Option D Flashcards
The internal structure of the earth
Crust
- Continental crust
- Oceanic crust
- On the outside is a very thin rigid crust it is composed from two parts:
- Continental crust – Thicker (20-60km), found under land masses or continents. It is generally older than oceanic crust and is less often destroyed.
- Oceanic crust – Thinner (5km), found underneath the oceans. It is denser than continental crust and can besubducted.
Mantle
Has properties of a solid but can flow very slowly
Makes up 82% of the volume of earth.
Outer core
Made from liquid nickel and iron
Inner core
Made from solid nickel and iron
Density of the layers is controlled by two things:
- Pressure
2. Temperature
Lithosphere
Rigid outer later that is made up of the crust and the top part of the mantle.
Radiogenic
(radioactive decay of materials in the mantle and crust )
Primordial heat
(heat lost by the earth as it continues to cool from its original formation)
How is the heat transported?
- Conduction Main heat lost through the crust
- Convection - Main source of internal heat with the rest mainly originating in the crust Main heat transfer
- Volcanic advection - only 1% - heat transfer through liquid
Advection
Is a lateral or horizontal transfer of mass, heat or other property
Large scale convection currents:
-Large scale convection currents occur in the earth interior
- The inner core heats up magma at the bottom of the outer core
- Magma rises from the core to the surface
- It then spreads out at mid-ocean ridges
- The cold solidifies crust sinks back into the earth’s interior because it is heavier and denser than the surrounding material.
- The CAUSE of the movement is radioactive decay of uranium and potassium in the mantel.
Subduction
Refers to the plunging of one plate beneath another.
Formation of subduction zones:
Lithosphere plate colliders with either a Continental OR Oceanic plate
Subduction zones form where an oceanic lithospheric plate collides with another plate – can be continental or oceanic. The density of the oceanic plate is similar to the asthenosphere; thus it can be easily pushed down into the upper mantle.
The sub-ducted oceanic crust remains cooler and therefore denser then the surrounding mantle for millions of years thus the subduction will carry on. The subducting plate drags or pulls the rest of the plate behind it. Plates are hot at the mid-ocean ridge but cool as they move away.
Rifting:
Where does it occur: Constructive plate boundaries, where the lithosphere is being pulled apart.
Cause: Hotspot activity is believed to be the main cause of rifting.
The rift valleys created consist of rock that is hotter and less dense than the older colder plate. Hot material wells up beneath the ridges to fill the gasps created by the spreading plate.
Plate margin
the region where two or more plate tectonics meet. It is a zone of intense seismic activity.
Types of plate boundaries
- Destructive (convergent)
- Collision
- Constructive (Divergent)
- cONSERVATIVE
Destructive plate margin (Convergent)
Involves an oceanic plate and a continental plate.
- The plates move towards one another; this can cause earthquakes.
- The plates collide
- The oceanic plate is forced beneath the continental plate. This is subduction. This happens because the oceanic plate is denser (heavier) than the continental plate.
- When the oceanic late sinks into the mantle it melts to form magma.
- The pressure of the magma builds up beneath the earth’s surface.
- The magma escapes through weaknesses in the rock and rises up through a composite volcano. (eruptions are often violent, with lots of steam, gash and ash)
Collision margin
When two continental plates collide neither can sink and so the land buckles up to form fold mountains. This Is called a collision margin.
*Earthquakes occur here
Constructive plate boundaries (Divergent
- Two plates move apart from one another
- Magma from the mantle rises up to fill the gap (constructing new land)
- This causes shield volcanoes (don’t erupt with force as the magma can escape easily)
*Both earthquakes and volcanoes are found here.
Example- Mid-Atlantic Ridge
Conservative plate margins (Transform)
- The plates move past each other or are side by side moving at different speeds.
- As the plates move, friction occurs and the plates become stuck.
- Pressure builds up because the plates are still trying to move.
- When pressure is released, it sends out huge amounts of energy, causing an earthquake.
- Earthquakes at conservative plate boundaries are very destructive because they are close to the surface
- There are no volcanos at a conservative plate margin.
Example- San Andreas Fault
Speed of onset-
is how quickly the peak of the hazard event occurs, with the starting time at the first point of the process towards it becoming a hazard.
Frequency-
refers to the number of hazard events in a given period of time
Duration
is how long the hazard event lasts.
Magma plumes
A column of hot rock rising through the mantle. The heat form the plume causes the rock to melt. . The lower lithosphere rocks are heated by theplumesand melt to form hot spots.
Hot spot-
A proportion of the earth’s surface which experience volcanism.
The types of volcanoes
Three most common types of volcanoes:
¬ Shield
¬ Composite
¬ Cinder
- SHIELD VOLCANOES
Formation: Shield volcanoes formed from very hot, runny basaltic lava. The lava is hot thus is can flow great distances therefore building up shield volcanoes.
Features: Gently sloping sides, a shallow crater and a large circumference. No explosive activity due to how they are built so no ejected fragments.
Plate boundary- constructive
Explosions: Not destructive
- COMPOSITE / STRATO
-Most common type of volcano
Formation: Formed by alternating eruptions of fragment material followed by lava outflow.
Features: Characterized by slopes by 3 degrees near the summit and 5 degrees near the base. The highest volcano in the word is composite. Steep-sided and cone shaped. Made up of ash and lava which doesn’t flow every far.
The main cone consists of layers of ash and lava fed from the main pipe which accumulates in the crater. Large explosions form due to cooled lava in the pipe which leads to gas build up,may blow the top off the cone and from a larger crater within which a secondary cone may develop. Frequently parasitic cones grow on the sides.
Explosions: Very destructive and violet: The pipe becomes plugged with cooled lava, the pressure of gas builds up and the result is a violent explosion.
3.CINDER VOLCANO
Formation: Formed by fragments of solid material which accumulate as a steep conical hill around the vent to form a cone.
Features: The shape depends on the nature of the material. It is usually concave and the material spreads out near the base and has a very steep angle of 30-40 degrees depending on the size of the material. Cinder and ash cones are not usually as high (300m)
Explosions: violent eruptions, lava is ejected into the atmosphere and breaks up into cinders, ash and other fragments.
Volcanoes are categorized on how active they are
Active- erupt frequently
Dormant – Temporarily inactive but not fully extinct
Extinct- Never likely to erupt again
Eruptions are categorized into two main groups:
1) Lava eruptions
2) Pyroclastic eruptions
Lava eruptions
The amount of silica in a lava eruption is what makes the difference between the volcanoes in Iceland and Hawaii (erupt frequently) and those in Japan and the Philippines (infrequent eruptions but violent).
Lava released where the oceans meet the continents absorb silica-rich sediments. This causes the lava to become more viscous and block the vents until enough pressure has built up to break them open.
Icelandic lava eruptions
• Persistent fissure eruptions
• Large qualities of basaltic lava may build up vast horizontal plaines.
Hawaiian eruptions
• Have a central vent
• Occasional pyroclastic activity occurs
• Runny basaltic lava flows down the sides of the volcano and gases escape easily