Carbonate Petrography

Carbonate petrography is the study of limestones, dolomites and associated deposits under optical or electron microscopes greatly enhances field studies or core observations and can provide a frame of reference for geochemical studies.

25 strangest Geologic Formations on Earth

The strangest formations on Earth.

What causes Earthquake?

Of these various reasons, faulting related to plate movements is by far the most significant. In other words, most earthquakes are due to slip on faults.

The Geologic Column

As stated earlier, no one locality on Earth provides a complete record of our planet’s history, because stratigraphic columns can contain unconformities. But by correlating rocks from locality to locality at millions of places around the world, geologists have pieced together a composite stratigraphic column, called the geologic column, that represents the entirety of Earth history.

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Showing posts with label Types of Margins. Show all posts
Showing posts with label Types of Margins. Show all posts

Geologic Contacts

A geologic contact is where one rock type touches another. There are three types of geologic contact:1. Depositional contacts are those where a sedimentary rock (or a lava flow) was deposited on an older rock
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 detail about fault here

Following are the some pictures showing each type of geologic contact

Depositional Contacts

1. Angular Unconformity, Siccar Point, Scotland

This place is known as Siccar Point which is 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 Devonian Old 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

Photo Courtesy: dunbriste.com 


Fault Contacts

1. Normal Faulting in the Cutler Formation near Arches National Park

Photo Courtesy: travelinggeologist.com

2. Normal Fault in Titus Canyon, Death Valley, California 

Photo Courtesy: travelinggeologist.com


3. 
Horst and Graben Structure in Zanjan, Iran

Photo Courtesy: Amazhda

Intrusive Contacts 

1. 
Pegmatite and aplite dikes and veins in granitic rocks on Kehoe Beach, Point Reyes National Seashore, California.


2. Spectacular mafic dyke from Isla de Socorro from Pep Cabré. The Isla de Socorro is a volcanic island off the west coast of Mexico and it is the only felsic volcano in the Pacific Ocean

Photo Courtesy: travelinggeologist.com

3. The margins of this Granite dyke cooled relatively quickly in contact with this much older Gabbro.
Photo near Ai-Ais Namibia

Photo Courtesy: travelinggeologist

Continental Accretion and Plate Tectonics Model

Continental Accretion

Accretion is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts or other igneous features, or blocks or pieces of continental crust split from other continental plates. Over "geologic time" (measured in millions of years), volcanic arcs form and may be crushed onto (or between) colliding continents with plate boundaries. Pieces of continental land masses may be ripped away and carried to other locations. For instance, Baja California and parts of southern California west of the San Andreas Fault are being ripped away from the North American continent and are slowly being carried northward. These rocks may eventually pass what-is-now San Francisco, and perhaps 70 to 100 million years from now will be crushed and accreted into the landmass currently known as Alaska!
Plate tectonics model: 
Subduction introduces oceanic crustal rocks (including sediments) back into the Asthenosphere. Water and gas helps low-temperature minerals to melt and rise as, forming new continental crust (less dense than oceanic crust). Floating on the Asthenosphere, the continental crustal materials accumulate, forming continents.

Plate Tectonic Model
Photo Courtesy: Phil Stoffer


The processes associated with subduction lead to the accretion (growth) of continents over time. As ocean crust is recycled back into the upper mantle, the lighter material "accumulates" along continental margins. Pieces of lithosphere are sometimes scraped off one plate and crushed onto and added to another plate.  
 

refining
Photo Courtesy: Phil Stoffer
   

The Sea Floor

The Sea Floor
Introduction

The Earth is covered by 71% ocean.

Most of what we know about the ocean floor was discovered after 1950, when advances in technology permitted its exploration.

We study the ocean floor using:
1)            Sonar
2)           Core drilling
3)           Submersibles
4)           Gravity and magnetic surveys

The ocean floor consists of sediment lying on top of basaltic crust

Therefore oceanic crust (basaltic) is composition-ally and structurally simpler than continental crust (chiefly granitic).
 Structure of the Ocean Floor 



Continental shelf
 – An underwater platform of continental crust at the edge of a continent.  It is inclined very gently seaward at an angle of less than 1°.

On the Atlantic coast of the US, the shelf is 500 km wide.  On the Pacific coast, it is only a few kilometers wide. 

The shelf is covered with young, loose, sediment derived from the land via rivers.

Continental slope – A relatively steep (~4-5°) slope extending from a depth of 100-200 meters at the edge of the continental shelf down to the deep ocean floor.

This is where the continental crust transitions into oceanic crust.

Abyssal plain – The very flat region of the deep-ocean floor, consisting of oceanic (basaltic) crust and overlying sediments.

The plain starts at the base of the continental slope.  The water depth is about 5 km.

This is the flattest feature on the Earth (overlying sediment “fills in” the rugged volcanic oceanic crust). 

Submarine Canyons- V-shaped erosional canyons incised in the continental shelf and slope, and end at the abyssal plain.

Sediment transported within these canyons is deposited in fan-shaped features called abyssal fans (analogous to alluvial fans on land).

These underwater canyons may have initially been carved by rivers during the most recent period of glaciation when sea level was lower.

Currents related to tides move up and down the canyons aiding in the transport of sediment and the erosion of the canyon.

Also, turbidity currents (underwater landslides triggered by earthquakes or strong storms) contribute to the formation of these canyons.
Types of Margins

Passive Continental Margins (East Coast of US)
 
Passive continental margin- A margin that connects continental crust to oceanic crust without any tectonic boundaries.

This is a geologically "quite" boundary without volcanoes, earthquakes, or young mountain belts.  The main activity is sediment deposition.

Passive margins include a large continental shelf, a continental slope, an abyssal plain, and a “continental rise”.

Continental rise- A wedge of sediment that lies at the base of the continental slope on passive margins.  It connects the continental slope to the abyssal plain, and has a gentler slope than the continental slope (~0.5°).

Active Continental Margins (West Coast of US)
 
Active continental margins are characterized by tectonic boundaries, volcanoes, earthquakes, and young mountain belts.

They include a continental shelf and continental slope.

The continental rise is typically absent.

Instead, oceanic trenches are present.

Oceanic trenches are the deepest (8-10 km) parts of the ocean.  They parallel the edge of a continent and are related to a subduction zone.

Trenches are characterized by earthquakes associated with the subducting slab of oceanic crust (the Benioff zone).  Volcanoes are produced above the subduction zone on the continent.

The continental slope occurs on the landward side of the trench.  The continental slope angle changes from 4-5° on the upper part to 10-15° or more near the bottom of the trench.

The Mid-Oceanic Ridge


Mid-ocean ridges are giant undersea mountain ranges.  There are 49,700 miles of mid-ocean ridges on earth.  They are 930-1550 miles wide and 1.2-1.8 miles high.

The crests of the mid-ocean ridges are rift valleys: normal fault-bounded, down-dropped areas where the crust is undergoing extension.  They are about the size of the Grand Canyon.

Mid-ocean ridges are characterized by:
1. Basalt eruption (pillow basalt's)
2. Shallow earthquakes
2. High heat flow
4. Black smokers (sulfide minerals) and associated exotic organisms (that survive toxic chemicals, high temperatures, high hydro-static pressure, and total darkness).  These organisms may give some evidence for how life first evolved on earth.
Sediments of the Sea Floor
Sea floor sediment varies in thickness but can be up to thousands of meters thick in spots.

Terrigenous sediment – sediment derived from land that finds its way to the sea floor (via turbidity currents).

Pelagic sediment – sediment that settles slowly from ocean water.  It is made of:
a)            Fine grained clay- washed to deep sea.
b)           Volcanic ash- airborne fallout
c)            Skeletons of microscopic organisms (foraminifera and radiolarian).
The Age of the Seafloor
The age of oceanic crust and seafloor sediments do not exceed 200 million years in age (~ Jurassic).  In contrast, the oldest crustal rocks are 3.7 - 4.3 billion years old.

The young ages reflect recycling of dense oceanic crust at subduction zones.