The rock types
There are 3 types of rock: igneous, sedimentary and metamorphic. They are all formed in different ways and have different characteristics that define them into the 3 types of rock. Knowing if a rock is igneous, sedimentary or metamorphic already gives us more information on how it formed. All 3 rock types can be linked by the rock cycle.
For more information about rocks and the rock cycle, visit Geological Survey Ireland’s website.
Igneous rocks form when hot liquid rock, called magma, cools. When the magma cools below the Earth’s surface, it cools slowly forming large crystals and is known as an intrusive igneous rock e.g. granite. When magma reaches the Earth’s surface through a volcano, it is called lava and it cools quickly and has small crystals. Rocks formed by this process are known as extrusive igneous rock e.g. basalt.
Sedimentary rocks form when small particles, known as sediments, build up over time, layer upon layer, and overlying pressure causes them to compress and cement together, eventually forming a consolidated rock.
The particles can be material that comes off rocks due to weathering and erosion. The resulting sediment particles are transported by various means (e.g. river, glacier, gravity), and finally deposited into layers before being buried and becoming a sedimentary rock.
The particles can also be the remains of ancient marine organisms which have accumulated over time and cemented to form rocks such as limestone and chalk.
Metamorphic rocks are rocks that have been changed by heat and pressure. When igneous, sedimentary or even other metamorphic rocks are exposed to high heat and pressure, they will change form and form a new rock. This process does not melt the rock but instead changes the composition of the rock by forming new minerals, from the heat and pressure, from the minerals that were already present in the rock.
This change in rock is known as metamorphism and the rock has now become a metamorphic rock. For example, limestone becomes marble, sandstone becomes quartzite, mudstone becomes slate and granite can become gneiss. The original rock, e.g. limestone, sandstone, mudstone or granite, is referred to as the parent rock. The crystals in metamorphic rocks are often aligned in a particular direction, which reflects the direction of pressure the rock was subjected to. This is known as foliation.
Metamorphism often occurs in response to high tectonic activity for example the collision of two plates, either where one plate is subducted beneath the other, or both plates collide folding upwards and forming mountains.
The structure of the Earth: Core, Mantle, Crust
The earth is made up of three distinctly different layers: the crust, the mantle and the core.
This is the outermost layer of the earth and is made of solid rock. It is mostly made up of two igneous rocks: basalt and granite. There are two types of crust; oceanic and continental. Oceanic crust is denser and thinner and mainly composed of the mafic igneous rock, basalt. Continental crust is less dense, thicker, and mainly composed of the felsic igneous rock, granite.
The mantle lies below the crust and is up to 2900 km thick. It consists of hot, dense, iron and magnesium-rich solid rock. The crust and the upper part of the mantle make up the lithosphere, which is broken into plates, both large and small. The semi-solid/ semi-molten state of the asthenosphere permits the tectonic plates above to move, due to the convection currents within.
The core is the centre of the earth and is made up of two parts: the liquid outer core and solid inner core. The outer core is made of nickel, iron and molten rock. Temperatures here can reach up to 50,000°C.
The Earth’s crust and upper mantle is broken into many plates called tectonic plates that are like pieces of a jigsaw puzzle. There are seven major plates: the Eurasian, North American, Pacific, South American, Antarctic, Indo-Australian and African plates.
The tectonic plates are in motion and it is thought that they have been in motion since early in Earth’s history. It is due to the movement of plates, i.e. plate tectonics, that some areas of the world experience phenomena such as volcanoes, earthquake and tsunamis.
The areas where these plates meet are known as plate boundaries. There are three types of plate boundary:
Divergent or constructive plate boundaries
The plates diverge, i.e. move away from one another, and this causes new rock to form. This happens when two tectonic plates pull apart and mantle rises up through the opening, forming a new rock when it cools. This is how new oceans form. Spreading, the diverging of the two plates, continues at the mid-ocean ridge whilst the ocean is opening. An example of this is the Mid-Atlantic Ridge; the Atlantic Ocean started forming when the North American and Eurasian plates diverged away from one another and new material was formed at the mid-ocean ridge between them.
Convergent or destructive plate boundaries
The plates converge, i.e. move towards one another, and collide. The result depends on the type of plates involved. It is possible to have the collision of two oceanic plates, an oceanic plate and a continental plate or two continental plates. The Himalayas are an example of the collision of two continental plates. The Indian plate is crashing into the Eurasian plate and is being forced upwards. They are continually growing at an average rate of 1 cm per year, this will be 10 km in 1 million years. In geology, the mountain building process is called an ‘orogeny’ (from the Greek word for mountain).
Passive plate boundaries (also known as strike-slip or transform boundaries)
The plates slide past each other. When the plates move, the jagged edges of the plate snag and catch each other. This can cause the plates to get jammed and create a build-up of pressure. When the plates eventually pass each other, the pressure is released suddenly in the form of an earthquake. The movement of the two plates can either be in opposite directions or in the same direction but at different speeds. The San Andreas Fault in California, USA, is an example of the latter.
Wilson Cycle (Oceans opening and closing)
The Wilson Cycle is a model of plate tectonics that operates over very long period of geological time and explains the opening and closing of oceans. Continents break up, due to rifting, and diverge away from one another, which forms a new ocean. Eventually the ocean can no longer open, and the two plates start converging towards one another, which closes the ocean that was just created.
Geological time is something that is very hard to picture or imagine. Earth is about 4.5 billion (4500000000) years old. Geologists have broken up this long length of time into a number of smaller time periods. These periods often coincide with major geological events and mark changes in rock types, mass extinctions of species of plants and animals and changes in climate. This is known as the stratigraphic column and it is what geologists, palaeontologists and many other Earth scientists use to understand and represent geological time and date certain historical events on Earth. It relates to stratigraphy (layers of rock) and can be viewed as a rock core taken from the Earth, with the oldest rock being at the bottom and the youngest rock being at the top. The diagram on the right is perhaps the most used stratigraphic column.
The ages of rocks were first determined by relative dating. Relative dating determines the relative order of past events by comparing the age of one object to another. This determines where in a timescale the object fits, without finding its specific age i.e. you can say one rock is older than the other. There are a few methods of relative dating, such as studying the stratigraphy (study of the order of the rocks) and cross dating. Studying stratigraphy is most effective for dating sedimentary rocks. If the layers of rock are undisturbed, you assume the relative age of rock as the oldest is at the base and the youngest is on top. Cross dating uses fossils to determine the relative age of a rock. Fossil remains have been found in rocks of all ages with the simplest of organisms being found in the oldest of rocks, therefore, the more basic the organism the older the rock is. There are some drawbacks to using relative dating, such as:
- It does not give the actual age of the rock in years.
- External forces from plate tectonics or erosion can change the sequence of the rock.
- Large gaps in geological information can make dating difficult.
The age of Earth has been estimated using radiometric dating. Radiometric dating is the main way to carry out absolute dating, where you find the absolute and actual age of an object. This is done by using the half-life of a radioactive isotope of a specific atom found in the minerals of the rocks. The half-life of an isotope is the time it takes for half of the unstable (radioactive) atoms to decay into a stable atom, for example the half-life of Uranium is 4.46 billion years! This means if you had 10g of uranium it would take 4.46 billion years for 5g of it to decay to lead. Using the ratio of uranium to lead as an example, the older the rock, the lower the ratio of uranium to lead. Young rocks will have high uranium content and low lead content whereas very old rocks will have low uranium content and high lead content. By studying the chemical composition of a rock and knowing the half-life of the radioactive isotopes present we can determine the age of the rock in years.
How do you read a geological map?
The different colours on a geological map represent the different rock types found within that area. In the aspiring Joyce Country & Western Lakes geopark, all three rock groups – igneous, sedimentary and metamorphic – are present: red represents granite (igneous), blue represents limestone (sedimentary), and beige represents quartzite, marble and schist (all metamorphic). Although the aspiring geopark is an area of complicated geology (this is what makes it internationally significant!), geologists can use clues such as igneous intrusions to help date rocks and get an idea of when they formed. For example, the rock the igneous intrusions are found within had to be there first so that they could intrude in to them. The other features seen on the map are faults, represented as black lines, which are fractures with significant displacement on either side of them, and the sites of interest, which are detailed here.
Seeing through the rocks
Another way of describing the local geology is to look at it in section; i.e. a representation of the various layers of rocks stacked over each other vertically. These are built thanks to rock cores drilled for 100s of metres and other lines of evidence on top of the surficial geological map of above and help understand the structure of the landscape and the contacts between the various types of rocks. All these allow us to reconstruct the geological story of a region.