Earth's Structure

Earth is an imperfect sphere, with a hot weird middle and a crust of rock around the outside.

The crust is a tiny part of our planet - it is just 50-100 km deep, on a planet with a radius of ~6400 km.

   Earth's rocky crust is constantly changing and adjusting from internal and external forces. We reconstruct past environments and continental configurations using geology.

## Rocks on Earth's continents reveal change over time.

Continents contain a mix of metamorphic, igneous, and sedimentary rocks.

If you hike in the Wasatch Mountains, you'll see all three kinds!

* Sedimentary rocks show signs of environmental change over time:
	+ Grain characteristics that show the amount or distance of travel.
	+ Packing and sorting conditions that show the depositional environment.
	+ Grain ingredients that show the source of the bits.
	+ Past presence of rivers, lakes, oceans, etc. are easy to spot in sedimentary rocks.
* Metamorphic rocks demonstrate wacky changes in physics and chemistry that acted on the rocks over time.
	+ Certain minerals only form at high pressure, or high temperature.
		- We can experiment with high pressures and temperatures in the laboratory using a diamond anvil cell (one of the specialties of our own Dr. Lowell Miyagi
		- A diamond anvil cell places a tiny experimental chamber between the tips of two diamonds.
		- The diamonds are strong enough to squeeze together and create just a tiny pocket of crazy-high pressure and/or temperature.
	+ People spend their entire careers studying how, where, how, and when metamorphic rocks form.
	+ We won't discuss them much in this class because metamorphic rocks usually jack up fossils. Boooooo!
	+ But their chemistry and physics require explanation! And that underpins many of the final details of plate tectonic ideas.
* Igneous rocks on continents are MOSTLY the intrusive kind.
	+ Intrusive igneous rocks formed by slow-cooling magma that doesn't pour over the Earth's surface.
		- Great examples are the granite and granodiorite found in the Wasatch and Uinta ranges of Utah, and in many historic SLC buildings, including the Temple.
		- The big rocks of Yosemite National Park (Half Dome, El Capitan, etc.) are made of granodiorite, too.
* But there are plenty of intrusive igneous rocks decorating the continents too!
	+ You can easily see cool igneous rocks from lava flows over continental areas:
		- All around Southern Utah.
		- In the Craters of the Moon National Monument in Idaho.
		- All around Northern Arizona and Eastern California.
		- Along the Columbia River in the Pacific Northwest.
		- Along the Palisades in New Jersey.

Earthquakes also show change in continental rocks - even in REAL TIME! If you've felt an earthquake, you've felt the consequence of rocks moving over time. It takes a long time for rocks to move, and they usually are stuck where the edges rub together. The tension is occasionally released as an earthquake that lets the rocks slide past each other, but feels very weird for the rest of us. We can easily observe signs of PAST earthquakes in lots of ways - rock layers cut by a highway that show offsets; trenches in sediments; and big shifts in the rocks displayed in our local mountains.

## Rocks on Earth's seafloors reveal change over time.

Ocean water sits low.

* Think of the Earth as a solid rock, like the moon, and then just POUR water onto its surface.
* Continents, made of thick low-density rock slabs, peak above the surface of water.
* Thinner slabs of denser crust made of extrusive igneous rock usually stays low, wrapped tightly around the Earth's mantle, so the seawater settles above.

Rocks at the bottom of the sea floor are usually made of extrusive igneous basalt.

* We can actually see new basalt forming at the sea floor and coming out as new rock.
* We can study the microbial life that flourishes on these fresh minerals.
* Sediments can build rock layers that layer on top of the basalt slab.
* We can use the texture, quality, and chemistry of the basalt rocks to measure how long they've been exposed to weathering.
* We can use the sediment piles or layers of rock to measure changes in environmental conditions during this time.

Some of the basalt that forms under the sea pokes above the ocean's surface.

* Basalt can pile into enormous undersea volcanoes.
	+ The islands of Hawai'i are an example: the islands poke above the ocean's surface.
	+ Colonies of coral and other carbonate-mineral-making animals coat the flanks of these islands, and add layers of sedimentary rock.
	+ People living there have to cope with volcanic eruptions.
* Basalt can make under-sea mountain ranges where new rock is born every day.
	+ A good example is the Mid Atlantic Ridge.
	+ The Mid Atlantic Ridge is a mountain range running N-S half-way between the Americans and Europe/Africa.
	+ Iceland is built on a part of the ridge that pokes above the surface of the ocean.
	+ People living there have to cope with volcanic eruptions.
* Magnetic particles form little compass indicators inside of basalt rocks.
	+ Basalt rocks have lots of iron-based minerals.
	+ These minerals can form little magnets that align to the Earth's magnetic poles as the rock cools and solidifies.
	+ If basalt rocks form layer on top of layer on top of layer, in a big stack...
		- we can measure the magnetic particles through the stack in order.
		- Typically, the older rocks are at the bottom of the stack.
	+ If basalt rocks form at the Mid Atlantic Ridge and then get pulled away...
		- they make room for new basalt rocks to enter the crack and form more fresh rock.
		- One consequence is that instead of building up in a vertical pile, they form side-by-side stripes.
		- Oldest rock stripes are farthest from the crack. Newest rock stripes are closest to the crack.
		- Here's an animation that's shows how this process works
	+ Layers of basalt rock can be matched to other layers far away that erupted at the same time, or from the same chemical soup.
	+ The little magnets aligned to poles tell us which way was magnetic north.
	+ In a stack of basalt flows, or in a series of neighboring basalt stripes, our measurement of North SWITCHES DIRECTIONS over and over!!
		- Lots of physics reasons explain why Earth's magnetic field SHOULD flip periodically, rather than stay exactly the same as today.
		- We can measure the field in detail today, and it's way more complex that we draw in books!
		- We can measure and compare the timing of basalt eruptions by comparing the magnetic north orientation.
* Earthquakes are also always happening in thin under sea basalt crust.
	+ Usually they don't impact life on land.
	+ They are always happening near the Mid Atlantic Ridge and other places where lava is forcing its way out.
	+ We can detect these a lot better with modern equipment.

**Many of basic observations of rocks on our planet were not available one hundred years ago.**

Many big surprises about rocks under the sea came to scientists' attention during the second world war, when submarine warfare required us to learn about the structure, bathymetry (underwater elevations of stuff) and physical traits of the rocks down there. We used magnets a lot because we were searching for submarines. The size of undersea mountains, and the stripes of rock with opposed magnetic signals baffled geologists at first.

## Plate Tectonics is a Framework

Observations that demonstrate motion and change of Earth's crust can be interpreted in the conceptual framework called Plate Tectonics.We recommend this explainer video with lots of diagrams, here.

Here are the basic ideas in Plate Tectonics.

* We represent the crust as a set of crunchy units that shift around on the Earth's surface.
* Each plate has a slab of rock that makes most of its material.
	+ Some plates contain thick slabs of low-density intrusive igneous rock (granite, diorite, etc.).
	+ Some plates contain thin slabs of high-density intrusive igneous rock (usually basalt).
	+ Some plates contain both kinds of rocks slabs.
* Water sits in low spots.
	+ Oceans nestle between continents.
	+ Basalt rocks deep under the sea form thin crusts of dense rock.
		- CHEMICAL COMPOSITION of the lava causes the basalt rocks to be dense.
		- The presence of water above them is NOT the reason the lava forms dense rock.
	+ Thick slabs of low-density intrusive igneous rocks poke above the ocean's surface and are easy to see as continents.
	+ High-density extrusive igneous rocks can also build up enough to poke above the ocean's surface: consider Iceland and Hawai'i.

* Sediments settle and form sedimentary rocks on these plates.
	+ Sediments can build rock on land, which adds to the thickness of continental slabs.
	+ Sediments can build rock on the coasts, which makes the slope transition between continent and sea floor a lot more gradual.
	+ Sediments can build rock on the seafloor, too.
* Metamorphic rocks form wherever the plates get nasty.
	+ Collision, scraping, or squishing of the Earth's crust plates changes the physics and chemistry acting on rocks.
	+ This can cause different minerals to fall apart or to form.
	+ If the same elements are there but they are in new mineral configurations, you'll get a different kind of rock out of the deal.
* Igneous rocks form continually where...
	+ plates are stretching away from each other, leaving a little crack for lava to push through.
		- This is always happening under the sea, in places like the Mid Atlantic Ridge.
	+ Or where plates are smashing together, making a thin slab of sea floor basalt slide under another plate.
		- This is happening in the Pacific Northwest of North America, and along the Andean Mountains of South America.
	+ Or where a pimple seems to let lava leak through.
		- This is happening all the time in Hawai'i, as lava is trying to make its way up through the pile.
		- The northwest islands of the archipeligo are no longer active, because the crust slab slid away from the pimple.
		- The fancy name for this concept is Hot Spot.
* Volcanoes form where the plates are crashing together.
* Earthquakes from where the plates are crashing together, AND where the plates are trying to slide past each other.
	+ Nasty earthquakes from plate collision happen in...
		- Tibet (continent plate crashing into continent plate)
		- Indonesia (ocean plate crashing into ocean plate)
		- the West Coast of South America (ocean plate crashing into continent plate)
		- the California Coast of North America
			* Here a plate that is MOSTLY ocean, the Pacific Plate, is dragging a sliver of continent on its edge.
			* Los Angeles is located on the Pacific Plate.
			* Los Angeles is moving NORTH with the Pacific Plate.
			* The North American Plate is carrying most of the North American continent, plus half the crust underneath the North Atlantic ocean.
			* San Francisco is moving SOUTH with the North American Plate.
			* If you measure the plate motion from the middle of each plate, say, a GPS unit in Hawai'i vs one in Nevada, the plates are moving apart about a 5 mm per year. That's about as fast as fingernails grow.
			* If you measure the plate motion right near the San Andreas Fault, you get zero motion. It's stuck. Until it's not stuck, and then we need Dwayne Johnson to save us.

ACTIVE MARGINS are continent edges that sit at plate boundaries that are crashing into or alongside each other, causing volcanoes and earthquakes.

PASSIVE MARGINS are continent edges that sit in the middle of a plate. Volcanoes and earthquakes are rare along passive margins.

## We can Reconstruct Past Geographic Conditions.

We can combine observations from geology, chemistry, paleontology, and physics with the concept framework of plate tectonics to estimate past configurations of Earth's continents, oceans, and water-driven climate systems.

Certain magnetic minerals can tell us the position of Earth's magnetic poles AT THE TIME that mineral formed.

* We compare magnetic minerals to relate stripes of seafloor basalt
	+ We can measure the order and spacing of each stripe.
	+ We can use the periodic flip-flop pattern of Earth's magnetic poles to organize basalt stripes into a relative timeline of eruptions.
* We compare magnetic minerals to relate layers of lava rock that piled up on continents of Earth's surface.
	+ We can measure the thickness and order of each lava rock layer. Older rocks should be at the bottom, newer rocks on top.
	+ We can use the periodic flip-flop pattern of Earth's magnetic poles to organize lava rock layers into a relative timeline of eruptions.
	+ If part of a continent has ROTATED while its plate moved through time, the magnets can show this, too.

It's taken decades to learn how to observe magnetic minerals correctly, and to interpret what they say about a rock's relative age, original orientation, and relationship to its neighboring rocks. Scientists like Utah's Dr. Pete Lippert still spend their entire careers unraveling the history of ancient rocks, so we can better understand how our planet works as a dynamic system.

Fossil are vital to reconstruct paleo geography in two main ways.

* Paleontologists compare fossils from around the world.
	+ Some fossils of plants or land animals are found on far-away continents today, but may have lived in a time when these continents were not so far apart.
	+ Fossils can show what habitats were present:
		- Echinoderms (sea urchins, sea stars, crinoids, etc.) generally only live in OCEAN water of normal salinity.
		- Sea snails can tolerate a wild variety of salinities.