Earth Structure and CompositionLayers, Rocks, Minerals and the Rock Cycle

The Layers of the EarthThe Earth is an oblate spheroid – the Solid Earth. It is composed of a number of different layers as determined by deep drilling and seismic evidence.

The Four Basic Layers The crust is the layer that you live on, and it is the most

widely studied and understood. The mantle is much hotter, has the largest mass, and

several layers (uppermost/rigid mantle, asthenosphere, lower mantle).

The outer core and inner core are even hotter with pressures so great you would be squeezed into a ball smaller than a marble if you were able to go to the center of the Earth!

A More Detailed View of Earth’s Structure

The Crust The Earth's

Crust is like the skin of an apple. It is very thin in comparison to the other three layers.

The crust is only about 3 - 5 miles thick under the oceans (oceanic crust) and about

25 miles thick under the continents (continental crust).

The Crust

The crust is composed of two different rocks. The continental crust is mostly granite. The oceanic crust is basalt.

Basalt is much denser than granite. Because of this the less dense continents ride on the denser oceanic plates.

The Mantle

The uppermost part of the mantle is rigid, and together with the crust, forms the Lithosphere

The middle part of the upper mantle is composed of very hot dense rock that flows like asphalt, and it is called – asthenosphere

The lower mantle is hot and dense.

The Mantle is the largest layer of the Earth (1800 miles thick – 2/3 of earth’s mass), it is hot (5100 - 3300◦ F), and the source of most magma → (lava)

The Lithosphere The crust and the uppermost

layer of the mantle together make up a zone of rigid, brittle rock called the Lithosphere.

The Lithospheric Plates

The crust of the Earth (which is part of the lithosphere) is broken into many pieces called lithospheric/crustal plates. The plates "float" on the soft, semi-rigid or plastic asthenosphere.

The AsthenosphereThe asthenosphere is the semi-rigid part of the upper mantle that flows like hot asphalt under a heavy weight.

Convection Currents The asthenosphere

"flows" because of convection currents.Convection currents are caused by the very hot material at the deepest part of the mantle rising, then cooling and sinking again – repeating this cycle over and over. When the convection currents flow in the

asthenosphere they also move the lithospheric/crustal plates.

The Outer Core

The core of the Earth has a radius of 2100 miles and contains 1/3 of Earth’s mass.

It is like a ball of very hot metals, with estimated temperatures of 12,400˚F at the center, and 8,600˚F at the outer limits.

The Inner CoreThe inner core of the Earth, about 700 miles thick, has temperatures and pressures so great that the metals are squeezed together and are not able to move about like a liquid, but are forced to vibrate in place like a solid.

The outer core (1400 miles thick) is so hot that metals in it are in liquid state. It is composed of mainly melted nickel and iron → Earth’s magnetic sphere is largely related to this outer core

Core to Crust

Earth Structure: Established Relationships

Density of materials (rocks, minerals) greatest in the center (core), and decreases towards the surface

Gravitational force strongest at the center, decreasing towards the surface

Temperature also decreases from center to the periphery

Pressure decreases from center to periphery as well

Layers in earth’s internal structure are differentiated by composition, density, temperature and other characteristics

Earth’s interior is an immense reservoir of minerals and geothermal energy

Questions1. What are the different layers in

Earth’s Structure that are indicated on this diagram?

2. Which layers together constitute the Lithosphere?

3. Which layer is characterized by convection currents?

4. What layers are solid? Which ones are liquid and/or plastic?

5. Have we ever seen part of the Mantle? Explain.

Detailed View of Earth’s Structure

Rocks, Minerals and the Rock Cycle

WHAT YOU LEARN

• To distinguish between minerals and rocks

• How rocks are classified

• How rocks are constantly recycled providing raw materials for other rocks

EARTH’S CRUST – Rocks and Minerals

The earth’s crust is composed of rocks. Rocks are primarily composed of minerals (but may also

contain organic materials). The granite and basalt rocks of the continental and

oceanic crusts were the original, igneous rocks.

What is a mineral? • A naturally occurring inorganic substance• With a definite chemical composition and atomic structure• Crystalline in nature, displaying consistent geometric

shapes• Commonly grouped into: Silicates, Oxides and Carbonates

• Also, metallic and non-metallic minerals

What is a Rock? A rock is an aggregate of mineral particles – but may

also contain organic materials

So, minerals are essentially the building blocks of rocks

Bedrock → Outcrop → Regolith → Soils

Rock Classification

• Igneous Rocksform due to the cooling

and crystallization of magma

• Sedimentary Rocksform through

lithification of sediments from other rocks

• Metamorphic Rocksform via

recrystallization of other rocks due to heat, pressure, and chemical alteration

IGNEOUS ROCKSIgneous Rocks Are Subdivided into Two Classes:

Volcanic (Extrusive) Igneous Rocks

Volcanic extrusive igneous rocks form on earth's surface as lava cools

Basalt Basalt is the most widespread volcanic rock It is a dark, fine-grained rock Basalt is the rock of the sea floor

Plutonic (or Intrusive) Igneous Rocks

Plutonic igneous rocks form deep underground where magma cools slowly; these rocks have a coarse crystalline texture

Granite Granite is the most widespread of plutonic igneous rocks.

It underlies much of the continental crust.

Igneous Rocks

SEDIMENTARY ROCKSMost sedimentary rocks are formed of layers of materials that have washed into lakes, rivers and oceans – • Sedimentary rocks form strata

• Often layers are tilted by earth movements

• Sedimentary rocks contain fossils

How do sediments turn into hard rock? Through Lithification Processes:

Compaction

Cementation

Crystallization

Subclasses of Sedimentary Rocks:

1. Clastic: form from bits and pieces of other rocks

2. Chemical: consist of minerals deposited from a solution

3. Organic: consist of organic matter such as plants and animal remains Organically-formed sedimentary rocks form from the remains of plants and animals (fossil limestone, coal)

Sedimentary Rocks

METAMORPHIC ROCKSMetamorphic rocks are rocks that have been changed in form due to heat, pressure, and chemical alteration.

FOLIATED NONFOLIATED  Slate Marble  Schist Quartzite

Gneiss

Slate: forms when shale is compressed by heat and pressure; splits easily

Schist: dominated by platy or needle-like minerals that form shiny layers

Gneiss: under pressure the minerals in granite recrystallize to form bands of light and dark minerals

Marble: Limestone recrystallizes into marble – a denser and more crystalline form of calcite

Quartzite: Sandstone changes into quartzite; Sand grains recrystallize to form a hard mass of quartz

Metamorphic Rocks

HOW ROCKS RECYCLE ? The rock cycle is a general model that describes how

various geological processes create, modify, and influence rocks

The origin of all rocks can be ultimately traced back to the

solidification of molten magma Magma consists of a partially melted mixture of elements

and compounds commonly found in rocks Magma exists just beneath the solid crust of the Earth in an

interior zone, the mantle

The Rock Cycle shows how rocks of any rock class can be recycled into rocks of any other rock class.

The Rock Cycle

Stages in the Rock Cycle

All rock types physically and chemically decomposed by a variety of surface processes collectively known as weathering

The debris thus created often transported by erosional processes via streams, glaciers, wind, and gravity

When this debris is deposited as permanent sediment, the processes of

burial, compression, and chemical alteration over long periods of time produce sedimentary rocks

Geologic processes like tectonic folding and faulting exert heat and pressure on both igneous and sedimentary rocks, altering them physically or chemically – rocks modified in this way are termed metamorphic rocks

Any of the rock types can eventually be returned to Earth's interior by tectonic forces at areas known as subduction zones

Once in Earth's interior, extreme pressures and temperatures melt the rock back into magma to begin the rock cycle again