Igneous Rocks and Magma

Igneous Rocks

• Igneous rocks derive from molten magma/lava from the Earth's interior.
• Igneous rocks are the source of all other rocks and soils.
• They are "fire-born" and created by the cooling and solidification of molten rock.
• The term "igneous" comes from the Latin word "ignis," meaning "fire".

Magma

• Igneous rocks come from magma or lava.
• Magma is a molten or semi-molten mixture found beneath the Earth's surface.
• It consists of silicate liquid and mixtures of carbonate and sulfide melts.
• Magma is produced when the mantle or crust melts, occurring in subduction zones, continental rift zones, mid-ocean ridges, and hotspots.
• Suspended crystals and unmelted rock fragments may be present in magma.
• Dissolved volatiles may separate as bubbles; some liquid may crystallize during movement.

Magma Types

• There are five magma types: felsic, intermediate, mafic, ultramafic, and alkaline.
• Felsic magmas' (or silicic) silica content is over 63%.
• Felsic magmas are very viscous, leading to explosive volcanic eruptions and fragmental deposits.
• Felsic magmas' temperature range is 700°C to 900°C.
• Intermediate magmas (or andesitic) contain 52% to 63% silica.
• Intermediate magmas erupt at temperatures from 850°C to 1100°C, which is higher than felsic magmas.
• Intermediate magmas are less viscous than felsic magmas.
• Mafic magmas (or basaltic) have a silica content of 45% to 52%.
• Mafic magmas have very low viscosity and eruption temperatures from 1100°C to 1200°C.
• Ultramafic magmas have a silica content below 45%
• Ultramafic magmas have very low viscosity and high eruption temperatures up to 1600°C.
• Alkaline magmas have elevated levels of alkali metal oxides, such as sodium and potassium, and are silicic/felsic.

Lava

• Igneous rocks derive from magma or lava.
• Lava is a molten or semi-molten material emerging onto the Earth's surface.
• Lava consists of silicate liquid and mixtures of carbonate and sulfide melts.
• Lavas are also the solidified rock formed by the cooling of molten lava flows.
• Lava temperature ranges from about 700°C to 1200°C.
• Lava material can be very fluid or extremely stiff.
• Higher silica content in lava equates to higher viscosity.

Lava Types

• There are five lava types: felsic, intermediate, mafic, ultramafic, and alkaline.
• Felsic lavas or silicic lavas have a silica content greater than 63%.
• Felsic lavas are extremely viscous, resulting in explosive volcanic eruptions that generate fragmental deposits.
• Temperatures range from 700°C to 900°C for felsic lavas.
• Intermediate or andesitic lavas have a silica content of 52% to 63%.
• Intermediate lavas erupt at temperatures ranging from 850°C to 1100°C, which is higher than felsic lavas.
• Intermediate lavas are less viscous than felsic lavas.
• Mafic, or basaltic lavas, contain 45% to 52% silica.
• Mafic lavas are very low viscosity, and have eruption temperatures from 1100°C to 1200°C.
• Ultramafic lavas have silica content lower than 45%.
• Ultramafic lavas have very, very low viscosity and very high eruption temperatures up to 1600°С.
• Alkaline lavas are silicic/felsic lavas, containing elevated levels of alkali metal oxides (sodium and potassium).

Types of Igneous Rocks

• There are two igneous rock types: intrusive and extrusive.
• Intrusive rocks (plutonic rocks) solidify within the Earth.
• Intrusive igneous rocks are crystalline and have large crystals visible to the naked eye, due to slow cooling.
• Central cores of major mountain ranges are made of intrusive igneous rocks, exposed by erosion or human activity.
• Extrusive rocks (volcanic rocks) erupt onto the surface or into the atmosphere before solidifying.
• Extrusive rocks generally have smaller crystal sizes and a fine-grained or glassy groundmass, formed by rapid cooling.
• Extrusive rocks erupt as lava flows via non-explosive eruptions or as shattered bits of rock volcanic ash/tephra/volcanic bombs called pyroclasts, via explosive eruptions.

Forms of Lava (Extrusive) Flow

• 'A'ā (or aa, 'a'a, a-aa) describes a basaltic lava flow with a rough or rubbly surface made of broken lava blocks.
• Pāhoehoe (or pahoehoe) is basaltic lava with a smooth, billowy, undulating surface and created by very fluid lava.
• Block lava is similar to aa, but is more viscous.
• Block lava forms smooth-sided angular blocks of solidified lava.
• Pillow lava forms when lava emerges from an underwater vent or enters the ocean.

Lava Landforms

• Volcanoes are landforms created by repeated eruptions of lava and ash, which enable gases, volcanic ash, and hot lava to escape from a magma chamber below the Earth's surface.
• Kīpuka is an area surrounded by younger lava flows, formed when lava flows around a hill or ridge.
• Lava domes are mound-shaped protrusions from the slow extrusion of viscous lava from a volcano.
• Lava tubes/pyroducts are natural conduits formed by flowing lava from a volcanic vent under a hardened lava flow surface.
• Lava lakes are large volumes of lava, usually basaltic, in a volcanic vent, crater, or depression.
• Solidified lava lakes are are called frozen lava lakes.
• Lava deltas form where subaerial lava flows enter standing water bodies; different from pillow lavas.

Igneous Intrusions

• Batholiths are large, irregularly shaped intrusive igneous rock masses.
• Batholiths form beneath the Earth's surface by intrusion and the solidification of magma.
• Laccoliths are intrusive rock bodies with a dome-shaped upper surface and level base.
• Laccoliths are fed by a conduit from below, forming when magma rising through the crust spreads out horizontally.
• Dikes/dykes are sheets of rock in a fracture of a pre-existing rock body
• Dikes form as magma flows through the crack and solidifies into an intrusion, cutting across rock layers or through a contiguous rock mass.
• Sills are tabular igneous sheet intrusions between older rock layers, along the foliation direction of a metamorphic rock.
• Sills do not cut across pre-existing rock beds, unlike dikes.
• Lopoliths are large igneous intrusion with roughly flat top and a shallow convex base, which may have a feeder dike below.
• Volcanic plugs/necks are volcanic objects that form when magma hardens within a vent or opening on a volcano.

Igneous Textures

• Igneous textures occur in igneous rocks.
• Final textures are affected by extrusive or intrusive cooling (slow or rapid)
• Water or gas content affect final texture.
• Textural terms: phaneritic, aphanitic, porphyritic, pyroclastic, vesicular, glassy.

Texture Types

• Phaneritic: texture describes coarse grains in rocks containing large crystals, found in intrusive rocks with slow cooling.
• Aphanitic: texture contains fine grains with mostly small crystals, common with extrusive rocks with rapid cooling.
• Porphyritic: texture contains both small and large crystals, always having two cooling phases: one slow-cooling then rapid cooling (or vice versa).
• Glassy (vitreous): texture shows formless glass and few to no crystals and are formed when lava is quenched very fast, which prevents crystallization.
• Vesicular: texture is characterized by the formation or presence of vesicles, or air-filled voids/sacs.
• Vesicular textures form due to air bubbles or steam present in the magma/lava during solidification.
• Pyroclastic: texture resulted from the compaction of volcanic ash, explosive debris, and other textures.
• Rocks belonging to pyroclastic texture have a gritty feel and contain bits of fragmented material.