Tectonics EQ1

What are the three main layers of the Earth?

Crust (outermost layer), Mantle (middle, widest layer) and Core (innermost layer)

What are the two types of crust and how do they differ?

Continental crust: thicker (45–50km), less dense, mostly granite. Oceanic crust: thinner (6–10km), denser, mostly basalt

What is the Mohorovičić discontinuity (Moho)?

The boundary between the crust and the mantle

What are the layers of the mantle?

Upper mantle: consists of a rigid layer (which with the crust forms the lithosphere) above the asthenosphere (a semi-molten, plastic layer that moves under high pressure). Lower mantle: hotter and denser, kept solid by intense pressure

What is the asthenosphere and why is it important for plate tectonics?

A semi-molten, plastic layer in the upper mantle. Tectonic plates move slowly over it — its behaviour under pressure allows plate movement to occur

What is the core made of?

Inner core: solid, mostly iron. Outer core: semi-molten, mostly liquid iron and nickel

What is convection and how does it drive plate movement?

Heat from radioactive decay in the core moves upward into the mantle, creating convection currents. These push up into spreading mid-ocean ridges, forcing plates apart (ridge push). Convection was the original explanation for plate movement but other processes are now also recognised

What is ridge push?

At divergent boundaries, magma rises at mid-ocean ridges forming new crust. The elevated ridge pushes the plates apart due to gravity — contributing to plate movement

What is slab pull?

At subduction zones, as oceanic crust cools it becomes denser and thicker. Gravity forces it down into the subduction zone and as it sinks it drags (pulls) the rest of the plate with it — considered a major driver of plate movement

What is seafloor spreading?

At mid-ocean ridges, magma rises and solidifies to form new oceanic crust, gradually pushing existing crust outward and widening the ocean basin

What is palaeomagnetism and how does it provide evidence for seafloor spreading?

As lava cools at mid-ocean ridges, magnetic minerals align with Earth's magnetic field at the time. As Earth's magnetic field periodically reverses, the pattern of magnetic striping on either side of the ridge is a mirror image — providing evidence that the seafloor has gradually spread apart

What are the four types of plate boundary?

Convergent (destructive), Divergent (constructive), Collision and Transform (conservative)

What happens at a convergent (destructive) boundary between oceanic and continental plates?

The denser oceanic plate subducts under the lighter continental plate, forming deep ocean trenches (over 6km deep) and a Benioff zone. Causes explosive volcanic eruptions (rhyolite magma) and powerful earthquakes. Also forms fold mountains (e.g. Andes — Nazca and South American plates)

What happens when two oceanic plates converge?

The denser of the two oceanic plates subducts, forming deep ocean trenches and island arcs (e.g. the Caribbean) through submarine volcanic eruptions building up above sea level

What happens at a collision boundary?

Two continental plates of similar density collide — neither can subduct so both are crushed, crumpled and forced upward forming collision fold mountains (e.g. Himalayas — Eurasian and Indian plates, still rising at ~1cm/year). No volcanic activity as there is no subduction. Earthquakes are the main hazard

What happens at a divergent (constructive) boundary?

Plates move apart. Magma rises to fill the gap forming new oceanic crust. The Mid-Atlantic Ridge is an example. Causes mild, shallow earthquakes and small effusive eruptions of low-viscosity basalt lava. Can also form rift valleys on land

What happens at a transform (conservative) boundary?

Plates move slowly past each other — either in opposite directions or the same direction at different speeds. Plates stick due to friction then suddenly snap past each other (strike-slip faults), releasing energy as powerful earthquakes. No volcanic activity as no subduction or magma generation occurs. Land is neither created nor destroyed

What is the Benioff zone?

The narrow area within a subduction zone where most friction and pressure builds up as the subducting plate descends. Site of powerful, deep-focus earthquakes at convergent boundaries

How do properties of magma affect the magnitude and type of volcanic eruption?

Low viscosity, low gas content, high temperature magma (basalt) at divergent boundaries produces gentle effusive eruptions. High viscosity, high gas content, lower temperature magma (rhyolite) at convergent boundaries traps gases causing explosive eruptions of much greater magnitude

Compare basalt, andesite and rhyolite magma

Basalt: dark, 45–55% silica, 1000–1200°C, low viscosity, gentle eruption, 80% of surface lava. Andesite: medium grey, 55–65% silica, 800–1000°C, medium viscosity, medium eruption, 10% of surface lava. Rhyolite: light coloured, 65–75% silica, 600–900°C, high viscosity, explosive eruption, 10% of surface lava

What is the global distribution of earthquakes?

About 95% occur at or near plate boundaries. Many cluster around the Ring of Fire surrounding the Pacific Ocean. The most powerful are associated with convergent or conservative boundaries. Intra-plate earthquakes also occur but are less common

What is the global distribution of volcanoes?

Most active volcanoes occur at or near plate boundaries — about 75% around the Ring of Fire. Occur at convergent and divergent boundaries. Also found at hotspots in the middle of plates (e.g. Hawaii)

What is the global distribution of tsunamis?

Over 70% occur around the Pacific Ocean. 15% in the Mediterranean Sea, 9% in the Caribbean and Atlantic, 6% in the Indian Ocean. Caused by tectonic activity — most at convergent boundaries

What is an intra-plate earthquake and what causes it?

Earthquakes that occur away from plate boundaries. Causes not fully understood but thought to involve tectonic stresses reactivating ancient fault lines, or zones of weakness caused by plates moving over a spherical surface. Examples: New Madrid earthquake (1812) and 2011 Virginia earthquake

What is a hotspot volcano and how does it form?

Occurs over a stationary magma plume (column of rising magma) in the asthenosphere. As the tectonic plate moves over the plume, a chain of volcanic islands forms — the oldest island is furthest from the plume (e.g. Hawaii in the central Pacific)

What are P waves (Primary waves)?

Body waves — the fastest seismic waves, reaching the surface first. Travel through both liquids and solids. Cause backwards and forwards shaking. Least damaging of the three wave types

What are S waves (Secondary waves)?

Body waves — slower than P waves. Can only travel through solids (not liquids). Cause sideways motion. More damaging than P waves

What are L waves (Love waves)?

Surface waves — the slowest seismic waves. Cause a side-to-side motion. Largest amplitude and energy focused on the surface. Most damaging of the three wave types

What are the primary hazards of earthquakes?

Ground shaking and crustal fracturing (movement causes the Earth's crust to crack)

What are the secondary hazards of earthquakes?

Landslides and avalanches (Earth movement triggers collapse of material on steep slopes), liquefaction (shaking causes ground particles to move apart making the ground behave like a liquid) and flooding caused by tsunami

What are the primary hazards of volcanic eruptions?

Pyroclastic flows (dense, fast-moving mix of hot rock, ash and gases), lava flows (slow-moving but reach over 1000°C), ash falls (travel many km causing injuries, deaths and transport disruption) and gas eruptions (toxic gas clouds released from magma)

What are the secondary hazards of volcanic eruptions?

Lahars (fast-flowing mixtures of rock, mud and water flowing down the volcano, destroying everything in their path) and jökulhlaups (glacial floods caused when glacial ice is rapidly melted by the eruption)

How are tsunamis formed by submarine earthquakes?

An earthquake occurs beneath the sea bed at a subduction zone — the sea bed jolts upward, displacing the water column above and creating a wave. As the wave approaches land it slows, wavelength compresses and wave height increases — frequently reaching 5–10m, sometimes up to 30m. As it reaches the shore a vacuum is created and water first recedes rapidly exposing the sea bed before the wave strikes

What other events can cause tsunamis?

Landslides (triggered by earthquakes or volcanic eruptions displacing water), underwater volcanic eruptions and rarely meteor strikes. Tsunamis most commonly occur near plate boundaries, especially around the Pacific Ring of Fire

Why are focal depth and the Benioff zone important in earthquake magnitude?

The Benioff zone is where friction and pressure build as a subducting plate descends — generating the most powerful earthquakes at convergent boundaries. Shallow focus earthquakes (near surface) tend to be more destructive than deep focus earthquakes as energy dissipates less before reaching the surface