Extrusive Igneous Rocks

Examples of Extrusive Igneous Rocks:

• Basalt (Iron-rich and Dark colored): It forms when the upper mantle melts and this low viscosity magma then rises to the surface and flows out as lava. This then solidifies and settles with small crystals and fine grained texture to form a new ocean crust and so is abundantly found beneath the ocean floor. 
• Obsidian / Volcanic glass (Silica rich and Black to deep green and purple colored): Forms when lava almost instantly due to direct contact with water. This has a glassy texture as can be understood from the name. 
• Pumice: lava which is full of gasses and air when cools down to a frothy texture, Pumice is formed. The formed rock has a rough texture compared to other extrusive igneous rocks.

Some other examples include Andesite, Dacite, Rhyolite, Scoria and Komatiite.

Since formation of extrusive igneous rocks all occur only due to eruption of lava, it is also important to discuss volcanic landforms that are a direct result of volcanic eruption.

Extrusive volcanic landforms

When magma rises from deep within the earth to the surface and cools as lava, it produces extrusive igneous landforms and extrusive igneous rocks. Lava can reach the surface in two different ways: explosively or slowly, depending on its chemical makeup and whether it has an easy route to reach the surface. The magma would be forced to explode as the pressure of the atmosphere built up (as in shaking soda).

Throughout Earth’s history, volcanic processes have altered its surface. During a volcanic eruption, islands can be formed, mountains built or destroyed, and landscapes altered. In our national parks, you’ll find a variety of volcanic activity, including active, dormant, and ancient eruptions.

During an eruption, volcanic material is released into the atmosphere. During volcanic eruptions, lava flows, pyroclastic debris, volcanic bombs, ash, dust, and gaseous products, such as nitrogen compounds, sulphur compounds, and a little chlorine, hydrogen, and argon, are produced.

Volcanic Landforms: Extrusive Igneous Rocks

During the solidification of lava, either within or outside the earth’s surface, volcanic landforms are formed. Landforms resulting from volcanic activity and eruptions are called volcanic landforms. Geological processes determine the characteristics of volcanic landforms. According to the degree of cooling of the magma, volcanic areas consist of extrusive igneous rocks and intrusive igneous rocks. As magma cools and solidifies in the earth’s crust, plutonic rocks are formed. During the cooling process of Lava above the surface of Earth, igneous rocks are extruded.

Types of extrusive igneous rocks

When lava and other volcanic materials are thrown onto the top of the Earth during volcanic eruptions, they form igneous landforms. In addition, volcanic ash and gas such as sulfur dioxide and nitrogen compounds appear in volcanic lava, pyroclastic debris, and volcanic bombs.

The conical vent and fissure vent

A conical vent occurs when lava flows through a narrow, cylindrical hole into the earth’s crust during volcanic activity. Composite volcanic features (or stratovolcanic features) are more likely to contain vents with conical shapes.

Volcanic eruptions are characterized by fissures that are narrow linear vents through which lava can spill out. It is common to find fissure vents associated with basaltic formations. Fissure vents are huge, ranging in size from a few meters to many kilometers.

Shield volcanoes

There are gentle upper slopes for shielding volcanoes, which are slightly steeper at the lower levels. During eruptions, these volcanic cones produce relatively fluid lava flows over a central vent. Basaltic lava that is high in fluidity and low in viscosity is the main component of shield volcanoes. As a consequence, igneous rocks are extruded.

The shield volcano usually does not erupt. However, if water gets inside the vent, the volcano can explode. World-wide, there are eight shield volcanoes. Volcanoes located in shields typically have greater heights and distances than other types. There are also shield volcanoes in Hawaii, such as Mauna Loa.

Cinder cone volcanoes

Cinder cones have the characteristics of steep conical hills with loose pyroclastic fragments around the vent, including volcanic clinker, cinder, and volcanic ash (scoria). There is no lava inside cinder cone volcanoes because they are made entirely of loose grains of cinder. In addition to a small crater on its top, cinder cones usually have very steep sides. They resemble small volcanoes.

Composite volcanoes

Composite volcanoes (stratovolcanoes) are mainly cone-shaped with moderately steep slopes. As hot andesitic lava flows toward vent openings, it is coupled with pyroclastic material and ash. Consequently, volcanic mounts appear as composite volcanoes because they are formed from layers.

Compound volcanoes are also called stratovolcanoes. Compound volcanoes are the most common and highest types of volcanoes. Stromboli, the Mediterranean Lighthouse, Mount Fuji, and so on are some examples.

In composite volcanoes, the lava eruptions are cooler and more viscous than in basaltic volcanoes. The eruptions in composite volcanoes can sometimes be explosive.

Flood basalt provinces (Lava plateaus)

Throughout history, thin, fluid layers of lava have broken into the Earth’s surface, and flowed for long periods of time, covering large areas. These layers, like undulations in a wave pattern, lead to layered flat surfaces with undulations.

Fluctuations of basalts or lavas are extrusive igneous rock types known as flood basalts or lava plateaus. The Deccan traps of India, the Snake basin in the United States, and the Canadian Shield are examples of Flood basalt provinces.

Mid-oceanic ridge volcanoes

Submerged beneath the ocean’s surface, the mid-oceanic ridges can be seen. An extensive system of 70000 km long mid-oceanic ridges exists across all major ocean basins. Mid-oceanic ridges commonly experience volcanic eruptions along their central regions.

The lava that comes out of these eruptions is basaltic and contains less silica, which is why it is less viscous. As a result of their lower viscosity, they can spread over a larger distance and cool more slowly. Due to volcanic eruptions, lava overflows from the earth, causing the seafloor to spread.

Caldera lake

Lava stops flowing when it stops forming a lake, called a caldera lake, which is formed following volcanic eruptions. As a result of these depressions, rainwater and snowmelt easily accumulate, forming lakes.

Among caldera lakes are Lonar in Maharashtra, Krakatoa in Indonesia, and Lake Caldera in southern Oregon. Nonetheless, there are many others.

During the divergence of boundaries between continents, rifts are formed which then turn into rift valleys. Mountain peaks such as Mount Kilimanjaro, Mount Kenya, and Mount Cameroon in Africa’s Great Rift Valley are examples. The divergent plate boundaries are normally located at oceanic ridges, which are formed between oceanic plates. There are constant changes of seafloor boundaries along the Mid-Atlantic Ridge, for example. Iceland is a spectacular location for viewing the mid-ocean ridge of the North Atlantic, making it an enviable spot. A divergent boundary creates gaps that molten lava fills when plates separate, which makes volcanic islands possible.

Subduction zones cause collisions of oceanic plates and continental plates when oceanic plates dive under continental plates. In times of intense heat and pressure, the oceanic plate ruptures beneath the surface of the earth. The molten rock material, magma, can then leak towards the surface where it accumulates to form volcanoes.

Conclusion

So, we can conclude that Extrusive rock is a type of igneous volcanic rock formed when hot magma from deep within the Earth flows out (extrudes) as lava or explodes violently into the atmosphere, falling back as pyroclastics or tuff. They have their presence all around the planet and lead to formation of flood basalt provinces