Formation and Types of Peatlands
Taken from Bord na Móna, Fact Sheet no. 2.
Peatlands (or Boglands) are biogenic deposits where the incompletely decayed remains from a variety of plants, and occasionally trees, have accumulated in waterlogged areas over hundreds or thousands of years. The deposits are a minimum of 45 cm deep but in the vast majority of instances they range from 2 metres to 12 metres in depth.
The three main types which occur in Ireland are : (See Fig. I below)
- Raised Bogs;
- Blanket Bogs;
FIG I: Peatland types
PEATLAND (OR BOG) FORMATION
The formation of peatlands will be discussed separately under each of the peatland types. (See Facts Sheets 3, 4, 5)
Peat forms in situations where a number of conditions exist over long periods of time.
(i) the continuous annual growth of vegetation;
(ii) the presence of moderate to relatively high levels of rainfall;
(iii) the existence of poor drainage which leads to waterlogging of the mineral soil surface;
(iv) the level of oxygen in the waterlogged soil is very low or absent (i.e. anaerobic conditions prevail).
INFLUENCE OF ICE AGE:
Any discussion on the formation of peatlands should make reference to the last glaciation period, or ice age, because of the major influence this had on the evolution of peatlands, especially in the central plain. About 13,000 years ago, the ice fields had melted and retreated northwards, due to a slight increase in temperature, and left behind glacial deposits (i.e. eskers, moraines, drumlins) which resulted in a very irregular topography or landscape. The glacial deposits were mainly composed of calcareous drift in the central plain and siliceous drift in the west of the country. The undulating landscape had many deep hollows or basins which filled with water due to the poor draining characteristics of the basin soils. Lakes were formed and between the lake basins were many gravel ridges (i.e. of esker and moraine origin) which impeded drainage and promoted the formation of peatlands. The formation of peat can be written in equation form and this can be compared to the normal decomposition process of plant material. (See Table I)
Peat Forming (incomplete) Decomposition Process,
Plant Residues +Anaerobic Bacteria = Peat +CH4
(In stagnant waters, poor in oxygen)
Normal (complete) Decomposition Process
Plant Residues + Soil Microbes +O2 = Humus +CO2+ H2O
WHY EXTENSIVE PEATLANDS FORMED IN IRELAND
The mild Irish climate with its relatively high rainfall and number of rain days per year has been important in promoting peatland development. Raised bogs form where rainfall is between 700mm and 1000mm and the number of rain days is between 150 and 175 per year. Blanket bogs form where rainfall is greater than 1250mm and the number of rain days is greater than 200 per year. (See Fig 2)
POOR DRAINING SOILS
These had a major influence in the formation of peatlands. After the last ice age many of the hollows or basins, particularly in the central plain, had poorly draining soils which were mainly composed of silty clays. These soils led to waterlogging of the soil surface and to the formation of lakes where a variety of plants grew annually. As the cycle of growth of vegetation and partial decay continued, peat formed at the bottom of the lakes each year and this led to the development of our extensive peatlands.
FIG 2:Mean annual rainfall (mm)
AGE OF PEATLANDS
The last glaciation period, or ice age, ended about 13000 years ago. This left a very uneven landscape of ridges and hollows throughout the country. In very many cases the hollows had poorly draining soils where water accumulated and formed lakes. As the climate gradually became warmer, a range of vegetation types grew in these lakes: Reeds grew along the margins, water lillies in shallow bays and floating pondweeds on the deeper waters. When these plants died they only partially decayed and their remains accumulated at the bottom of the lakes. Over time the plant residues built up and the lakes became shallower and gradually infilled. Therefore the oldest peats within the vast areas of Ireland's peatlands were formed about 9000 years ago, and younger peats derived from different vegetation sources have formed in periods since that time. (See Fig 3)
FIG 3: Peatlands calendar
BORD NA MONA