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Volcanism has continued throughout the Cenozoic on land and at the major oceanic ridges, such as the Mid-Atlantic Ridge and the East Pacific Rise, where new seafloor is continuously generated and carried away laterally by seafloor spreading.
Iceland, which was formed in the middle Miocene, is one of the few places where the processes that occur at the Mid-Atlantic Ridge can be observed today.
The Paleogene Period, the oldest of the three divisions, commences at the onset of the Cenozoic Era and includes the Paleocene Epoch (66 million to 56 million years ago), the Eocene Epoch (56 million to 33.9 million years ago), and the Oligocene Epoch (33.9 million to 23 million years ago).
The Neogene spans the interval between the beginning of the Miocene Epoch (23 million to 5.3 million years ago) and the end of the Pliocene Epoch (5.3 million to 2.6 million years ago).
It is only in more recent times that the international geologic community has formulated a philosophical framework for stratigraphy.
By specifying the lower limits of rock units deposited during successive increments of geologic time at designated points in the rock record (called stratotypes), geologists have established a series of calibration points, called Global Boundary Stratotype Sections and Points (GSSPs), at which time and rock coincide.
The Paleocene-Eocene boundary has an estimated age of 56 million years; its GSSP is located near Luxor, Egypt.
The GSSP associated with the Miocene-Pliocene boundary is located in Sicily and has been dated to about 5.3 million years ago, although the location of this boundary may be repositioned in the future.
By 2009 the larger intervals (periods and epochs) of the Cenozoic had been formalized by the ICS and the Neogene Period (23 million to 2.6 million years ago), and the Quaternary Period (2.6 million years ago to the present).
Under this paradigm, the Paleogene and Neogene span the interval formerly occupied by the Tertiary.
In the shallower parts of the ocean (above depths of 4.5 km [about 3 miles]), sediments are calcareous (made of calcium carbonate), siliceous (derived from silica), or both, depending on local productivity.
Below 4.5 km the sediments are principally siliceous or inorganic, as in the case of red clay, due to dissolution of calcium carbonate.