Tectonic Plates

From the deepest ocean trench to the tallest mountain, plate tectonics explains the features and movement of Earth’s surface in the present and the past.

Plate tectonics is the theory that Earth’s outer shell is divided into several plates that glide over the mantle, the rocky inner layer above the core. The plates act like a hard and rigid shell compared to Earths mantle. This strong outer layer is called the lithosphere.

The concept of plate tectonics was formulated in the 1960s. According to the theory, earth has a rigid outer layer, known as the lithosphere, which is typically about 100 km (60 miles) thick and overlies a plastic layer called theasthenosphere. The lithosphere is broken up into about a dozen large plates and several small ones. These plates move relative to each other, typically at rates of 5 to 10 cm (2 to 4 inches) per year, and interact along their boundaries. Such interactions are thought to be responsible for most of Earth’s seismic and volcanic activity, although earthquakes and volcanoes are not wholly absent in plate interiors. Plate motions cause mountains to rise where plates push together, or converge, and continents to fracture and oceans to form where plates pull apart, or diverge. The continents are embedded in the plates and drift passively with them, which over millions of years results in significant changes in Earth’s geography.

 

Pine Oak Forest

Central American pine-oak forests

The Earth’s Atmosphere

The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth’s gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night (the diurnal temperature variation).

Composition

The three major constituents of air, and therefore of Earth’s atmosphere, are nitrogen, oxigen, and argon. Water vapor accounts for roughly 0.25% of the atmosphere by mass.

Structure of the atmosphere

The atmosphere of the Earth may be divided into several distinct layers, as the following figure indicates.

 

 

TROPOSPHERE

This is the layer of the atmosphere closest to the Earth’s surface, extending up to about 10-15 km above the Earth’s surface. It contains 75% of the atmosphere’s mass. The troposphere is wider at the equatorthan at the poles. Temperature and pressure drops as you go higher up the troposphere.

STRATOSPHERE

This layer lies directly above the troposphere and is about 35 km deep. It extends from about 15 to 50 km above the Earth’s surface.

MESOSPHERE

Directly above the stratosphere, extending from 50 to 80 km above the Earth’s surface, the mesosphere is a cold layer where the temperature generally decreases with increasing altitude. Here in the mesosphere, the atmosphere is very rarefied nevertheless thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky.
THERMOSPHERE

The thermosphere extends from 80 km above the Earth’s surface to outer space. The temperature is hot and may be as high as thousands of degrees as the few molecules that are present in the thermosphere receive extraordinary large amounts of energy from the Sun. However, the thermosphere would actually feel very cold to us because of the probability that these few molecules will hit our skin and transfer enough energy to cause appreciable heat is extremely low.