- An extensive area of basaltic lava, often with a layered structure.
- Formed by major eruptions from vents or, more usually, form a fissure.
- The layered structure is cause by the accumulation of lava from a series of eruptions over time.
- The plateau itself tende to be flat and featureless, with limited soil and vegetation cover.
- Eruptions from oceanic ridges produce huge abyssal plains on the sea floor.
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- Occur in a series of broad bands.
- Tend to be either along the edge of continental land masses (e.g. the west coast of South America), or throught the middle of oceans (e.g mid-Atlantic ridge).
- There are exceptions to this pattern, e.g. the volcanoes of the Hawaiian islands, which are more isolated.
- This pattern can be explained by the positions of the various type of plate margin, as volcanoes are produced at divergent and convergent margins, as well as hot spots.
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Volcanoes at divergent margins
- Divergent plate margins tend to give rise to fissure eruptions and shield volcanoes.
- Fissure eruptions occur along fault and fracture lines, while shield volcanoes erupt from a vent.
- Shield volcanoes are typically low in height with long, gently sloping sides and a wide base.
- The lava that erupts from them is usually mafic (or basaltic), which means it has low viscosity due to its low silica content. This lava flows quickly and covers long distances before it cools and solidifies.
- Eruptions are frequent but low in magnitude - magma is able to reach the surface relatively easily since the plates are diverging and the crust is fracturing.
- Such volcanoes also occur at hot spots.
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Volcanoes at convergent margins
- Volcanoes at convergent margins are different in character with contrastinc cone shapes as a result.
- Eruptions tend to be less frequent and much more explosive.
- The lava itself is intermediate or silicic, with more than 50% silica content. This, allied to a lower temperature of about 800 degrees, makes it much thicker and more viscous.
- It flows slowly and cools quickly, giving rise to a cone with a narrow base and a greater height.
- Sometimes these have secondary or parasitic cones on their sides. There form when the passage of rising magma through the main vent is blocked, probably as a result of magma from earlier eruptions solidifying in the vent before it can escape. Pressure builds up and the magma forces its way through cracks in the sides of the vent.
- These volcanoes are often composite in their structure, with alternating layers of ash and lava. The ash is produced by a highly explosive eruption, often after blocking of the vent, fragmenting parts of the cone or the plug of solidified magma.
- There is no volcanic activity at conservative or collision margins, as no new crust is being created by rising magma or through the destruction of existing crust by subduction.
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- Large masses of intrusive rock that may cause a general doming-up of the surface as they are forming. However, they are only exposed after the gradual weathering and erosion of the less resistant overlying country rock.
- This is facilitated by the fractures and cracks formed at the surface as it is streched during uplift.
- The heat transferred from the magma to the country rock causes metamorphic rock to be produced around the intruding magma. Examples of this include sandstone being metamorphosed into quartzite, and limestone into marble.
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- Intrusions that are formed parallel to bedding planes in the country rock (i.e. concorfant).
- Often, but not always, lying horizontally.
- The bedding planes provide a line of weakness along which the magma flows before cooling and solidifying.
- As it cools, the magma contracts, producing cracks in the resultant rock.
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- Discordant because they cut across the bedding planes of the country rock, often vertically.
- Magma flows through cracks and weaknesses but again cools and solidifies before reaching the surface.
- Contraction joints develop parallel to the surface as the magma solidifies.
- Once exposed, the dykes can appear as linear outcrops of resistant rock.
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- When stressed, the outer part of the Earth tends to break if pushed too quickly.
- These breaks, across which slip has occurred, are called faults.
- They tend to occur along the boundaries between plates, but faulting can also happen in the middle of the plates, particularly in the continents.
- In general, faulting is restricted to the top 10-15km of the Earth's crust.
- Normal faults occur when the hanging wall moves downwards. The fault plane on the normal fault is generally very steep. The exposed block forms a cliff-like feature known as a fault scarp.
- Theses faults are common at divergent plate margins.
- Reverse faults form when the hanging wall moves upwards. The forces creating reverse faults are compressional, pushing the sides together. These tend to be found at convergent plate margins.
- Together, normal and reverse faults are called dip-slip faults, because the movement on them occurs along the dip direction - either down or up respectively.
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- At divergent plate margins, rift valleys may form.
- This can be where two oceanic plates are diverging (e.g. the mid-Atlantic ridge), or where an area of continental crust is being rifted by divergence (e.g. east Africa).
- Typical rift features are a central linear down-dropped fault segment, called a graben, with parralel normal faulting and rift-flank uplifts on either side forming a rift valley, where the rift remains above sea level. The axis of the rift area commonly contains volcanic rocks, and active volcanism is a part of many, but not all, active rift systems.
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- Sometimes explained as a process of small-scale failures, on areas perhaps the size of a dinner plate, releasing stress under high-strain conditions.
- It is only when sufficient microfractures link up into a large slip surface that a significant seismic event or earthquake can occur.
- According to this theory, after a large earthquake most of the stress is released and the frequency of microfracturing is much lower.
- A related theory, accelerating moment release (AMR), suggests that theb seimicity rate increases prior to large earthquakes. This could be a promising tool for earthquake prediction in the future.
- Microseismisity is increasingly being used to predict rock failures in mines, and applications are being attempted for the portions of faults within brittle geological conditions.
- Similar behaviour is observed in the tremors preceding volcanic eruptions.
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