Dating Geological Events
The Auckland Islands – Investigating evidence related to dating geological events
The Auckland Islands lie approximately 375km south of Stewart Island on the Campbell Plateau. These islands were formed by multiple geological events. Geological events being significant occurrences as a result of the earth system. Whilst these islands have been shaped over many thousands of years by events such as weathering and erosion, particularly due to glaciation, the main events which formed the islands were two volcanic eruptions. The way that it can be determined that the islands were formed from volcanic eruptions, and when they were formed, is by looking at the record that is contained in the rocks of the islands.
One of the first things you can do to determine the geological history of an area is to look at the types of rocks that are present. The Auckland Islands consist mainly of basalt, but there are also some occurrences of granite, gabbro, sandstone, conglomerate and limestone.
Basalt, granite and gabbro are all igneous rocks – they form when magma rises from the mantle and cools. This cooling can occur either within the Earth’s crust, or on the surface. The different cooling locations form different types of rocks, and also give an indication as to how the rocks were formed. Rocks that cool within the Earth’s crust, intrusive igneous rocks, cool slowly and thus have coarse crystalline grains, whereas, rocks that cool on the surface of the Earth, extrusive igneous rocks, cool relatively quickly. As the cooling happens much faster in extrusive rocks than intrusive rocks, the minerals do not have time to congregate with one another and thus form fine-grained rock. The granite and gabbro found on the Auckland Islands are intrusive igneous rocks, whereas the basalt is extrusive igneous rock, therefore of volcanic origin.
As well as the igneous rocks found on the Auckland Islands, there are also sedimentary rocks – sandstone, conglomerate and limestone. Sedimentary rocks are rocks that form from the deposition, compaction and cementation of small fragments of rock, or dissolved substances from other rocks, or in some cases marine animals. Sandstone is created when individual sand fragments are deposited in thick layers by wind and water, which are then compacted and cemented together. Conglomerate is a type of sandstone that contains a wide variety of particle sizes. Both sandstone and conglomerate are what is known as clastic sedimentary rocks, where mineral fragments from any of the three major rock groups accumulate and lithify (compact and cement). Limestone is what is known as a chemically precipitated sedimentary rock. These rocks form when mineral compounds accumulate at the bottom of oceans or inland lakes. Limestone is a rock that consists of over 50% calcium carbonate and can be either formed through carbonate precipitation, or the accumulation of marine organisms. Limestone is often abundant in fossils and therefore geologists can determine the environmental conditions when the sediments were initially deposited.
Once the types of rocks present have been determined, and therefore you have some idea of the processes involved in the formation of the landscape, you now may wish to figure out when these processes occurred. There are two main aspects to geological dating – relative dating and absolute dating.
Relative dating does not give a precise age of a rock, but determines whether it is older or younger than another rock – placing rocks in their sequence of formation. There are laws of relative dating which guide geologists to be able to determine the relative age of rocks.
- The first law of relative dating is
the Law of Original Horizontality. This law takes into account the
understanding that sedimentary rocks are originally deposited horizontally. If
they are tilted, folded or broken, this occurred after deposition.
- The second aspect of relative dating
is the Law of Superposition. This law states that in an un-deformed sequence of
sedimentary rocks, the oldest strata will be at the bottom of the sequence.
- The third aspect is the Law of
Cross-Cutting Relationships. This law states that if an igneous intrusion or
fault cuts through existing rocks, the intrusion or fault is younger than the
rocks that it cuts through.
- The forth law is the Law of Lateral
Continuity. This law states that layers of rock are continuous until they
encounter other solid bodies that block their deposition or until they are
acted upon by agents that appeared after deposition took place.
- Rock strata can also contain
inclusions which can assist in determining the relative age of the rocks.
Inclusions are pieces of rock or minerals enclosed within another rock. These
inclusions must be older than the surrounding rocks to enable them to be
enclosed within them.
- Unconformities can also be found in rock strata. Unconformities are missing sections of the geological record in the strata. These occur due to rocks being eroded before deposition commenced again, or due to long periods of no deposition occurring.
Using all of these aspects, geologists can look at a cross section of rock strata and determine in relative order which rocks were deposited when. This gives an idea of the geological history of the area, but does not give a definitive timeframe as to when each event occurred. In order to gather this information geologists need to use absolute dating techniques.
Absolute dating needs only a small sample of the rock or fossil and can give an ‘exact’ age for that sample. This dating method works by looking at the atomic structure of the sample. All rocks are composed of atoms, each atom having a nucleus containing protons and neutrons. Most atoms are stable, but some are not. Those that are not are called radioactive isotopes. In these isotopes, particles within the nucleus break apart and the atom decays into a different element, emitting radiation in the process. Each isotope decays at a different known rate. The known rate is expressed as a half-life, which is the amount of time it takes for half of the parent isotopes to decay into daughter atoms. So in order to determine the age of a rock sample, geologists compare the amount of the original isotope in the sample with the amount of the decayed end product – the daughter atoms. Different radioactive isotopes are useful for dating different rock samples, as those with a shorter half-life are only useful for dating younger rocks.
Absolute dating using radioactive isotopes provide a running time clock for the history of the earth, so geological events can be dated and their sequence understood.
Stratigraphic column showing a basement section on Musgrave Peninsula, Carnley Harbour (Gamble & Adams, 1985).
Whilst relative dating using stratigraphy is useful in showing the sequence of events in a particular location, such as Musgrave Peninsula at the Auckland Islands, the true age of the rocks can be determined using absolute dating. Absolute ages and relative stratigraphy can then be cross correlated to justify the evidence related to dating the geological events.
At the Auckland Islands rock samples have been dated using radiometric potassium-argon dating. Samples (mainly basalt) were taken from both the Ross and the Carnley volcanic fields to show when the two volcanic events took place. This study showed that the Ross volcano is at least 19.2 Million years old, but has widespread younger flows which are 15-17 Million years old. It also showed that the Carnley volcanic field had its first activity between 24-26 Million years ago, but has had more recent volcanic activity beginning about 19 Million years ago. Samples of granite were also taken and dated using potassium-argon dating, showing an age of 93.7 and 95.6 Million years old. Cross-correlation of the different techniques, relative stratigraphy and absolute potassium-argon dating, can then conclude that the formation of the granite came before that of the basaltic volcanism, at least 95.6 Million years ago, and the basaltic volcanism had two centres, the Carnley and the Ross volcanos, with the Ross volcano being the younger of the two.
Histogram of potassium-argon ages from the Auckland Island volcanic rocks (Adams, 1983)
C. J. Adams (1983) Age of the volcanoes and granite basement of the Auckland Islands, Southwest Pacific, New Zealand Journal of Geology and Geophysics, 26:3,227-237
J.A. Gamble & C.J. Adams (1985) Volcanic geology of Carnley volcano, Auckland Islands, New Zealand Journal of Geology and Geophysics, 28:1, 43-54
D.D. Ritchie & I.M. Turnbull (1985) Cenozoic sedimentary rocks at
Carnley Harbour, Auckland Islands, Campbell Plateau, New Zealand Journal of
Geology and Geophysics, 28:1, 23-41