| Eycott Volcanic Group Stratigraphic range: Ordovician (Llandeilo to Caradoc epochs) | |
|---|---|
| Type | Group |
| Sub-units | Potts Ghyll Formation, Overwater Formation |
| Underlies | unconformity |
| Overlies | Skiddaw Group |
| Thickness | to 3200m |
| Lithology | |
| Primary | andesitic lavas and sills |
| Other | volcanic breccias, tuffs, sandstone |
| Location | |
| Region | Cumbria |
| Country | England |
| Extent | northern Lake District & Melmerby Fell inlier |
| Type section | |
| Named for | Eycott Hill |
The Eycott Volcanic Group is a group of volcanic rock formations of Ordovician age (Llandeilo to Caradoc epochs) named after the locality of Eycott Hill in the English Lake District. [1] The group overlies the Skiddaw Group and is unconformably overlain by a variety of different Devonian and Carboniferous age rocks.
A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are generally divided into individual formations. Groups may sometimes be divided into "subgroups" and are themselves sometimes grouped into "supergroups".
The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.2 million years from the end of the Cambrian Period 485.4 million years ago (Mya) to the start of the Silurian Period 443.8 Mya.
In geochronology, an epoch is a subdivision of the geologic timescale that is longer than an age but shorter than a period. The current epoch is the Holocene Epoch of the Quaternary Period. Rock layers deposited during an epoch are called a series. Series are subdivisions of the stratigraphic column that, like epochs, are subdivisions of the geologic timescale. Like other geochronological divisions, epochs are normally separated by significant changes in the rock layers to which they correspond.
This rock sequence has previously been known as the Eycott Group. [2] It consists largely of andesitic lavas and sills with tuffs, breccias and volcaniclastic sandstones. Its main outcrop is in an east-west oriented band of country in the northern part of the Skiddaw range in the northern Lake District stretching from the village of Bothel east to form the hill of Binsey and further east, the more extensive Caldbeck Fells. A smaller outcrop underlies Greystoke Park just to the east of this area and forms Eycott Hill east of Mungrisdale. There is a further inlier at Melmerby Fell in the North Pennines. The outcrops are intensely faulted and in the Caldbeck area are crossed by numerous mineral veins yielding arsenic, barium, copper and lead which have been worked in the past.
Andesite ( or ) is an extrusive igneous, volcanic rock, of intermediate composition, with aphanitic to porphyritic texture. In a general sense, it is the intermediate type between basalt and rhyolite, and ranges from 57 to 63% silicon dioxide (SiO2) as illustrated in TAS diagrams. The mineral assemblage is typically dominated by plagioclase plus pyroxene or hornblende. Magnetite, zircon, apatite, ilmenite, biotite, and garnet are common accessory minerals. Alkali feldspar may be present in minor amounts. The quartz-feldspar abundances in andesite and other volcanic rocks are illustrated in QAPF diagrams.
Lava is molten rock generated by geothermal energy and expelled through fractures in planetary crust or in an eruption, usually at temperatures from 700 to 1,200 °C. The structures resulting from subsequent solidification and cooling are also sometimes described as lava. The molten rock is formed in the interior of some planets, including Earth, and some of their satellites, though such material located below the crust is referred to by other terms.
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or along the direction of foliation in metamorphic rock. A sill is a concordant intrusive sheet, meaning that a sill does not cut across preexisting rock beds. Stacking of sills builds a sill complex and a large magma chamber at high magma flux. In contrast, a dike is a discordant intrusive sheet, which does cut across older rocks. Sills are fed by dikes, except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related to foliation in metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations.
