Spreading Explained

• R.S. Dietz and H.H. Hess proposed sea-floor spreading to explain continental drift.
• Oceanic crust is created from the mantle at mid-ocean ridges.
• Confirmation came from the discovery of symmetrical magnetic anomalies by F.J. Vine and D.H. Matthews.

Paleomagnetism

• When lava cools, magnetic grains in the rock record the direction of the Earth's magnetic field at the time of cooling, known as paleomagnetism.
• Elevated ridges are comprised of hotter, less dense rock.

Spreading Rates

• Spreading rates vary throughout the mid-ocean ridge system.
• Slow-spreading ridges have a pronounced rift, while fast-spreading ridges lack a central rift.
• Mid-ocean ridges are broken into segments by transverse faults.

Subduction Zones

• Subduction zones form when an oceanic plate collides with another plate and is pushed into the upper mantle.
• Oceanic crust remains cooler and denser.
• The amount of subduction generally balances the amount of crust production at mid-ocean ridges.
• Subduction zones dip at angles between 30° and 70°.
• Older crust tends to dip more steeply.

Evidence for Subduction

• Evidence includes the existence of deep-sea trenches, folded sediments, Benioff zones, and the distribution of temperature at depth.
• Deep-sea ocean trenches are found at subduction zones and are long, narrow depressions in the ocean floor.
• The depth can range from 6000 meters to 11,000 meters.

Benioff Zone

• The Benioff zone occurs on a plane that dips at an angle of about 45° away from the underthrusting oceanic plate.
• Earthquakes on the Benioff zone extend to a maximum depth of about 680 kilometers.

Fold-Mountain Building

• Plate tectonics is associated with mountain building where linear or arcuate chains are formed on land.
• Where an ocean plate meets a continental plate folding can occur.
• Where two continental plates meet, folding and buckling can form mountains like the Himalayas.

Himalayas Formation

• The Himalayas formed by the collision of the Eurasian and Indian plates.
• Oceanic crust pieces thrusting onto continental crust is known as Ophiolite.
• Crustal thickening leads to the uplift of the Himalayan mountain range.

Andes Formation

• The Andes were formed by the subduction of oceanic crust under continental crust.
• The Andes contain many active volcanoes.
• Sediments accumulated on the continental shelf and slope.

Ocean Ridges

• Ocean ridges are giant submarine mountain chains, 60,000 kilometers long, with crests that rise 2 to 3 kilometers above the ocean floor.
• These mid-ocean ridges are where new lithosphere is created.

Volcanic Island Arcs

• Island arc systems are formed when oceanic lithosphere is subducted beneath oceanic lithosphere.
• They are located at the margins of oceans.
• Typical arc characteristics are low bulge (the 'trench outer rise'), trench that marks the plate boundary largely filled with sediments, and faults on the trench's outer slope.
• Subduction complex forms from the descending slab, potentially forming landforms like Trinidad, Tobago and Barbados.
• The overriding plate contains a volcanic island arc.