Tectonics EQ1 Flashcards
What is a tectonic hazard?
A natural hazard caused by the movement of tectonic plates (including volcanoes and earthquakes). Can either be seismic or volcanic. A natural hazard is either hydro-meteorological (caused by climatic processes) or geophysical (caused by land processes)
Where do geographical hazards occur?
Near plate boundaries. These plates move at different speeds and directions which can cause collisions, earthquakes and volcanic activity.
What is the general distribution of volcanoes?
- Most earthquakes are found on plate boundaries.
- The most powerful is on Conservative margins.
- The ocean fracture zone is earthquake activity along mid-ocean ridges.
- The continental fracture zone is earthquake activity along mountain ridges.
- The Pacific Ring of fire is the earthquake and volcanic activity around the Pacific Ocean.
- Earthquakes can also occur intra- plate - inside the plate boundaries.
- Volcanoes occur in constructive and destructive margin areas where there is an opportunity for magma to rise from the mantle.
- Where there is a volcano there is an earthquake.
What is the general distribution of volcanic eruptions?
These also occur at, or close to tectonic plate boundaries with some occurring in the middle of the plate boundary known as hotspots, such as Hawaii. There are about 500 active volcanoes throughout the world and on average, about 50 erupt each year. Where there is a volcano there is an earthquake.
What is the general distribution of tsunamis?
The global distribution of tsunamis is fairly predictable in terms of source areas, with around 90% of all events occurring within the Pacific Basin, associated with activity at the plate margins. Most are generated at subduction zones (convergent boundaries), particularly of the Japan-Taiwan island arc, South America and the Aleutian Islands (25% of all historical events have been recorded in this geographic region.)
What is the general distribution of earthquakes?
Over 80% of large earthquakes occur around the edges of the Pacific Ocean, an area known as the ‘Ring of Fire’; this is where the Pacific plate is being subducted beneath the surrounding plates. The Ring of Fire is the most seismically and volcanically active zone in the world. Where there is a volcano there is an earthquake. Earthquakes occur along all types of plate boundaries. However, there are earthquakes that occur within the plates.
What are the causes of earthquakes?
Earthquakes are sudden releases of stored energy. As two plates move past each other they inevitably ‘stick’. This allows the strain to build up over time and the plates are placed under increasing stress. Earthquakes are generated because of the sudden release of stress - ‘slip-stick behaviour’. A pulse of energy radiates out in all directions from the earthquake focus. In some cases the earthquake motion displaces the surface, so a fault scarp can be seen. (see below for causes at different boundaries)
What are the causes of tsunamis?
Tsunamis are generated when a sub-marine earthquake displaces the sea bed vertically as a result of movement along a fault line at a subduction zone. The violent motion displaces a large volume of water in the ocean water column, which then moves outwards in all directions from the point of displacement. Submarine earthquakes that occur close to the shoreline can generate intense ground shaking damage, followed by damage from the subsequent tsunami.
What are the causes of volcanic eruptions?
Earthquakes are sudden releases of stored energy. As two plates move past each other they inevitably ‘stick’. This allows the strain to build up over time and the plates are placed under increasing stress. Earthquakes are generated because of the sudden release of stress - ‘slip-stick behaviour’. A pulse of energy radiates out in all directions from the earthquake focus. In some cases the earthquake motion displaces the surface, so a fault scarp can be seen.
Deep within the Earth, it is so hot that some rocks slowly melt and become a thick flowing substance called magma. Since it is lighter than the solid rock around it, magma rises and collects in magma chambers. Eventually, some of the magma pushes through vents and fissures to the Earth’s surface.
What is a plate boundary?
What is a plate margin?
The place where two plates meet
Areas adjacent to plate boundaries
What are the 4 plate boundaries?
- Divergent
- Conservative (transform)
- Convergent
- Collision
Where are (Constructive) Divergent plates located?
This typically occurs along a mid-oceanic ridge, such as the mid-Atlantic rift that extends from the north to the south of the Atlantic ocean.
Where are Conservative (transform) plates located?
Conservative margins are also known as transform faults. Transform faults are mainly found on the ocean floor, where they offset mid-ocean ridges and enable to ocean to spread at different rates. e.g San Andreas fault zone in California
Where are (destructive) Convergent plates located?
Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere.
Where are Collision plates located?
The best place to see two continental plates converging is in the Himalaya Mountains.
What are divergent (constructive plate margins)?
- At a divergent plate boundary - the plates move apart from one another. When this happens the magma from the asthenosphere rises up through the gap to make (or construct) a new crust.
- The movement of the plates over the mantle can cause gentle earthquakes. Since the magma oozes out through the gap small shield volcanoes are made.
- New land forming on the ocean floor by lava filling the gaps is known as sea - floor spreading (as the floor spreads and gets wider).
- If a divergent plate boundary happens underwater (oceanic crust) it can create mid-ocean ridges.
If a divergent plate boundary occurs on land (continental crust), the crust stretches and breaks into sets of parallel faults which can form rift valleys.
- Earthquakes along the Benioff zone
What are conservative plate margins (transform)?
At a conservative/transformplate boundary, the plates slide past each other in opposite directions, or in the same direction but at different speeds. As the plates try to move, friction occurs and plates become stuck. Pressure builds up because the plates are still trying to move.
When the pressure is released, it sends out huge amounts of energy causing a violent earthquake. There are no volcanoes at a conservative plate boundary as the plates do NOT move apart or there is no subduction. Causes frequent earthquakes.
What are convergent (destructive plate margins)
- As the plates collide, the oceanic plate is forced beneath the continental plate. This is known as subduction and results in the formation of an ocean trench. This happens because the oceanic plate is denser (heavier) than the continental plate.
- When the plate subducts into the mantle it melts to form magma. The force of this generates violent earthquakes.
- The pressure of the magma builds up beneath the Earth’s surface. The magma is forced out violently as a composite volcano. The volcanic eruptions are often violent, with lots of steam, gas and ash. Fold mountains occur when sediment is pushed upwards during subduction.
- If two oceanic plates converge and hit each other the plate that is heavier and older will subduct. Built-up pressure causes underwater volcanoes to burst through the oceanic plate. Lava cools and creates new land called island arcs.
- Continental earthquakes are rare but explosive earthquakes along the Benioff zone and batholiths.
What are collision plate margins (destructive/convergent)
If two continental plates collide, neither can sink because continental crust is light - and so the land buckles upwards and uplifts to form mountains. They are called fold mountains are the crust crumbles upwards. Earthquakes are the main hazard. There are no volcanoes as there is no subduction or gap created. Cause shallow earthquakes.
What are intra- plate earthquakes?
Earthquakes can occur in mid-plate settings, usually associated with major ancient fault lines being re-activated by tectonic stresses. For instance, the New Madrid Seismic Zone on the Mississippi River generates earthquakes up to magnitude 7.5 but is thousands of miles from the nearest plate boundary.
About 5% of earthquakes take place within a plate, away from plate boundaries. These intraplate earthquakes are caused by stresses within a plate. Since plates move over a spherical surface, zones of weakness are created. Intraplate earthquakes happen along these zones of weakness. The earthquakes may take place along ancient faults or rift zones.
What are intra-plate volcanoes
Intraplate volcanoes are thought to be associated with ‘hot spots’ in the mantle, which remain stationary as plates move over them. This may happen in areas that are distant from plate boundaries. The mechanism by which such hot spots produce volcanic activity is a subject of much debate amongst Earth scientists. Some have suggested they are caused by mantle plumes – cylindrical bodies of material hotter than the surrounding mantle. The plumes are thought to originate at the outer core/lower mantle boundary, rising upwards to the base of the lithosphere over many millions of years. They’re located within tectonic plates.
What are volcanic hotspots?
The earth’s volcanic eruptions take place inside plate boundaries. They happen due to the fracture in the crust, and magma plume in the mantle creates the eruption. This creates islands such as Hawaii.
Mantle plumes are concentrated areas of heat convection. At plate boundaries, they are sheet-like, whereas at hot spots they are column-like.
What happens at the locations of hotspots?
Some volcanic eruptions are ‘intra-plate’ meaning there are distant from a plate boundary at locations called mid-plate hotspots, such as Hawaii and the Galapagos Islands.
At these locations:
Isolated plumes of convecting heat, called mantle plumes, rise towards the surface, generating basaltic volcanoes that tend to erupt continuously.
A mantle plume is stationary, but the tectonic plate above moves slowly over it.
Over millennia, this produces a chain of volcanic islands, with extinct ones most distant from the plume’s location.
Most hotspot volcanoes are basaltic (e.g., Hawaii, Tahiti) despite the nearest plate margin being 3,200 km away.
Hot spot volcanoes occur far from plate boundaries.
What is the process of a plume?
Radioactivity in the Earth’s core creates a huge column of upwelling magma, known as a “plume”.
The plume from the Asthenosphere (upper mantle) pushes upwards; pressure drops and the plume become molten, melting and pushing through the crust above.
This lies at a fixed position under the Tectonic Plate. As the plate moves over this “hot spot”, the upwelling magma creates a steady succession of new volcanoes that migrate along with the plate.
The plume also eats into or melts the plate above, so that the thickness of the crust at this point is much smaller than the average. This produces smaller shield/basaltic volcanoes
What are the layers of the earth?
- The crust
- The lithosphere
- The asthenosphere (upper mantle)
- The lower mantle
- The outer core
- The inner core
What are the properties of the crust?
It is the thinnest and coldest layer of the earth on which we live. It is divided into tectonic plates that are moving (lithosphere) due to the upper mantle (asthenosphere) that it sits on.
It is made up of granite (sedimentary) and basaltic (volcanic) rocks. There are two types of crust:
Oceanic crust (crust under the oceans). This crust is thinner (5-8km) but heavier (denser 3.0p). Made up of basalt, silicon, and magnesium.
Continental crust (crust of the land). This crust is thicker (30-70km) but lighter (less dense 2.6p) and made of mainly granite, silicon, and aluminium.
The temperature is around 400°C.
What is the properties of the lithosphere?
Together the crust and upper mantle. It is the zone where tectonic plates are formed (rigid).
What is the moho discontinuity?
The thin layer separates the crust from the top of the mantle.
What are the properties of the asthenosphere?
Lies directly below the Lithosphere and includes the rest of the mantle where rocks are much softer (plastic). The asthenosphere is almost solid, but a slight amount of melting (less than 0.1% of the rock)
What are the overall properties of the mantle?
- The mantle is the widest section of the earth composed mainly of Silicate rocks, Iron and Magnesium.
- The mantle is between 700-2900km thick
- The upper mantle is known as the asthenosphere
The lower mantle (mesosphere) - Temperatures range from 500°C – 4,500°C and therefore cause convection currents.
As you go to the centre of the earth it gets hotter and denser (heavier).
What are the properties of the lower mantle?
Solid magma is the largest area. The lower mantle is much less ductile than the upper mantle and transition zone. Although heat usually corresponds to softening rocks, intense pressure keeps the lower mantle solid.
