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8 Jun 2020

June 08, 2020 Comment

How Do Extrusive and Intrusive Environments Differ?

With a background on how melts form and freeze, we can now introduce key features of the two settings intrusive and extrusive in which igneous rocks form.

Different volcanoes extrude molten rock in different ways. Some volcanoes erupt streams of low-viscosity lava that ﬂood down the ﬂanks of the volcano and then cover broad swaths of the countryside. When this lava freezes, it forms a relatively thin lava ﬂow. Such ﬂows may cool in days to months. In contrast, some volcanoes erupt viscous masses of lava that pile into rubbly domes. And still others erupt explosively, sending clouds of volcanic ash and debris skyward, and/or avalanches of ash tumbling down the sides of the volcano.

Examples of eruptions and extrusive materials.

Which type of eruption occurs depends largely on a magma’s composition and volatile content. Volatile-rich felsic lavas tend to erupt explosively and form thick ash and debris deposits (figure above a, b). Maﬁc lavas tend to have low viscosity and spread in broad, thin ﬂows (figure above c, d).

Intrusive Igneous Settings

Magma rises and intrudes into pre-existing rock by slowly percolating upward between grains and/or by forcing open cracks. The magma that doesn't make it to the surface freezes solid underground in contact with pre-existing rock and becomes intrusive igneous rock. As we noted, geologists commonly refer to the pre-existing rock into which magma intrudes as wall rock. The boundary between wall rock and an intrusive igneous rock is called an intrusive contact.

Igneous sills and dikes, examples of tabular intrusions.

Geologists distinguish among different types of intrusions on the basis of their shape. Tabular intrusions, or sheet intrusions, are planar and are of roughly uniform thickness. Most are in the range of centimetres to tens of meters thick, and tens of meters to tens of kilometres long. A dike is a tabular intrusion that cuts across pre-existing layering (bedding or foliation), whereas a sill is a tabular intrusion that injects parallel to layering (figure above a–d). In places where tabular intrusions cut across rock that does not have layering, a nearly vertical, wall-like tabular intrusion is called a dike, and a nearly horizontal, tabletop-shaped tabular intrusion is called a sill. Some intrusions start to inject between layers but then dome upward, creating a blister-shaped intrusion known as a laccolith.

Plutons are blob-shaped intrusions that range in size from tens of meters across to tens of kilometres across (figure below a–e). The intrusion of numerous plutons in a region creates a vast composite body that may be several hundred kilometres long and over 100 km wide; such immense masses of igneous rock are called batholiths. The rock making up the Sierra Nevada of California is a batholith formed from plutons that intruded between 145 and 80 million years ago.

Igneous plutons, "blob shaped" intrusions.

Making room for an igneous intrusion.

Where does the space for intrusions come from? Dikes form in regions where the crust is being stretched horizontally, such as in a rift. Thus, as the magma that makes a dike forces its way up into a crack, the crust opens up sideways (figure above a). Intrusion of sills occurs near the surface of the Earth, so the pressure of the magma effectively pushes up the rock above the sill, leading to uplift of the Earth’s surface (figure above b).

How does the space for a pluton develop? Some geologists propose that a pluton is a frozen “diapir,” meaning a light-bulb-shaped blob of magma that pierced overlying rock and pushed it aside as it rose (figure above c). Another explanation involves stoping, a process during which magma assimilates wall rock, and blocks of wall rock break off and sink into the magma (Fig. 4.11d). If a stoped block does not melt entirely, but rather becomes surrounded by new igneous rock, it is a xenolith, after the Greek word xeno, meaning foreign. More recently, geologists have proposed that plutons form by injection of numerous superimposed dikes or sills, which coalesce and recrystallize to become a single, massive body.

Credits: Stephen Marshak (Essentials of Geology)

7 Jun 2020

June 07, 2020 Comment

How Do You Describe an Igneous Rock?

Different parameters are used to describe an igneous rock which are described in detail.

If you wander around a city admiring building facades, you’ll ﬁnd that many facades consist of igneous rock, for such rocks tend to be very durable. If you had to describe one of these rocks to a friend, what words might you use? You would probably start by noting the rock’s colour. Overall, is the rock dark or light? More speciﬁcally, is it gray, pink, white, or black? Describing colour may not be easy, because some igneous rocks contain many visible mineral grains, each with a different colour; but even so, you’ll probably be able to characterize the overall hue of the rock. Generally, the colour reﬂects the rock’s composition, but it isn't always so simple, because colour may also be inﬂuenced by grain size and by the presence of trace amounts of impurities. (For example, the presence of a small amount of iron oxide gives rock a reddish tint.) Next, you would probably characterize the rock’s texture. A description of igneous texture indicates whether the rock consists of glass, crystals, or fragments. If the rock consists of crystals or fragments, a description of texture also speciﬁes the grain size. Here are the common terms for deﬁning texture:

Textures and types of igneous rocks.

Crystalline texture: Rocks that consist of minerals that grow when a melt solidifies interlock like pieces of a jigsaw puzzle (figure above a). Rocks with such a texture are called crystalline igneous rocks. The interlocking of crystals in these rocks occurs because once some grains have developed, they interfere with the growth of later-formed grains. The last grains to form end up filling irregular spaces between already existing grains. Geologists distinguish subcategories of crystalline igneous rocks according to the size of the crystals. Coarse-grained (phaneritic) rocks have crystals large enough to be identified with the naked eye. Fine-grained (aphanitic) rocks have crystals too small to be identified with the naked eye. Porphyritic rocks have larger crystals surrounded by a mass of fine crystals. In a porphyritic rock, the larger crystals are called phenocrysts, while the mass of finer crystals is called groundmass.
Fragmental texture: Rocks consisting of igneous chunks and/ or shards that are packed together, welded together, or cemented together after having solidified are fragmental igneous rocks (figure above a).
Glassy texture: Rocks made of a solid mass of glass, or of tiny crystals surrounded by glass, are glassy igneous rocks (figure above a). Glassy rocks fracture conchoidally (figure above b).

What factors control the texture of igneous rocks? In the case of nonfragmental rocks, texture largely reﬂects cooling rate. The presence of glass indicates that cooling happened so quickly that the atoms within a lava didn’t have time to arrange into crystal lattices. Crystalline rocks form when a melt cools more slowly. In crystalline rocks, grain size depends on cooling time. A melt that cools rapidly, but not rapidly enough to make glass, forms ﬁne-grained rock, because many crystals form but none has time to grow large (figure above c). A melt that cools very slowly forms a coarse-grained rock, because a few crystals have time to grow large.

Because of the relationship between cooling rate and texture, lava ﬂows, dikes, and sills tend to be composed of fine grained igneous rock. In contrast, plutons tend to be composed of coarse-grained rock. Plutons that intrude into hot wall rock at great depth cool very slowly and thus tend to have larger crystals than plutons that intrude into cool country rock at shallow depth, where they cool relatively rapidly. Porphyritic rocks form when a melt cools in two stages. First, the melt cools slowly at depth, so that phenocrysts form. Then, the melt erupts and the remainder cools quickly, so that groundmass crystallizes around the phenocrysts.

There is, however, an exception to the standard cooling rate and grain size relationship. A very coarse-grained igneous rock called pegmatite doesn't necessarily cool slowly. Pegmatite contains crystals up to tens of centimetres across and occurs in dikes. Because pegmatite occurs in dikes, which generally cool quickly, the coarseness of the rock may seem surprising. Researchers have shown that pegmatites are coarse because they form from water-rich melts in which atoms can move around so rapidly that large crystals can grow very quickly.

Because melts can have a variety of compositions and can freeze to form igneous rocks in many different environments above and below the surface of the Earth, we observe a wide spectrum of igneous rock types. We classify these according to their texture and composition. Studying a rock’s texture tells us about the rate at which it cooled, as we've seen, and therefore the environment in which it formed. Studying its composition tells us about the original source of the magma and the way in which the magma evolved before ﬁnally solidifying. Below, we introduce some of the more important igneous rock types.

Crystalline igneous rocks

The scheme for classifying the principal types of crystalline igneous rocks is quite simple. The different compositional classes are distinguished on the basis of silica content ultramaﬁc, maﬁc, intermediate, or felsic whereas the different textural classes are distinguished according to whether the grains are coarse or ﬁne. As a rough guide, the colour of an igneous rock reﬂects its composition: maﬁc rocks tend to be black or dark gray, intermediate rocks tend to be lighter gray or greenish gray, and felsic rocks tend to be light tan to pink or maroon. Figure above provides images of some of these rocks.

Igneous rocks are classiﬁed based on composition and texture.

Note that, according to figure above, rhyolite and granite have the same chemical composition but differ in grain size. Which of these two rocks develops from a melt of felsic composition depends on the cooling rate. A felsic lava that solidiﬁes quickly at the Earth’s surface or in a thin dike or sill turns into ﬁne-grained rhyolite; but the same magma, if solidifying slowly at depth in a pluton, turns into coarse-grained granite. A similar situation holds for maﬁc lavas a maﬁc lava that cools quickly in a lava ﬂow forms basalt, but a maﬁc magma that cools slowly forms gabbro.

Glassy igneous rocks

Glassy texture develops more commonly in felsic igneous rocks because the high concentration of silica inhibits the easy growth of crystals. But basaltic and intermediate lavas can form glass if they cool rapidly enough. In some cases, a rapidly cooling lava freezes while it still contains a high concentration of gas bubbles these bubbles remain as open holes known as vesicles. Geologists distinguish among several different kinds of glassy rocks.

Obsidian is a mass of solid, felsic glass. It tends to be black or brown (first figure b). Because it breaks conchoidally, sharp-edged pieces split off its surface when you hit a sample with a hammer. Pre- industrial people worldwide used such pieces for arrowheads, scrapers, and knife blades.

Pumice, a vesicles filled volcanic rock, is so light that paper can hold it up. The vesicles it contain tend to be small.

Pumice is a felsic volcanic rock that contains abundant vesicles, giving it the appearance of a sponge. Pumice forms by the quick cooling of frothy lava that resembles the head of foam in a glass of beer. In some cases, pumice contains so many air-filled pores that it can actually float on water, like styrofoam (figure above).

Scoria is a mafic volcanic rock that contains abundant vesicles (more than about 30%). Generally, the bubbles in scoria are bigger than those in pumice, and the rock, overall, looks darker.

Pyroclastic igneous rocks

As we have noted, when volcanoes erupt explosively, they spew out fragments of lava. Geologists refer to all such fragments as pyroclasts. Accumulations of fragmental volcanic debris are called pyroclastic deposits, and when the material in these deposits consolidates into a solid mass, due either to welding together of still-hot clasts or to cementation by minerals precipitating from water passing through, it becomes a pyroclastic rock. Geologists distinguish among several types of pyroclastic rocks based on grain size. Let’s consider two examples.

Tuff is a fine-grained pyroclastic igneous rock composed of volcanic ash. It may contain fragments of pumice.
Volcanic breccia consists of larger fragments of volcanic debris that either fall through the air and accumulate, or form when a lava flow breaks into pieces.

Credits: Stephen Marshak (Essentials of Geology)

14 May 2020

May 14, 2020 Comment

10 of the Best Learning Geology Photos of 2016

A picture is worth a thousand words, but not all pictures are created equal. The pictures we usually feature onLearning Geology are field pictures showing Geological structures and features and many of them are high quality gem and mineral pictures. The purpose is to encourage students and professionals' activities by promoting "learning and scope" of Geology through our blogs.

In the give up of 2016, we're sharing with you the 10 satisfactory snap shots of 2016 which we've got published on our page.

P.S: we usually try our best to credit every and each photographer or website, however now and again it?S not possible to track some of them. Please go away a remark in case you recognize approximately the lacking ones.

1. Folds from Basque France

Image Credits: Yaqub ShahYaqub Shah

2. Horst and Graben Structure in Zanjan, Iran

Image Credits:https://www.Instagram.Com/amazhda

three. A specific Normal Fault

four. The Rock Cycle

Therock cycle illustrates the formation, alteration, destruction, and reformation of earth materials, and typically over long periods of geologic time. The rock cycle portrays the collective system of processes, and the resulting products that form, at or below the earth surface.The illustration below illustrates the rock cycle with the common names of rocks, minerals, and sediments associated with each group of earth materials: sediments, sedimentary rocks, metamorphic rocks, and igneous rocks.

Image Credits: Phil Stoffer

5. An amazing Botryoidal specimen for Goethite fans!

Image Credits: Moha Mezane

6. Basalt outcrop of the Semail Ophiolite, Wadi Jizzi, Oman

Image Credits: Christopher Spencer

Christopher Spencer is founder of an high-quality technological know-how outreach program named as Traveling Geologist. Visit his website to research from him

7. Val Gardena Dolomites, Northern Italy

8. Beautiful fern fossil found in Potsville Formation from Pennsylvania.

The ferns maximum generally found are Alethopteris, Neuropteris, Pecopteris, and Sphenophyllum.

Image Credits: Kurt Jaccoud

9. Snowball garnet in schist

Syn-kinematic crystals in which ?Snowball garnet? With noticeably turned around spiral Si.

Porphyroblast is ~ five mm in diameter.

From Yardley et al. (1990) Atlas of Metamorphic Rocks and their Textures.

10. Trilobite Specimen from Wheeler Formation, Utah

The Wheeler Shale is of Cambrian age and is a international famous locality for prolific trilobite stays.

Image Credits: Paleo Fossils

6 May 2020

May 06, 2020 Comment

Geologic Contacts

2. Intrusive contacts are those where one rock has intruded another

3. Fault contacts are those where rocks come into contact across fault zones.

Learn in element about fault right here

Following are the a few snap shots displaying each kind of geologic contact

Depositional Contacts

1. Angular Unconformity, Siccar Point, Scotland

This place is known as Siccar Point whichis the most important unconformity described by James Hutton (1726-1797) in support of his world-changing ideas on the origin and age of the Earth.

Here gently sloping strata of 370-million-year-old Famennian Late DevonianOld Red Sandstone and a basal layer of conglomerate overlie near vertical layers of 435-million-year-old lower Silurian Llandovery Epoch greywacke, with an interval of around 65 million years.

2. Cretaceous Sandstone overlying Conglomerate Kootenai Formation, SW Montana

Photo Courtesy: marlimillerphoto.Com

3. Dun Briste Sea Stack, IrelandDun Briste is a truly incredible site to see but must be visited to appreciate its splendour. It was once joined to the mainland. The sea stack stands 45 metres (150 feet) tall.

Dun Briste and the surrounding cliffs were formed around 350 million years ago (during the 'Lower Carboniferous Period'), when sea temperatures were much higher and the coastline at a greater distance away. There are many legends describing how the Sea Stack was formed but it is widely accepted that an arch leading to the rock collapsed during very rough sea conditions in 1393. This is remarkably recent in geological terms.