A 6.3 earthquake has just struck the city of Christchurch, New Zealand, killing dozens and leaving dozens more buried in rubble with rescue workers trying to dig them out. On the TV this morning, the mayor of Christchurch told his story: Having just left a series of meetings, he was sitting on a balcony outside the city offices in a tall building with his executive assistant planning their next activities when the quake struck. They tried to re-enter the building but were repeatedly thrown back away from the entrance way. When the powerful earthquake stopped, he picked himself up off the floor of the balcony and gazed across his city to see absolute devastation including numerous collapsed buildings and destroyed infrastructure.
You hear things like this from New Zealand now and then, including just a short time ago when the same region was struck buy a somewhat larger quake, but one apparently located farther from the center of population. Also, you hear about a number of volcanoes in the southern Pacific island nation.
Here’s New Zealand:
The most recent earthquake there, as I write this, was actually tomorrow, because New Zealand is on the other side of the international dateline so it occurred officially at 1:21 AM on the 23rd local time, but that was the most recent of several aftershocks. According to the GeoNet archives New Zealand has experienced 296 earthquakes of 3.0 magnitude or greater over the last 30 days. During the same period, an equivalent size area that includes one of the more earthquake prone states in the US, California, had 22 magnitude 3.0 or higher events (Data from the USGS).
So, yes, it is true that New Zealand has a lot of earthquakes, and one could even say that tectonic activity in New Zealand is ten times more frequent than it is in California.
Earthquakes tend to be concentrated at the edges of tectonic plates, as these areas build up energy as plates move in relation to each other. Earthquakes represent the release of that energy. Both New Zealand and California sit on the edge of the Pacific Plate, but in very different places and on very different types of plate boundaries. California borders the northeastern edge where the Pacific plate, rotating counter-clockwise grinds against the plate that makes up that part of the continental US. This is pretty energetic and can create substantial movements of earth, very large and complex fault systems, and plenty of earthquakes. But that is nothing compared to what happens to New Zealand as well as Japan and several other places.
There are areas where one plate is moving towards another, or two plates move towards each other. When that occurs there are two major effects that may occur: One of the plates dives under the other (subduction) and the other buckles, forming mountains. Look at this blow-up of a map from Wikipedia showing the relative movements along boundaries of several tectonic plates:
I’ve circled three things on this map, in green. On the upper right is the California coast. Note that the relative movement of the plates, indicated by the arrow, is as described, with one plate grinding past the other. To the lower right I’ve circled the western coast of South America. Here the Nasca plate is pushing into South America. Thus, the Andes, a major mountain range, rises as the South American plate buckles. To the lower left, also circled in green, is New Zealand. Note here that the pacific plate is moving northwest, and the Australian plate is moving (actually, spreading) southeast, the two plates slamming into each other. Also note, if you look really close, that New Zealand is not restricted to either plate, but rather, straddles the two.
Australia and North America are continents because they are huge land masses sitting on top of plates. Hawaii is not on a continent. That island state consists of a series of volcanoes coming up from the middle of a plate, but not on a continental mass. The Hawaiian Islands are true oceanic islands. But what is New Zealand? Is it a bit of continent isolated from other larger masses, as is the case with Madagascar? Is it an oceanic island?
Well, both. New Zealand might be thought of as a very large oceanic island, but it really isn’t. It has a long (half a billion years) geological history consisting of volcanism, uplift, subduction (the opposite of uplift) followed by the formation of sediments, more uplift, and so on. But this complexity can be characterized in a rather simple way that probably describes the geology of New Zealand quite accurately: Modern New Zealand is the froth that forms on the surface of the earth over a subduction zone, where one continental plate is diving under another. New Zealand, Japan, and many other areas that form the so-called “Ring of Fire” making up the boundary of the Pacific plate consist of a combination of volcanic material and sediments related to the instability of subduction zones.
But it is a little more complicated than that. New Zeland started out as a fragment of the ancient southern continent, Gondwana, and made up part of the land mass that is now Australia. Much of the rock that makes up New Zealand is from the edge of that ancient continent, and uplift and subduction occurred several times because of the relative movement of the plates. After about 100 million years ago, New Zealand started to move increasingly far from major continents, with a full scale separation from Gonwanaland happening about 85 million years ago. During this time, what was to become New Zealand separated from what is now Australia. After about 50 million years of this, New Zealand sat low in the ocean and, mainly underwater, accumulated extensive marine sediments. Then, starting about 24 million years ago, the Australian plate (still spreading) and the Pacific plate started to interact more energetically, both moving towards each other and rinding past each other, causing the formation of the massive mountain ridges seen today in New Zealand, as well as a great deal of volcanic activity.
The following drawing represents a schematic east-west cross section through New Zealand:
The green curve on the right is the sea floor of the Pacific Plate diving beneath the part of the Australian plate that carries much of New Zealand. New Zealand consists in part of fragments of continent (in green) carried away from Australia millions of years ago. As subduction happens, interesting things happen that cause the formation of lava (in red) along the margins of the plates, which moves upwards pushing through and expanding the cracks that inevitably form when continents are mushing together, often forming pools of lava much nearer the surface as well. These pools … those at the subduction juncture itself and those formed secondarily higher up … are the source of lava for numerous volcanoes.
The plate on top of this unholy geological intercourse is forced upwards, forming mountains, and the whole process is highly energetic and causes frequent earthquakes, which can be quite severe.
New Zealand has so many earthquakes and volcanoes because it is in the wrong place (at the juncture of two tectonic plates) at the wrong time (while one plate is diving beneath the other).
For the most recent news on the earthquake in New Zealand, click here.