Below the mantle is the core. The outer part of the core is known to be liquid and the inner part solid, though they are made of similar substances, thought to be mostly the metal iron with nickel and various sulfur and oxygen compounds. Although nobody has ever directly seen these very deep layers, we know they are there because of the way that earthquake waves bounce around inside the earth. This is an example of an inference.

Mantle convection and Plate Tectonics

The mantle is solid, but it can move slowly like hot tar on the road. This is called plastic flow.

In places where the mantle is hot, it rises. In places where it is cold it sinks.

The top of the rising mantle partly melts to make new ocean floor at a Mid Ocean Ridge:

This new ocean floor then spreads sideways, carried by the convection currents, until it comes to a place where the mantle is sinking. The ocean crust is then carried down again in a process called subduction.

A subduction zone is usually marked by a trench in the ocean floor.

The crust that is carried down again causes some of the plate boundary to melt. This makes a line of volcanoes. The volcanic line between Mt Ruapehu and Tonga is such a line, and is part of the Pacific Ring of Fire.

The material from the volcanoes builds up over time. It makes a different type of crust, called continental crust. This is lighter than ocean crust, and can't be subducted. Instead, it gets eroded and deposited and otherwise mover about by processes on the surface.

Where two areas of continent are pushed together, you get mountains pushed up. An example of this is found in the north-east of the South Island. This type of plate movement usually produces a 'rumpled' landscape of parallel mountain ranges and valleys.

Tectonic Plates

Continents cannot sink into the mantle because the rock they are made of is too light. Instead, they get carried along the surface of the Earth by the mantle currents. As a result, the continents 'drift' around over very long periods of time

The mantle convection currents therefore cause the Earth's surface to be broken into large areas sliding along between the ridges and the trenches. These are called tectonic plates. 'The plates act like 'rafts' floating on the mantle and they are more-or-less rigid.

New Zealand lies on the boundary between two of these plates: the Pacific Plate and the Australian Plate. This has shaped much of the geology of New Zealand and we will look at it in more detail later. The edges of the Pacific Ocean are surrounded by the boundary of the Pacific Plate (and Nazca Plate in South America) and are a zone of volcanoes known as the "Ring of Fire"

Plates in New Zealand

New Zealand lies on the boundary between the Pacific Plate and the Australian Plate:

he plate boundary in NZ is unusually complicated. In the North Island, the Pacific Plate is subducted beneath the Australian Plate:

The process of subduction causes volcanoes and deep earthquakes.

Volcanoes happen where magma (melted rock) reaches the surface and is erupted..

Rock can melt for several reasons:

• chemical melting: the chemical that causes melting is usually water. This is the melting that produces plate boundary volcanoes. When ocean floor gets subducted, the water that is chemically combined with ocean floor rocks gets released when it gets down to a depth of 120 km or so. This water then melts the mantle rock above it and produces cone volcanoes such as Mt Ruapehu and White Island.

• heat melting: this happens when extra hot rock from deep in the mantle rises to higher in the mantle and melts it (a "mantle plume"). The volcanoes of Hawaii and Iceland are this sort of volcano. These volcanoes are usually made of very runny basalt lava.

• pull-apart (decompression) melting: when the mantle gets pulled apart by convection currents or tectonic stresses, it can cause melting. This is the cause of spreading plate boundaries such as the Mid-Atlantic Ridge. Pull-apart forces can also happen in places where plate movement is twisting the plate and causes volcanoes away from plate boundaries. The volcanoes of Auckland and some of the ones in Australia are of this sort. They are also often made of runny basalt.

• Below is a diagram of what volcanologists think is happening under the volcanoes of the Tongariro Volcanic Centre. The thickness of the crust is exaggerated compared to the distance down to the sinking Pacific Plate.

Mount Ruapehu is a subduction volcano. The magma is formed when mantle rock deep under Ruapehu is made to melt. The mantle melts because of water from the Pacific Plate. This water is held by minerals in the rock of the seafloor, but gets released by the heat and pressure when it is carried down between 100-200 km depth by subduction.

As you can see in the diagram, the 'plumbing' of a volcano can be quite complicated. Ruapehu is like many volcanoes in that more than one type of magma is erupted. Most of Ruapehu is made of andesite, but basalt has been erupted near the ski resort at Ohakune.

Ruapehu and Tongariro may have the same deep source but separate shallower sources; this is not yet clear.

Ngauruhoe is just a vent of Mt Tongariro. A vent is a place where the magma comes out.

Many volcanoes have multiple vents; only 'simple' cone volcanoes have a single vent.

Magma that is erupted becomes .lava, ejecta and ash. Ash is not burnt lava, it is lava that has been blasted into a fine powder by exploding gases and then carried up into the air by lighter than air gases. When it falls on the ground in a thick layer it is called tephra.

Lava is a flow of liquid molten rock. The runnier the lava, the less steep the volcano that is formed. Lava can only form when most of the gas has escaped from the magma. The stickier the volcanic rock, the harder it is for the gas to escape. This means that lave flows are most often found in very runny magma, but can also be formed from stickier magma after it has lost most of its gas. Very sticky lava makes steep, dome shaped volcanoes.

Below is a diagram showing how different sorts of volcano are formed depending on the stickiness and gas content of the volcano.

The first seismometers where purely mechanical, and drew the output onto a piece of paper rolled around the drum (sometimes run by clockwork). As the drum rotated (usually twice per hour), it would move down so as to create a continuous spiral around the drum representing 24 hours of signal. When the piece of paper was taken off, it was a 'seismograph' . It had 48 lines on it, each representing half an hour.

Modern seismometers use digital detectors and record all their data into computer storage. However, the output is still converted into a picture of what a 'drum seismograph' would look like:

You can see on the image above a large quake on the third line from the bottom. The start of this line is one hour before the time stamp (2.15 pm) so that quake was detected at 1.13 pm. The actual quake happened at 1.06 pm in the Kermadec Islands - 2500 km from this seismic station; the time difference is the time it took the earthquake vibrations to reach Birch Farm from the Kermadecs travelling at about 6 km per second.

Earthquake scales

As mentioned above, a small local earthquake can feel similar to a larger more distant one. For example: on the seismograph above, the other quake showing 'off the scale' shaking (about 6 hours 57 minutes before timestamp) had a similar level of local shaking to the Kermadecs one as felt at Birch Farm. However, it was a very local quake, only about 20 km from the seismic station at Waipukurau.

To allow for this, we use two scales to describe earthquakes. The Richter Scale is based on the earthquake's energy. Each number of the Richter Scale is nearly 32 times more energy than the number below it. On the Richter Scale, the Kermadec quake on the seismograph above was 6.1, and the Waipukurau one 2.1

The difference of four on this scale means that the Kermadec quake released about a million times as much energy as the Waipukurau one. It was therefore felt over a much wider area. Although this scale is 'open ended', rocks can only store a limited amount of energy so there is limit of about 9.5 for tectonic earthquakes. Meteorite impacts could cause quakes of over 10.

The other scale in common use measures the effects of a quake in a particular place. It is called the Modified Mercalli, or MM scale. Strictly speaking, MM numbers should always use a Roman numeal (e.g. MMVI) to distinguish them from the Richter/magnitude scale.

The Kaikoura 2016 quake did not produce as high a value on the MM scale as some people would expect from such a big (magnitude 7.8) quake. However, the energy of this earthquake was spread out in both space and time which reduced the maximum MM scale effect (but increased the distance over which the shaking was felt).

What causes earthquakes?

The surface layers of the Earth are in constant but very slow motion due to plate tectonics. This means that areas of land can move together, apart or sideways. As the land moves, the rocks bend, and as the rocks bend they store up a good deal of energy in the form of strain energy,

Rocks are not very bendy. In a similar way to a bent wooden ruler, they will eventually snap. The sudden release of energy during the 'snap' is what causes the earthquake. This is called elastic rebound.

The sequence above shows the case for a sideways (or 'strike-slip) movement fault. The Alpine Fault is such a fault. Most faults are a combination of sideways and up-and-down movement.

If the movement across the fault is vertical it often leaves a trace called a scarp, visible in the photo above. Scarps generally erode away after time, and the ponded stream will turn into a bog. A geologist, however, can learn to recognise these signs in the landscape and uses such information as part of earthquake hazard assessment.

Faults which show signs of recent movement are termed active. Faults which seem not to have moved for tens of thousands or more years are termed inactive. Active faults are more common near plate boundaries, but also occur far away from plate boundaries.

Locating an earthquake

A seismograph can tell you how far away an earthquake is. This is because there are two types of waves, P (primary) and S (secondary) waves. Primary waves travel about 2.5 km/s faster than secondary waves. This means that for every 10 km away the waves get another four seconds further apart.

In the snapshot above, the P and S wave traces are 40 seconds apart. This means that the earthquake was 40 x 2.5 = 100 km away.

That means you could draw a circle 100 km radius around the seismic station; the quake was somewhere on the circumference of the circle. If you repeat the process for two other seismic stations, you get three circles which will overlap at a point. The map below shows an example based on three stations at Waitara (green cross), Waipukurau (red) and Lower Hutt (blue). The circles intersect just off the coast of Whanganui. They intersect at a point, indicating this is a shallow quake.

The point they overlap is called the epicentre of the quake. In practice, the three circles more usually overlap in a triangle. This is because most earthquakes actually happen some distance underground. The size of the triangle where the three circles overlap tells you the depth to the focus. The focus is the place from which the earthquake waves originate.

Earthquakes with a very deep focus may not be felt at the surface.

A peculiar feature of subduction in the North Island is that some deep earthquakes are felt more strongly some distance from the epicentre, such the example below:

This is a nice illustration of plate tectonics in action. Rotorua, near the epicentre, is on the Australian Plate. The Pacific plate is subducting and is about 200 km underneath Rotorua. The quake was therefore on the Pacific Plate.

The vibrations of the quake were transmitted along the Pacific plate and were most strongly felt in Gisborne and Hawkes Bay, close to where the Pacific Plate comes to the surface. It was weakly felt in the eastern Bay of Plenty; this has to do with the nature of the rocks in that area.