What is Whin Sill made of?
The Whin Sill is made of a hard, dark, crystalline rock called dolerite (known locally as whinstone), which is mainly composed of the minerals pyroxene and feldspar. As it cooled, the Whin Sill contracted, producing vertical cracks along which the rock breaks into rough columns.
What are volcano sills?
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.
What is sill or sheet?
sill, also called sheet, flat intrusion of igneous rock that forms between preexisting layers of rock. Sills occur in parallel to the bedding of the other rocks that enclose them, and, though they may have vertical to horizontal orientations, nearly horizontal sills are the most common.
How is basalt sill formed?
Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill.
Where is dolerite formed?
Formation. Dolerite cools under basaltic volcanoes, like those at mid-ocean ridges. It cools moderately quickly when magma moves up into fractures and weak zones below a volcano.
Where is the Great Whin Sill?
The Whin Sill or Great Whin Sill is a tabular layer of the igneous rock dolerite in County Durham, Northumberland and Cumbria in the northeast of England.
What is the thickness of a sill?
All sills have a thickness of 50 m.
How is a sill formed?
Sills: form when magma intrudes between the rock layers, forming a horizontal or gently-dipping sheet of igneous rock.
What are sills examples?
A sill is a flat sheet-like igneous rock mass that is formed when magma intrudes into between the older layers of rocks and crystallizes. A renowned example of the sill is the tabular mass of quartz trachyte found near the summit of Engineer Mountain near Silverton, Colorado.
What elements is basalt made of?
Basalt generally has a composition of 45–52 wt% SiO2, 2–5 wt% total alkalis, 0.5–2.0 wt% TiO2, 5–14 wt% FeO and 14 wt% or more Al2O3. Contents of CaO are commonly near 10 wt%, those of MgO commonly in the range 5 to 12 wt%.
What minerals make up dolerite?
Dolerite. Fine grained, ophitic texture. Primary minerals – plagioclase, pyroxene, hornblende and quartz. Acicular and lath-like plagioclase, equant grains of pyroxene – some alteration to chlorite along cleavage and fissures.