Flashcards in C2a Deck (20):
How is the structure of the earth described?
As a sphere with a thin rocky crust, a mantle and an iron core.
What does the movement of tectonic plates result in?
Volcanic activity and earthquakes where the plates meet. It's the movement of the plates against each other that causes them.
What is the movement of the tectonic plates like?
Very slow, at about 2.5cm a year.
What are the timescales involved in the movement of continents?
300 million years ago there had just been one continent.
What have been put forward to explain the nature of the Earth's surface?
Who accept the theory of plate tectonics?
What is the lithosphere?
The relatively cold rigid outer part of the earth that includes the crust and part of the mantle. Made of tectonic plates that are less dense than the mantle below.
What are the problems associated with studying the structure of the earth?
The crust is too thick to drill through and there is the need to use seismic waves produced by earthquakes or manmade explosions.
Why is the theory of plate tectonics now widely accepted?
It explains a wide range of evidence and it has been discussed and tested by a wide range of scientists.
What is the mantle?
The zone between the crust and the core which is cold and rigid just below the crust and hot and non rigid at greater depths and therefore able to move.
What is the theory of plate tectonics?
Energy transfer involving convection currents in the semi rigid mantle causing the plates to move slowly.
Oceanic crust more dense than continental crust.
Collision between oceanic plate and continental plate leads to subduction and partial melting.
Plates cooler at ocean margins so sink and pull plates down.
What is the development of the theory of plate tectonics?
In1914, Wegener hypothesised that Africa and South America had previously been one continent which had then split. Evidence was that there were matching layers of rocks on different continents. His theory of 'continental drift' supposed that about 300 million years ago there was just one supercontinent, which he called Pangaea. According to Wegener, it broke into smaller chunks, the continents today, and they are still drifting apart today. The theory wasn't accepted at first because his theory explained things that couldn't be explained by the 'land bridge' theory, e.g. the formation of mountains, which Wegener said happened as continents smashed together. The main problem was that his explanation of how they drifted apart wasn't very convincing, and the movement wasn't detectable. He claimed the continents movement could be caused by tidal forces and the Erath's rotation - other geologists showed this was impossible.
In the 1960's, scientists investigated the Mid-Atlantic ridge, which runs the whole length of the Atlantic. They found evidence that magma rises up through the se floor, solidifies and forms underwater mountains that are roughly symmetrical either side of the ridge. The evidence suggested that the sea floor was spreading at about 10cm per year. This was convincing evidence that continents were moving apart. All evidence supported Wegener's theory so it was gradually accepted.
What does the type of volcanic eruption depend on?
The composition of the magma.
How is the size of crystals in an igneous rock related to the rate of cooling of molten rock?
If it cools quickly, the crystals are small but if it cools slowly, the crystals are large.
What are the different forms of molten rock?
Below the surface of the Erath it's called magma, but when it erupts from a volcano it's called lava.
What are the two ways volcanoes erupt?
Some volcanoes erupt runny lava fairly safely, while some erupt thick lava violently and catastrophically.
What are the different types of igneous rock that are formed from lava?
Iron rich basalt is formed from runny lava from a fairly safe volcanic eruption. Silica rich rhyolite is from thick lava from an explosive eruption.
Why do some people choose to live near volcanoes?
The lava and ash deposited during an eruption breaks down to provide valuable nutrients for the soil. This creates very fertile soil which is good for agriculture.
The high level of heat and activity inside the Earth, close to a volcano, can provide opportunities for generating geothermal energy.
Why do geologists study volcanoes?
To be able to forecast future eruptions and to reveal information about the structure of the Earth.